Applied Geochemistry xxx (2013) xxx–xxx

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Applied Geochemistry

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An experimental model approach of biologically-assisted silicatedissolution with olivine and Escherichia coli – Impact on chemicalweathering of mafic rocks and atmospheric CO2 drawdown

0883-2927/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apgeochem.2013.01.007

⇑ Corresponding author. Tel.: +33 1 47 52 53 17.E-mail address: bruno.garcia@ifpen.fr (B. Garcia).

Please cite this article in press as: Garcia, B., et al. An experimental model approach of biologically-assisted silicate dissolution with olivine and Eschcoli – Impact on chemical weathering of mafic rocks and atmospheric CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/1j.apgeochem.2013.01.007

Bruno Garcia a,⇑, Laurence Lemelle b,c, Estelle Rose-Koga d, Pascal Perriat e, Romain Basset b,Philippe Gillet b,f, Francis Albarède b

a IFP Énergies nouvelles, Direction Géosciences, 1-4 Avenue Bois Préau, 92500 Rueil-Malmaison, Franceb Ecole Normale Supérieure de Lyon, Laboratoire de Géologie de Lyon, 15 Parvis René Descartes, 69342 Lyon Cedex 07, Francec Ecole Normale Supérieure de Lyon, Laboratoire Transdisciplinaire Joliot Curie, 15 Parvis René Descartes, 69342 Lyon Cedex 07, Franced Université Blaise Pascal Clermont-Ferrand, Laboratoire Magmas et Volcans, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, Francee Insa de Lyon, Laboratoire de Recherche Matériaux, Ingénierie et Science, 7 Avenue Jean Capelle, 69621 Villeurbanne Cedex, Francef Earth and Planetary Science Laboratory, Ecole Polytechnique Fédérale de Lausanne, Switzerland

a r t i c l e i n f o

Article history:Received 3 September 2012Accepted 15 January 2013Available online xxxxEditorial handling by D. Fortin

a b s t r a c t

Chemical weathering of Mg, Ca-silicates and alumino-silicates contributes significantly to the drawdownof atmospheric CO2 over long time scales. The present work focuses on how this mode of weathering maychange in the presence of free-living bacteria in oligotrophic waters, which compose most of the surfacefreshwaters of the Earth. Forsterite (Fo90) was reacted for 1 week with a stable Escherichia coli populationin water maintained at 37 �C and neutral pH in a batch reactor. Control samples with suspensions of pureolivine powders and E. coli cells in pure water were also used for reference. Olivine controls reproduce theMg, Si and Fe release in solutions predicted from rates published in the literature with pH shifts of lessthan 0.5 unit. After 1 week, under abiotic conditions, weathered surfaces are enriched in Fe and Fe3+ rel-ative to the initial composition of the mineral. Bacterial controls (without minerals) show decreasing Ehwith increasing cell concentrations (�50 mV with 7 � 107 cells/mL and �160 mV with 8 � 108 cells/mL).Magnesium concentrations in bacterial control solutions are in the lg/L range and can be accounted forby the release of Mg from dead cells. More than 80% of the cells were still alive after 1 week. The solutionsobtained in the experiments in which olivine reacts in the presence of cells show Mg and Si concentra-tions a few tens of percent lower than in the mineral control samples, with a prominent depletion ofFe(III) content of the mineral surfaces. Magnesium mass balance discounts both significant bacterialuptake and inhibition of the Mg dissolution rates as a consequence of changing pH and Eh. Coating bybacterial cell layers is also negligible. E. coli reduces the chemical weathering of olivine. This study infersthat the presence of free-living Proteobacteria, a prevalent group of subsurface bacteria, should decreasethe amount of riverine Mg released by chemical weathering of mafic rocks.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The impact of microbial–mineralogical feedback mechanismson the long-term response of the Earth system is unclear. Global-scale models verify that chemical weathering of Mg and Ca-bearingsilicates and alumino-silicates present in the continent directly im-pact climate. Their weathering products releasing alkalinity intothe ocean, which in turn is primarily removed from seawater byprecipitation of marine carbonates (Garrels and MacKenzie,1971; Broecker and Peng, 1982) leading to draw down of CO2.The rates of chemical weathering of Ca-silicates, Mg-silicates, and

Ca–Mg-silicates determine the rate of supply of Ca and Mg tooceans and thus affect the magnitude of this critical feedbackmechanism. If microorganisms significantly affect mineral dissolu-tion rates (e.g. Kalinowski et al., 2000a,b; Liermann et al., 2000;Santelli et al., 2001), then the evolution of the biosphere (domi-nated by microorganisms for the majority of time), atmosphere,hydrosphere, and lithosphere must have been closely coupled(Banfield et al., 1999). It is, for example, widely accepted thatearly in Earth’s history (between 1 and 3 Ga, Kasting, 1992, 1993;Kasting et al., 1992), the atmosphere was dominated by CO2 andthat O2 concentrations only increased with the evolution of effi-cient oxygenic photosynthetic microbial populations. Indeed, fromthe following equation:

CO2 þH2A ðelectron sourceÞ ! CH2Oþ 2A

erichia0.1016/

2 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

it can be shown that one C atom (from one molecule of atmosphericCO2) becomes a C in organic matter (CH2O) and, during sedimentationprocesses, this C in organic matter could become hydrocarbon imply-ing that one C atom from one molecule of CO2 is utilized in hydrocar-bon production. With photosynthesis (CO2 + H2O ? CH2O + O2),for each C atom stored in sediments, one molecule of O2 is liberatedto the atmosphere.

Moreover, early inorganic rock weathering resulted in the accu-mulation of Ca and Mg in ocean waters, leading to precipitation ofCa-, Mg-carbonates and draw down of CO2. At present, silicateweathering is known to account for a global CO2 drawdown rateof �108 tons/a (Gaillardet et al., 1999; Hilley and Porder, 2008).Basalts are responsible for one third of this consumption, eventhough they represent less than 5% of the continental area coveredby silicates (Dessert et al., 2003; Navarre-Sitchler and Brantley,2007). Chemical release of Mg and Ca into rivers is dominated bycarbonate dissolution (Meybeck, 1987; Stallard and Edmond,1983; Louvat et al., 2008) but dissolution of mafic silicates by run-off and subsurface waters still represents a major sink for CO2

(Regnier et al., 2005). Currently, quantitative impact of microbeson this draw down is not predictable. Chemical weathering resultsfrom intricate relationships between biological and geochemicalprocesses in soils, sediments and aquifers. Biomass modifies thechemistry of soil solutions (Lucas, 2001) and affects the composi-tion of river waters (Benedetti et al., 2003; Pogge von Strandmannet al., 2008). Vegetal cover and associated mycorrhiza affect the pHand organic ligand abundances in soil solutions and consequentlychange the dissolution rates of silicates (White and Brantley,1995). Biomass fluctuations modify the storage and release intosoil solutions of Mg and Ca, which are major elements in cells. Bac-teria and solutions are washed out from soils during floods andover the rainy season causing microbial concentration (Palijanand Fuks, 2006) peaks in rivers.

The elemental composition of bacteria has not been determinedextensively (Jones et al., 1979). Although increasing interest hasdeveloped in the role of mineral elements in the physiology ofmicrobial cells (e.g. Epstein and Schultz, 1965) and their transfor-mations in nature (e.g., Jernelov and Martin, 1975), data on the bio-accumulation of mineral cations by bacteria have remained sparse(e.g. Bowen, 1966). Washing procedures have been variable, if per-formed at all. The conditions of growth such as pH, elemental com-position of the medium, growth rate, and the source of the electrondonor in photosynthetic organisms influence the elemental com-position of bacteria. Magnesium is the most abundant divalent cat-ion in living cells and mediates in numerous cellular activities. Theuptake of this ion in most prokaryotes is through the action of theCorA family, which is also one of the most studied families of diva-lent cation transporters (Guskov et al., 2012). Jasper and Silver(1977) reported that total cellular Mg was generally in the rangeof 360–840 mg/L (wet cells).

The present work focuses on the influence of free-living hetero-trophic, Proteobacteria, that are often predominant in rivers, on thedissolved Mg load into runoff during chemical weathering of acommon and fairly soluble mafic mineral, olivine under oligo-trophic conditions. Experiments on silicate dissolution performedin the presence of microbial cells in low-nutrient culture mediasuggest a limited geochemical impact of free-living bacteria onchemical weathering of mafic rocks in subsurface oligotrophicwaters. Oligotrophic refers to bodies of water with very low nutri-ent levels, nutrients being here considered as any chemical sub-stance providing energy or supporting metabolism that must beextracted from the environment by the organism to live. Under oli-gotrophic conditions, basaltic glass has been suggested to remainunaltered in the presence of bacteria isolated from marine basalts(Einen et al., 2006), whereas dissolution was enhanced in mediaamended with glucose (Thorseth et al., 1995). Similarly, under

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

oligotrophic conditions, no apparent effect was observed of a pop-ulation presence of a model soil strain belonging to Proteobacteria(Welch and Ullman, 1999) on Ca-rich feldspar dissolution rate wasreported and glucose addition increased the dissolution rate due tometabolic processes. Such observations illustrate a limitation ofthe general inference that the presence of microorganisms en-hances the rate of silicate weathering (White and Brantley,2003). This is widely interpreted as a result of bacterial populationsdecreasing the pH and increasing the concentrations of organic li-gands (Barker et al., 1997; Banfield et al., 1999; Liermann et al.,2000; Wu et al., 2007) when they are grown in nutrient-rich media.Experiments in the presence of Proteobacteria treated with azide, ametabolic inhibitor, show that the dissolution rate of fayalite isindistinguishable from that measured under abiotic acidic(pH < 4) conditions (Santelli et al., 2001). Extrapolating the lackof impact of a stable population of bacteria on the weathering offayalite to the nearly neutral pH of natural subsurface waters needssome re-evaluation. Both the physics and chemistry of the cell wall(Norde and Lyklema, 1989) and the dissolution kinetics (Wogeliusand Walther, 1991) are indeed strongly pH dependent.

The present study sets out to investigate the influence of a stablefree-living bacteria population of Escherichia coli, a well-understoodstrain of Proteobacteria, on the fraction of dissolved Mg originatingfrom chemical weathering of olivine, an ubiquitous and fairlysoluble mineral of mafic rocks, in near-neutral and oligotrophicwaters. Mass-balance of Mg will be presented both for olivine/E. coli suspensions and for abiotic and bacterial reference samples.

2. Methodology

The present study aims to investigate the influence of a fixedsized-population of bacteria (E. coli) on the amount of Mg releasedby chemical weathering of olivine powder in pure water. In orderto facilitate the assessment of elemental mass balance, the runswere conducted in open air and in batch reactors. Olivine is animportant Mg-silicate as it is a major mineral of various maficrocks (Delvigne et al., 1979; Smith et al., 1987; Banfield andHamers, 1997; Welch and Banfield, 2002; Fisk et al., 2006). Its maincomposition ranges through Mg2SiO4 (Forsterite) to Fe2SiO4 (Faya-lite). As a result of its nesosilicate structure, it dissolves faster thanother common silicates and its dissolution rate is well documentedunder different conditions (see the legend of Fig. 1 for references).The choice of olivine and open-air batch reactors was driven by theapparent lack of variation of Mg fluxes with CO2 pressure at neutralpH and ambient temperature (Wogelius and Walther, 1992). E. coliare Gram-negative bacteria as are Proteobacteria a major group ofbacterial communities common in subsurface freshwaters (Sigee,2005). Although the ecology of this particular enteric bacteriumis not directly representative of normal subsurface environments,the intracellular cationic composition of E. coli, its variability withculture conditions and cellular geometry are well documented (e.g.Jones et al., 1979; Neidhardt and Umbarger, 1996; Outten andO’Halloran, 2001) which makes E. coli a unique bacterium modelfor mass balance studies. In addition, E. coli is a better Gram-nega-tive bacterium model than any suspension of dilute organic com-pounds found in the bacteria wall for studying Mg cationexchange between the environment and the bacterium compart-ments as the structure of the envelopes is fundamental to achieveits essential function of selective barrier permeability (Amro et al.,2000). The Gram-negative wall (Beveridge, 1999) is made of anoutside membrane of proteins, lipoproteins and an asymmetric li-pid bilayer with an outer leaflet composed by lipopolysaccharides.Therefore, suspending a population of E. coli in ultra-pure water,devoid of nutrients, simulates oligotrophic conditions of rivers aswell as rainfall and floods under which heterotrophs may occur(3 � 106–1 � 1010 cells/mL in Basu and Pick, 1997). No other

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

10-14

10-13

10-12

10-11

10-10

10-9

0 2 4 6 8 10 12 14

a = 1.46m2 g-1

a = 0.09 m2 g-1

Rat

es (m

ol c

m-2

s-1

)

pH

Fig. 1. Calculated and experimental dissolution rates of natural olivine versus pH inpure and neutral water at 37 �C. Calculated rates were estimated from the empiricalrelationships describing the aqueous dissolution rate of a forsteritic and a fayaliticolivine at 25 �C, JT=25�C (Wogelius and Walther, 1992; Stopar et al., 2006; Rosso and

Rimstidt, 2000; Pokrovsky and Schott, 2000): (1) JT¼25�CMg2 SiO4

ðmol cm�2 s�1Þ ¼9:07� 10�12 � a0:54

Hþ þ 5:25� 10�15 þ 2:33� 10�17 � a�0:31Hþ . (2) JT¼25�C

Fe2 SiO4ðmol cm�2 s�1Þ ¼

1:1� 10�10 � a0:69Hþ þ 3:22� 10�14 þ 1:2� 10�16 � a�0:30

Hþ � YðpH-8Þ where aH+ = 10-�pH and Y(pH-8) is the Heavyside function (=1 if pH > 8, =0 elsewhere), assuming

the following linear combination: (3) JT¼25�CðMgx Fe1�x Þ2 SiO4

¼ xJT¼25�CMg2 SiO4

þ ð1� xÞJT¼25�CFe2 SiO4

where

x is the forsterite content. The temperature is taken into account, by using the

integrated Arrhenius equation: (4) Ln JTðMgx Fe1�x Þ2 SiO4

¼ Ln JT¼25�CðMgx Fe1�x Þ2 SiO4

þ Eað298�1�T�1 ÞR .

where Ea, is the activation energy (kJ mol�1 K�1), R the universal molar gas constant(8.314 J K�1 mol�1). The values of the experimental activation energies is quitevariable, 79.7 ± 10 kJ/mol (Wogelius and Walther, 1991), 38.1 ± 1.7 kJ/mol (Grand-staff, 1986), and 30 ± 2 kJ/mol (Van Herk et al., 1989), which results in a range ofdissolution rates. An average value equal to 78.8 ± 2.6 kJ/mol is also obtained byaveraging the Ea values measured down the a, b and c crystallographic axes of aforsteritic olivine, 115.2 ± 14.7 kJ/mol, 67.1 ± 8 kJ/mol and 80.9 ± 9.1 kJ/mol, respec-tively. This figure plots olivine dissolution rates versus pH for forsterite (solid line),olivine (dashed line) and fayalite (dotted line). The values fall between2.2 ± 0.05 � 10�14 (Grandstaff, 1986) and 4.3 ± 0.7 � 10�14 mol cm�2 s�1 (Wogeliusand Walther, 1991) at a pH of 7 and 37 �C. Dissolution rates measured at 37 �C forfayalite in culture medium (in circles, Santelli et al., 2001), for bronzite in water(triangle, Schott and Berner, 1983), and olivine powders of this study are alsoshown (in squares). An idea of the scattering of the dissolution rates measured inforsterite dissolution experiments is given in Fig. 1 of Rosso and Rimstidt (2000).

B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx 3

source of reduced C is present besides the bacteria themselves. Thetemperature of the suspension was maintained at 37 �C. Eventhough this is not typical of ambient subsurface temperature, thisexperimental choice permits fast dissolution kinetics and alsoavoids additional phenomena related to bacteria heat shocks. Theduration of the experiments (�1 week) was chosen to be shorterthan the doubling-times observed for microbial communities oflake sediments, surface soils, aquifers and aquifer sediments(Boone and Balkwill, 1988; Onstott et al., 1999; Mailloux andFuller, 2002), which allows minimizing the effects of populationgrowth and the primary production expected in oligotrophicwaters.

2.1. Sample preparation

Millimeter-sized San Carlos olivine crystals with �90% forste-rite (defined as 100 � [Mg]/([Fe] + [Mg])) and analyzed by Induc-tively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)giving the formula: Mg1.82Fe0.18SiO4 with a space group Pbnmand cell parameters a = 4.7620 (1) A�, b = 10.2235 (2) A�, c = 5.9936

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

(3) A�(Merli et al., 2000); consistent with the composition givenby electron microprobe: 38.36% SiO2, 0.24% Al2O3, 10.52% Fe2O3,0.14% MnO, 49.34% MgO, 0.10% CaO, giving the formula: Mg1.84-

Fe0.22Si0.96O4, were selected for their optical quality and lack ofinclusions. Two sets of powders were prepared to test the effectof grain size distribution and specific surfaces, noted a below inthe text. The range of grain sizes of the first set, unsieved andsilt-rich, was: 50 wt% < 20 lm, 30 wt% between 20–40 lm,12 wt% between 40–63 lm, and 8 wt% > 63 lm. The second setwas sieved to keep the 90 ± 10 lm sand fraction. One g of olivinepowder, freshly crushed, was introduced in each experimentalrun. No cleaning process was applied. The specific surfaces wereobtained from Kr adsorption BET multipoint isotherms, with valuesof 1.46 ± 0.04 m2/g for the first set and 0.09 ± 0.01 m2/g for the sec-ond set, respectively. The diffraction patterns show no broadeningafter grinding inferring the absence of a significant amount of con-taminating phases and that defects were not induced by the grind-ing process, so that the difference between the two investigatedsamples is the particle size.

The ampicillin resistant E. coli strain DH5a pGREEN-TIR (Millerand Lindow, 1997) was chosen to limit potential contaminationof the biotic solutions. It also expresses Green Fluorescent Protein,which can be used to identify this particular species. The colonieswere grown first on agar plates then liquid cultures were carriedout using Difco LB growth medium (approximate formula: 10 g/Lof pancreatic digest of casein, 5 g/L of yeast extract and 5 g/L ofNaCl). A flask of 200 mL of LB medium was inoculated with a bac-terial colony, then kept overnight in a spinning incubator, at 37 �Cand 180 rpm, with 200 lL of a solution of ampicillin (0.3 lM). Bac-teria were harvested in the late exponential phase of their growth,centrifuged for 10 min at 4000 rpm and washed twice with double-distilled water at room temperature. To determine the Mg contentof the E. coli cells, the bacterial pellets were transferred into a Ptcrucible, then evaporated to dryness at 95 �C for 24 h and finallyweighed. Pellets were then combusted in Pt crucibles in a mufflefurnace. Temperature was ramped from room temperature to800 �C with a heating rate of 1 �C/min. The final temperature waskept at 800 �C for 3 h. The residual solid was dissolved in 2 mL of0.3 N HNO3 for analysis by ICP-MS.

Experiments were conducted in polypropylene beakers (1 L)cleaned in a laboratory washing machine, sterilized in an autoclaveand rinsed in series with KOH, HCOOH and distilled water. Runsconsisted of 200 mL initial volumes of five aqueous solutionssimultaneously prepared in 1 L flasks: (1 and 2) two aqueous solu-tions with different bacterial cells concentrations, (3 and 4) thesame two bacterial solutions mixed with 1 g of olivine powder,and (5) one solution of double-distilled water with 1 g of olivinepowder. The pH of the solutions was adjusted to 7, when needed,by adding NaOH (4 N). Aqueous bacterial solutions were obtainedby suspending the E. coli cells in double-distilled water with cellconcentrations of 0.1 OD600nm (optical density at 600 nm) and 1OD600nm. These values correspond approximately to 8 � 107 and8 � 108 CFU/mL (colony-forming unit per mL), respectively. Allthese experiments were run in an incubator held at 37 �C. Experi-ments were conducted under two contrasting stirring regimes, nostirring and stirring at 130 rpm on a rotary mixer.

Aliquots of solutions were sampled at different times. Ten mL ofsolution were sampled after 24, 48, 96 and 168 h of experimentsusing polypropylene syringes and further cultivated or filtered at0.22 lm for chemical analyses. The volume loss by aliquot removalaffects the result by less than 5% and left the results within limitsof analytical uncertainties (Table 1). On the last aliquot, after 168 h,powder was also recovered. It was sampled in suspended form in a50 mL polypropylene syringe and filtered on a nylon filter with a5 lm mesh size. The solid was gently rinsed with double-distilledwater and sampled for XPS analysis.

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

Table 1Concentrations of dissolved Mg, Si and Fe (lmol/L) as a function of the duration of the dissolution experiment (h). Concentrations are reported for the experiments performed atlow (OD600nm � 0.1) and high cell concentrations (OD600nm � 1), for olivine powders with a high (a = 1.46 m2/g) or a low (a = 0.09 m2/g) specific surface area, and under differentstirring conditions (s = 0 and 130 rpm). Two experiments were done with the low specific area (Exp. #1 and Exp. #2), but only Exp. #2 was analyzed for Mg, Si and Fe. The volumeloss by aliquots removal affects the result by less than 5% and the results remain within limits of analytical uncertainties.

a = 0.09 m2/g a = 1.46 m2/g a = 1.46 m2/gs = 130 rpm s = 130 rpm s = 0 rpm

Exp. #1a Exp. #2

[Mg] (lmol/L) [Mg] (lmol/L) [Si] (lmol/L) [Fe] (lmol/L) [Mg] (lmol/L) [Si] (lmol/L) [Fe] (lmol/L) [Mg] (lmol/L) [Si] (lmol/L) [Fe] (lmol/L)

Abiotic controlt = 20 h 62 ± 2 50 ± 4 32 ± 4 n.d. 466 ± 11 272 ± 8 0 ± 0.1 128 ± 4 78 ± 4 1.7 ± 0.2t = 48 h 75 ± 2 59 ± 4 36 ± 4 n.d. 577 ± 14 323 ± 9 0 ± 0.1 140 ± 4 91 ± 4 1.6 ± 0.2t = 120 h 102 ± 3 77 ± 4 47 ± 4 n.d. 663 ± 15 375 ± 11 1.2 ± 0.2 146 ± 4 95 ± 4 1.6 ± 0.2t = 168 h 129 ± 3 102 ± 4 60 ± 4 n.d. 758 ± 22 428 ± 14 1.0 ± 0.1 149 ± 4 98 ± 4 1.4 ± 0.2

Biotic dissolution (�0.1 OD600nm)t = 20 h 49 ± 1 37 ± 4 25 ± 4 n.d. 437 ± 10 264 ± 7 0 ± 0.1 87 ± 2 56 ± 4 4.9 ± 0.4t = 48 h 55 ± 2 42 ± 4 27 ± 4 n.d. 482 ± 14 290 ± 9 0.50 ± 0.1 97 ± 2 66 ± 4 5.4 ± 0.5t = 120 h 67 ± 2 52 ± 4 34 ± 4 n.d. 564 ± 18 343 ± 11 4.2 ± 0.2 107 ± 3 71 ± 4 5.1 ± 0.3t = 168 h 74 ± 3 68 ± 4 42 ± 4 n.d. 643 ± 21 393 ± 16 4.4 ± 0.2 112 ± 3 75 ± 4 5.2 ± 0.3

Biotic dissolution (�1 OD600nm)t = 20 h 28 ± 1 n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.t = 48 h 30 ± 1 n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.t = 120 h 39 ± 2 n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.t = 168 h 49 ± 4 n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.Bacterial control0.1 OD600nm 0.03 ± 0.01 n.d. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.1 OD600nm 3.7 ± 0.1 n.d. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.

n.d.: not detected.n.m.: not measured.

a Only Mg concentration was measured for this experiment.

4 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

2.2. Analytical methods

2.2.1. ICP-AES and ICP-MSMagnesium, Si and Fe concentrations were determined by

Inductively Coupled Plasma-Atomic Emission Spectrometry(ICP-AES) on a Perkin Elmer Optima 3000 DV equipped with anAS-90/AS-91 controller. The analytical reproducibility (1r) is 1%for Mg, 3% for Si, and 2% for Fe. Magnesium concentrations werealso occasionally measured on a Thermo Optek Quadrupole ICP-MS X7 CCT. In-run uncertainties were on the order of 1% or lessand analytical reproducibility (1r) of 2%. The wavelengths usedfor the Perkin Elmer Optima 3000 DV were 279.079 nm and279.553 nm for Mg, 212.412, 252.851 and 251.611 nm for Si, and234.349 and 273.955 nm for Fe. Calibration of the mass spectrom-eter is performed with standard solutions prepared at concentra-tions between 0 and 20 mg/L by diluting commercial standardsolutions of Si, Mg, and Fe (1000 mg/L from Johnson Matthey) in0.5 N HNO3. The Thermo Optek Quadrupole ICP-MS X7 CCT wascalibrated for Mg analysis using 0–40 lg/L standard solutions pre-pared by diluting commercial standard solutions. Blank solutionsof pure 2% HCl in double-distilled water were also measured beforeeach sample. In-run uncertainties were on the order of 1% or lessand analytical reproducibility (1r) of 2%.

2.2.2. X-ray Photoelectron Spectroscopy (XPS)Samples were prepared for surface analysis from the powder

fraction with a specific surface area a of 1.46 m2/g. Five mg olivinecollected on 5 lm nylon filters were gently rinsed with 5 mL ofpure water and mounted on a 1 cm � 1 cm In sheet. The XPS spec-tra were obtained on a VG Scientific ESCA LAB 200 R using a 240 WAl Ka radiation (1486.6 eV). Samples were positioned at an angle of30� with respect to the incoming beam. The detectors were posi-tioned to collect the photoelectrons that emerged perpendicularto the surface of the In sheet. Resolution (width at mid-height ofthe carbon peak) was �2.6 eV. The spectra of the C1s, O1s, Si2p,Mg2s and Fe2p photoelectrons were measured at binding energies

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

of 285.0 eV (1s), 532 eV (1s), 101.8 eV (2p1/2 and 2p3/2), 88.6 eV(2s) and 710.6 eV (2p1/2 and 2p3/2), respectively. The spectra weresmoothed and the background removed by the Shirley routine(Shirley, 1972). Concentrations were calibrated against total peakarea, including satellite peaks. Quantitative analysis (Briggs,1983) was carried out correcting the intensities for the inelasticmean free path, l, according to Seah and Dench (1979), the photo-ionization cross-section, r, according to Scofield (1976) and theanalyzer transmission function, T (Seah, 1980). For instance the to-tal areas of Si and Fe, noted RSi and RFe, respectively, are related tothe atomic Si/Fe-ratio through the formula:

nSi

nFe¼ kFe

kSi

RSi

RFeð1Þ

where ki is the sensitivity factor calculated as the product of l, r andT for the element i. The expected accuracy on Fe/Si ratios is on theorder of 10% and the experimental reproducibility is better than 3%.The 2p3/2 peaks are distinct for Fe2+ and Fe3+. The Fe2+ and Fe3+ con-tributions to the spectrum have been fitted using Gauss-Lorentzsum functions and the XPS Peak software (http://www.phy.cuhk.e-du.hk/~surface/XPSPEAK/). The fit is complicated by the presence ofa Fe2+ satellite. An energy difference of 1.6 eV was, therefore, as-sumed between the 2p3/2 Fe2+ and Fe3+ peaks and a same widthat half-height and proportions of Gaussian and Lorentzian compo-nents for the peaks. The energy of the Fe2+ satellite peak varies be-tween 714.4 and 715.0 eV. Uncertainties on the Fe3+/Fe2+-ratio are�20%.

2.2.3. Counting bacterial cellsThree different conventional methods (Madigan et al., 2008)

were applied to estimate the concentration of the bacterial cells:(i) optical density measurements at 600 nm. The total number ofviable and dead cells was obtained by averaging OD600nm on three1-mL aliquots obtained on a UV 1601 Shimadzu spectrophotome-ter and (ii) Malassez chambers measurements. Ten-lL drops ofsolution were diluted to obtain about 20–30 cells loaded in 20 lL

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

olivineolivine + E. coli (OD=0.1)olivine + E. coli (OD=1)

ppm

)

olivineolivine + E. coli (OD=0.1)olivine + E. coli (OD=1)

3.5

3

2.5

2

B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx 5

counting chambers. The reported counts represent an average ofthree measurements. (iii) Counts on plates to estimate the viablecells. Three 10lL-drops from 105, 106 and 107 dilutions wereplaced on nutrient agar plates and incubated at 30 �C for 60 h.The numbers of CFU/mL were determined from plates containing�70 colonies. The reported counts again represent triplicatemeasurements.

Magnesium mass balance was also achieved in this study to cal-culate the number of broken cells.

0 50 100 150 200

[Mg]

(

t (hours)

1.5

1

0.5

0

2.2.4. pH and EhpH was measured with a precision of ±0.01 pH unit on a Knick-

Calimatic 761, Mettler Toledo calibrated with three phosphate buf-fers before each measurement session. Eh was measured at ± 5 mVon a Mettler Toledo Pt4805-DXK-S8/120. The calibration used theelectrolytic redox Ingold 220 mV buffer at a pH of 7. The Eh valuesare reported versus the hydrogen reference. The pH and Eh data areaverage values of triplicate measurements performed at 21 �C inthe batch reactors after temperature was allowed to equilibratefor an 1 h.

Fig. 2. Magnesium concentration in the biotic and abiotic solutions as a function ofduration of the experiment (s = 130 rpm) with calibrated olivine grains (a = 0.09m2/g). The dotted lines correspond to experiments at low cell concentrations(OD600nm � 0.1), and the dashed line to experiments at high cell concentration(OD600nm � 1), respectively. Error bars are analytical uncertainties.

3. Results

3.1. Mg, Si and Fe concentrations of the solutions

The Mg, Si and Fe concentrations of the different solutions arereported in Table 1. The Mg concentrations in the mg/L range inthe stirred solutions are similar to those reported by Wogeliusand Walther (1991) for batch experiments at pH = 6, in reactorsof similar volume and specific surface of the powder. Once cor-rected for temperature differences, the initial dissolution ratesare consistent with these literature values (Fig. 1). After 48 h, theMg and Si release rates are constant (Fig. 2). Given the present ana-lytical precision, their ratios and concentration ratios indicate thatolivine dissolution is almost congruent with stirring (Mg/Si � 1.7for large surface area and �1.8 for small surface area) whereas itis not stoichiometric without stirring (Mg/Si � 1.3) (obtained fromTable 1). The Fe concentrations (<1.5 � 10�6 mol/L) are a factor 10smaller than those estimated from stoichiometric dissolution withno Fe precipitation (Fig. 3a). No Fe was detected in the stirredexperiments at low specific surface area. For large surface area orno stirring, Fe concentrations increased with run duration andreached a maximum. Iron concentrations in the solutions are high-er when the solutions are not stirred and the maximum is attainedfaster (Fig. 3a).

The Mg, Si and Fe concentrations of the filtered biotic solutionsare reported in Table 1. For the two calibrated olivine powders, Mgand Si were in the mg/L range, but less concentrated than in thecorresponding abiotic reference. Upon completion of the experi-ments, Mg and Si concentrations were about 30% lower in low cellconcentration-solutions, and 60% lower in high cell concentration-solutions (Fig. 4) than in the abiotic experiments. The rates of thelinear increase of the Mg and Si concentration in the stirred exper-iment are lower than in the abiotic experiment, in particular at lowsurface area, and slightly higher in the non-stirred experiments.Their ratio is nearly congruent in the stirred experiments Ratesdo not vary with the bacterial concentration in the stirredexperiments at low specific surface. The Fe/Mg and Fe/Si ratios ofbiotic solutions (Fig. 3b) are about three times those of abioticexperiments.

The Mg concentrations of the filtered bacterial control solutionsare less than 3 lmol/L (Table 1). Iron and Si contents are below therespective detection limits.

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3.2. Olivine surface characterization

Chemical compositions and Fe3+/Fe2+ ratios were estimatedfrom the XPS spectra (Tables 2 and 3). The chemical compositionof the original surface of the powders is indistinguishable fromthe bulk composition measured by ICP-MS. Up to 17% of the sur-face Fe is Fe(III).

The compositions of the surfaces recovered after abiotic disso-lution are significantly different than those of the original surfaceswith a Fe3+/Fe2+ ratio of 1.46 ± 0.12, and Fe/Mg and Fe/Si molar ra-tios are 1.3 higher (Table 2). The surface of the powder is, therefore,enriched in Fe upon abiotic alteration. After the abiotic dissolution,the +0.7 eV shift of the binding energy peak maxima for Fe2p 3/2relative to Si2p (�609.4 eV) better matches that of goethite thanthose of hematite, magnetite or Fe-silicate (see Table 5 in Schottand Berner, 1983). This is, therefore, evidence for the presence ofa hydrous ferric oxide secondary phase. The Mg/Si molar ratio of1.6 (Table 2) is consistent with preferential leaching of the divalentcations at the initial stages of the surface alteration (Table 2), how-ever, the precision of the XPS analyses does not warrant the con-clusion that olivine dissolution is non-stoichiometric. The lowmodification of the binding energy peak maxima for O1s relativeto Si2p after abiotic dissolution do not allow detection of aSiO2�nH2O layer on the surface of an altered olivine (Seyamaet al., 1996). The present observations are consistent with thoseof Schott and Berner (1983) on the surface of Fe-rich olivine weath-ered under oxic conditions at pH = 6. These authors used O and FeXPS to conclude similar Fe excesses and similar nFe3þ=nFe2þ ratios.

The surface of the minerals recovered after biotic dissolutionare significantly different from both the original surfaces and fromthose produced in abiotic experiments, with less than half of the Febeing Fe(III) (Fe3+/Fe2+ = 0.74 ± 0.04). The Fe/Mg and the Fe/Si ra-tios of these samples are lower (0.07 ± 0.02 and 0.12 ± 0.02, respec-tively) than those measured in the corresponding abiotic reference(0.15 ± 0.03 and 0.24 ± 0.04, respectively) whereas the Mg/Si molarratio of 1.7 suggests it is higher than that of the abiotic referenceand close to that of olivine. This indicates that the initial surfacesare depleted in Fe during alteration with respect to the originalmaterial (Table 3).

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 50 100 150 200

0.09m2/g 130rpm

1.46m2/g 130rpm

1.46m2/g 0rpm[F

e]Ab

iotic

(ppm

)

t (hours)

a

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 50 100 150 200

t (hours)

[Fe]Biotic (ppm)

OD = 0.1 b

Fig. 3. Fe concentrations in the solutions as a function of the duration of dissolution experiments of olivine grains without (i.e., abiotic, Fig. 3a) or with (i.e., biotic, Fig. 3b)bacteria. The plain lines refer to the stirred experiments on low-specific surface area minerals (a = 0.09 m2/g). The dotted lines designate the unstirred experiments on high-specific surface area minerals (a = 1.46 m2/g). The dashed lines refer to stirred experiments on high-specific surface minerals.

0 50 100 150 200

[X] Bi

otic /

[X] Ab

iotic

OD600nm

= 0.1

OD600nm

= 1

OD600nm

= 0

x

t (hours)

MgSi

OD = 0.1 a = 0.09m2g-1s =130rpm

OD = 1 a = 0.09m2g-1 s=130rpm

OD = 0.1 a = 1.46m2g-1s =130rpmOD = 0.1 a = 1.46m2g-1s =0rpm

x1.2

1

0.8

0.6

0.4

0.2

0

Fig. 4. The ratios of Mg (solid squares) and Si (crosses) concentrations of the bioticsolutions normalized to the values in the abiotic reference solutions, as a function ofduration of the dissolution of calibrated olivine powders. The dash-dotted linecorresponds to runs of biotic dissolution at high cell concentration (OD600nm � 1).The shaded area highlights experiments at low cell concentrations (OD600nm � 0.1).The dashed lines are for the experiment performed without stirring.

6 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

3.3. Bacterial cells counts and bacterial Mg contents

The bacteria counts are reported in Fig. 5 and the data are re-ported in Table 4. The optical densities (0.06 < OD600nm < 0.1) ofthe abiotic samples are constant and slightly positive, which showsthat lm-sized olivine fines to remain in suspension. For the bioticexperiments, the optical density decreases by about 30% or less forsmall concentrations of bacterial cells (OD600nm � 0.1), and byabout 15% for larger concentrations (OD600nm � 1). In the bacterialcontrol solutions, optical densities start decreasing after �48 h.Although the concentrations of viable cells obtained from countingplates are consistently smaller than those of the total cells countedin the Malassez chamber, the differences remain within error bars(1 � 107 at OD600nm � 0.1 and 5 � 107 at OD600nm � 1). The concen-tration of both viable and total cells was found to decrease in thecourse of all experiments.

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Magnesium contents of the cultivated cells used in the presentstudy are reported in Table 5 (�2 mg Mg/g of dry weight) and com-pared with literature values (0.5–5 mg per mg of dry weight) fordifferent studied strains, culture media and growth phases.

3.4. pH and redox potential measurements

The pH and Eh values are plotted in Fig. 6 and the data are re-ported in Table 6. The pH of all experiments, initially at 7.0, firstdropped by 0.6 pH units, then eventually increased back to valuesgreater than 7.0 after 2 days. The Eh of the abiotic solution, con-stant at 229 ± 10 mV within errors, remained between the valueof aerobic (O2/H2O) and anaerobic (H2O/H2) redox couples. The re-dox potential decreased when the concentration of bacterial cellsincreased (on average 183 ± 9 mV at OD600nm � 0.1 and69 ± 43 mV at OD600nm � 1).

4. Discussion

4.1. Mass balance in abiotic control solutions

The purpose of this section is to assess the distribution of Mg, Siand Fe between coexisting solution, olivine and other potentialsolid phases. The first parameter to evaluate is the proportion ofolivine dissolved in the different experiments. Loss of Mg andSi does not affect the average grain size of the powders, butmarkedly alters and dismantles the surface layers. The Mg andSi contents of the stirred solutions correspond to less than 1019

(a = 0.09 ± 0.01 m2/g) and 1020 (a = 1.46 ± 0.04 m2/g) atoms of Mgdissolved from olivine, respectively. This is equivalent to a layerof a few nm or a few tens of unit cells. Given that the amount ofFe in solution does not account for the amount of Fe expected tobe lost by stoichiometric dissolution, a new Fe-rich phase(s), mostlikely hosted by a hydrous ferric oxide at the surface of olivinecrystals, must have formed in the experiments. Such low Fe con-centrations in solutions reflect the very low solubility of Fe3+ underaerobic conditions and are consistent with the high Fe/Mg, Fe/Si,and Fe3+/Fe2+ ratios measured on olivine of the abiotic sample(Table 2). If this apparent Fe surplus is homogeneous at the surfaceof the powder with a = 1.46 m2/g, it is equivalent to a uniform 3.3 Åthin layer. Such a thickness is consistent with the present XPSobservations. The molar nFe3þ=nFe2þ ratio measured by XPS can beapproximated by the following equation:

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

Table 2Fe/Mg, Fe/Si, (Mg + Fe)/Si, and Fe3+/Fe2+ atomic ratios in samples of untreated (a = 1.46 m2/g) and treated (for 168 h) olivine powder. Ratios were obtained from XPS spectralanalyses (see text for details). The optical density (OD) at 600 nm of biotic solutions is 0.1. The expected ratios of the untreated olivine are given in parentheses for comparison.

Experiment Fe/Mg Fe/Si Mg/Si Fe3+/Fe2+

a = 1.46 m2/g, s = 130 rpm

Olivine powder 0.1 ± 0.02 0.18 ± 0.02 1.8 ± 0.56 0.17 ± 0.04(0.11) (0.2) (1.8) (0)

Weathered powder from abiotic control 0.15 ± 0.03 0.24 ± 0.04 1.6 ± 0.58 1.46 ± 0.12t = 168 hWeathered powder from biotic dissolution 0.07 ± 0.02 0.12 ± 0.02 1.7 ± 0.77 0.74 ± 0.04�0.1 OD600nm and t = 168 h

Table 3XPS spectral peak fitting parameters for the Fe peak of the untreated olivine powder (specific surface area a = 1.46 m2/g) and of the powders soaked for 168 h in abiotic or bioticsolutions (OD600nm � 0.1). The Fe2+, Fe3+ and Fesat peak positions, the full width at half-height (FWHM) are given with the proportions of Gaussian (G) and Lorentzian (L)components, G/L, in each peak fit. v2 is the square of the ratio of the spectrum area out of the total fit area. The Fe peak of the untreated powder of olivine was fitted assumingboth pure ferrous iron (�) and a mixture of ferrous and ferric iron.

Experiment Fe2+ peakposition (eV)

Fe3+ peakposition (eV)

Fesat peakposition (eV)

FWHM of the2p peaks

% G/L 2ppeaks

FWHM ofthe satellite

% G/L ofthe satellite

v2

a = 1.46 m2/g, s = 130 rpm

Olivine powder� 709.9 n.d. 713.6 3.7 35 4.7 35 1.0336Olivine powder 709.89 711.49 714.43 3.8 40 3.4 38 0.9154Weathered powder from abiotic control 709.7 711.3 714.6 3.3 36 3 42 0.9337t = 168 hWeathered powder from biotic experiment 709.8 711.4 714.6 3.2 38 3.2 40 1.0403�0.1 OD600nm and t = 168 h

0

0.1

0.2

0.8

0.9

1

1.1

0 50 100 150 200

olivineE. coli (OD=0.1)olivine + E. coliE. coli (OD=1)olivine + E. coli

OD

600n

m

t (hours)

a

viable cellstotal cells

OD 600nm

0

10

20

60

70

80

90

0 50 100 150 200

107

cells

x m

l-1

t (hours)

b

Fig. 5. Estimation of cellular concentrations in the solutions as a function of duration of the experiment (s = 130 rpm). Optical densities (Fig. 5a) and corresponding cell counts(Fig. 5b) reported for the abiotic and the biotic solutions (OD600nm � 0.1 for dotted lines and OD600nm � 1 for dashed lines). The solid and open symbols refer to theexperiments with and without calibrated olivine powder (a = 0.09 m2/g). The triangles in Fig. 5b refer to the viable cells counts in the solutions, the circles to the total countson Malassez chambers.

B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx 7

nFe3þ

nFe2þ¼ qFe3þ lFe3þ þ qFe2þ ðlFe2þ � lFe3þ Þ

qFe2þ ðlFe2þ � lFe3þ Þ ð2Þ

where qFe3þ and qFe2þ are the average densities of Fe in ferric oxide(3.5 g/cm3) and in olivine (0.2 g/cm3), respectively, lFe3þ is the thick-ness of the ferric oxide layer. lFe2þ is the mean free path of electronsemitted by Fe2+ in olivine (about 6 nm) calculated from the semi-empirical formula proposed by Seah and Dench (1979). Given thecrude nature of this model, the measured (1.5) and calculated molarredox ratios (�2) are close enough to support the previous statementthat a significant fraction of the Fe lost by the olivine surface forms aFe(III)-rich secondary phase layer on the olivine surface itself. In the

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unstirred experiment, for which an incongruent slower dissolutionwas observed, a surface phase causing a preferential release of Sishould also be considered (Luce et al., 1972). Thin (<10 nm) residualdepleted surfaces phases have been observed with a preferentialleaching of Mg and Fe at low to medium pH (Schott and Berner,1983; Seyama et al., 1996). Secondary mineral phases such as smec-tite, hematite, goethite, laihunite have been reported from a fieldsample of olivine weathered under oxidizing conditions at low tem-perature (Delvigne et al., 1979; Smith et al., 1987; Banfield et al.,1999; Welch and Banfield, 2002; Fisk et al., 2006).

In contrast, in the stirred experiments, Mg concentrations do notseem to be limited by secondary phases. Likewise, Si concentrations

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

Table 4Cellular concentration in the biotic and abiotic solutions as a function of duration of the dissolution experiment. Concentrations and relative evolutions are reported forexperiments performed at low (OD600nm � 0.1) and high cell concentrations (OD600nm � 1). They were estimated from OD600nm (a) and direct counts on Malassez chambers (c).Viable cells were counted from ‘‘plate form’’ (b) (see Section 2 for details).

Experiment OD600nma Total cells concentrationc (107 mL�1) 100-loss of total cells (%) Viable cellsb (107 mL�1) Viability (%)

a = 0.09m2/g, s = 130 rpm

Abiotic controlt = 0 h 0.06 ± 0.01 0 0t = 5 h 0.06 ± 0.01 n.d. 0t = 20 h 0.06 ± 0.01 n.d. 0t = 48 h 0.07 ± 0.01 0 0t = 120 h 0.10 ± 0.01 n.d. 0t = 168 h 0.07 ± 0.01 0 0

Bacterial control (�0.1 OD600nm)t = 0 h 0.14 ± 0.01 7.0 ± 0.4 6.8 ± 0.5 97 ± 9t = 5 h 0.14 ± 0.01 n.d. 7.1 ± 0.4t = 20 h 0.15 ± 0.01 n.d. 7.5 ± 0.8t = 48 h 0.15 ± 0.01 n.d. 7.3 ± 0.4t = 120 h 0.08 ± 0.01 n.d. 5.4 ± 1.2t = 168 h 0.09 ± 0.01 6.6 ± 0.7 94 ± 11 5.9 ± 0.6 90 ± 13

Biotic olivine dissolution (�0.1 OD600nm)t = 0 h 0.14 ± 0.01 7.4 ± 0.3 7.1 ± 0.5 96 ± 8t = 5 h 0.17 ± 0.01 n.d. 7.3 ± 0.6t = 20 h 0.17 ± 0.01 n.d. 7.3 ± 1.2t = 48 h 0.16 ± 0.01 n.d. 7.5 ± 1.0t = 120 h 0.12 ± 0.01 n.d. 6.4 ± 1.1t = 168 h 0.12 ± 0.01 7.1 ± 0.9 96 ± 13 6.6 ± 0.8 93 ± 16

Bacterial control (�1 OD600nm)t = 0 h 0.97 ± 0.02 82 ± 7 82 ± 6 100 ± 11t = 5 h 0.95 ± 0.02 n.d. 80 ± 4t = 20 h 0.90 ± 0.03 n.d. 73 ± 9t = 48 h 0.89 ± 0.03 n.d. 72 ± 3t = 120 h 0.78 ± 0.04 n.d. 62 ± 12t = 168 h 0.82 ± 0.02 75 ± 8 91 ± 12 63 ± 10 84 ± 16

Biotic olivine dissolution (�1 OD600nm)t = 0 h 0.97 ± 0.02 87 ± 5 84 ± 4 96 ± 7t = 5 h 0.95 ± 0.02 n.d. 82 ± 7t = 20 h 0.91 ± 0.03 n.d. 77 ± 6t = 48 h 0.90 ± 0.02 n.d. n.d.t = 120 h n.d. n.d. n.d.t = 168 h 0.85 ± 0.02 77 ± 7 88 ± 9 64 ± 12 83 ± 12

n.d.: not detected.

Table 5The Mg concentration of E. coli cells estimated in literature and in the present study, reported in both the original units and in mg/g dry weight. The initial Mgconcentrations of the culture medium and the culture phase (E for exponential and S for stationary) at which cells were harvested are indicated.

References Original unit [Mg] (mM) E/S mg Mg/g dry weight

This study 2.3 ± 0.6 mg/g DW 0.1 S 2.3 ± 0.6Outten et al. (2001) 4 107 at/cells 1 E 2.25

8 107 at/cells 0.1 4.5

Kung et al. (1976) 2.8 mg/g DW 0.6 E 2.8

De Médicis (1986) 2.33 ± 0.06 mg Mg/g DW ? E 2.33 ± 0.061.32 ± 0.05 mg Mg/g DW E 1.32 ± 0.050.86 ± 0.01 mg Mg/g DW S 0.86 ± 0.01

Chang et al. (1986) 205 ± 50.2 mmol/kg DW 0.2 4.9 ± 1.2

Silver and Levine (1968) 15–28 lmol/1012 cells 0.08 0.5–4

Lusk et al. (1968) 0.195 lmol/mg Proteins 1 E 2.430.145 lmol/mg Proteins 1 S 1.80.16 lmol/mg Proteins 0.01 E 20.08 lmol/mg Proteins 0.01 S 1

Beveridge and Koval (1981) 0.256 lmol/mg DW envelopesHoyle and Beveridge (1983) 0.019 ± 0.003 lmol/mg DW outer membrane

8 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

(<1 mg/L) in the solutions exposed to the sieved olivine(a = 0.09 m2/g) remain well below the solubility of quartz andamorphous silica (>6 mg/L at 25 �C and >10 mg/L at 37 �C,Gunnarson and Arnorsson, 2000). Mass balance calculations will,therefore, be restricted to Mg and Si data on this size fraction.

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4.2. Mass balance in bacterial control solutions

Most of the Mg of biological origin is hosted in the cells. The to-tal amount of Mg introduced into a batch reactor can be estimatedby multiplying the Mg cellular contents (�2.3 � 107 Mg atoms/

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

6

6.5

7

7.5

-100

0

100

200

0 50 100 150 200

olivineE. coli (OD=0.1)olivine + E. coli E. coli (OD=1)olivine + E. coli

E (m

V)

t (hours)

Fig. 6. Calibrated pH and relative Eh variations of the biotic and abiotic solutions asa function of duration of the dissolution experiment of olivine grains. The dottedlines concern the experiments at low cell concentrations and the dashed line theexperiments at high cell concentration, respectively. The solid squares refer to theexperiments performed with olivine powder (a = 0.09 m2/g). Reproducibility isestimated from triplicated measurements.

Table 6pH and Eh (reading values) of the biotic and abiotic solutions as a function of durationof the dissolution. Concentrations and relative evolutions are reported for experi-ments performed at low (OD600nm � 0.1) and high cell concentration (OD600nm � 1).The reproducibility is estimated from triplicate measurements.

Experiment pH E (reading value mV)a = 0.09m2/g, s = 130 rpm

Abiotic controlt = 0 h 6.98 ± 0.05 244 ± 12t = 5 h 6.33 ± 0.06 237 ± 14t = 20 h 6.77 ± 0.04 223 ± 16t = 48 h 7.04 ± 0.06 229 ± 8t = 120 h 7.07 ± 0.04 222 ± 14t = 168 h 7.09 ± 0.04 218 ± 11Average 229 ± 10

Bacterial control (�0.1 OD600nm)t = 0 h 7.00 ± 0.03 234 ± 15t = 5 h 5.95 ± 0.12 190 ± 13t = 20 h 6.29 ± 0.04 166 ± 15t = 48 h 6.71 ± 0.05 116 ± 11t = 120 h 6.81 ± 0.05 113 ± 9t = 168 h 6.97 ± 0.04 155 ± 10Average 162 ± 46

Biotic olivine dissolution (�0.1 OD600nm)t = 0 h 6.98 ± 0.04 182 ± 18t = 5 h 6.39 ± 0.06 168 ± 21t = 20 h 6.73 ± 0.03 188 ± 14t = 48 h 6.84 ± 0.02 177 ± 15t = 120 h 7.02 ± 0.04 191 ± 13t = 168 h 7.09 ± 0.03 192 ± 11Average 183 ± 9

Bacterial control (�1 OD600nm)t = 0 h 6.96 ± 0.03 165 ± 12t = 5 h 6.51 ± 0.08 79 ± 11t = 20 h 6.80 ± 0.06 -18 ± 5t = 48 h 6.65 ± 0.05 79 ± 7t = 120 h 7.18 ± 0.06 67 ± 8t = 168 h 7.37 ± 0.07 117 ± 12Average 82 ± 61

Biotic olivine dissolution (�1 OD600nm)t = 0 h 6.99 ± 0.04 117 ± 13t = 5 h 6.79 ± 0.06 118 ± 12t = 20 h 6.82 ± 0.05 25 ± 7t = 48 h 6.94 ± 0.04 23 ± 9t = 120 h 7.13 ± 0.04 54 ± 8t = 168 h 7.06 ± 0.03 78 ± 12Average 69 ± 43

B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx 9

cell) by the initial number of cells introduced into the reactor (Ta-ble 4). This calculation leads to values in excess of those measuredin filtered solutions (Table 1), which indicates that the cells keptpart of their initial Mg inventory during the experiment. Besides,the amount of Mg present in the solutions is much more consistentwith the dead cell contribution. Using the concentrations of deadcells (difference between the viable cells counts and the totalcounts, see Table 4) and the Mg cellular content (Table 5), concen-trations of �0.02 lM of Mg for OD600nm = 0.1 and to �0.5 lM withfor OD600nm = 1 are obtained, respectively. These values are consis-tent with those of Table 1.

Magnesium is tightly bound within the cellular material (Changet al., 1986) and what was found in the solutions mostly repre-sented Mg from dead cells. Osmotic shock in distilled water, theend-member of oligotrophic media, causes mechanical injury tothe wall (Jacobson et al., 1976) and induces cellular Mg releasewhen the cells break and die. However, most E. coli cells survivesuccessive washes in distilled water (Neu and Heppel, 1965;Pembrey et al., 1999) and retain all their Mg (Silver and Clark,1971; Kung et al., 1976), or a large fraction of it (Lusk et al., 1968).

4.3. Mass balance in open-air suspensions of olivine grains and E. coli

The Mg and Si concentrations are lower in samples with sus-pended bacterial cells than in those of abiotic experiments. In addi-tion, the larger the optical density, the lower the Mg and Siconcentrations in solution. Such a negative correlation reflectseither that bacteria inhibit olivine dissolution or that they takeup significant amounts of cations from the solution.

Magnesium mass balance does not support bacterial uptake asthe main process responsible for lower Mg concentrations in thesolutions with suspended bacteria. A minimum uptake of0.01 mol of Mg per gram of dry cells would be necessary to accountfor the Mg deficit in the experiments at low cell concentrations (0.1OD600nm, a = 0.09 m2/g). Such a strong Mg uptake would requirecellular Mg contents about two orders of magnitude above the ob-served concentrations (Table 1). The question then arises of

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whether most of the missing Mg could be adsorbed on the cellularenvelope. Some literature Mg concentrations in E. coli envelopesare listed in Table 5. Using these data and assuming that envelopesconstitute about 30% of the total dry weight of the cell (Bratbakand Dundas, 1984), the maximum amount of Mg accounted forby envelopes is less than 40% of the total cellular Mg content, i.e.,much smaller than the 2 orders of magnitude required to satisfythe Mg mass balance. A preferential adsorption of the cationicMg versus the anionic Si species on the anionic bacterial surfaceis possible but not discernible with the present resolution. Indeedthe maximum decrease of the Mg/Si ratio calculated, consideringthe highest possible Mg content in the envelopes taken up fromthe solution, is equal to 0.05. Contrary to the mass balance of Sithat requires analytical developments, and to that of Fe that re-quires some evaluation of the highly variable level of the intracel-lular storage and siderophores releases, the mass balance of Mgallows interpreting the missing Mg in the biotic solution and thedecrease of the accumulation rate by a lower Mg release fromthe mineral surface.

The observations, therefore, show that the introduction of a sta-ble population of E. coli into a solution containing suspended oliv-ine powder inhibits olivine dissolution. pH variations are too small

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

Biotic olivine dissolution

y = 202.07x0,7746

R2 = 0.9546

y = 227.69x0,9098

R2 = 0.9974

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25 30 35

(Total cells - Viable cells) at equilibrium (107 per mL)

Cal

cula

ted

adso

rbed

cel

ls

(107

per g

)

Fig. 7. Freundlich sorption isotherms for biotic olivine dissolution (�0.1 OD600nm

and (�1 OD600nm) where ‘‘calculated adsorbed cells’’ (which represents the productof (i) the dead cells and (ii) the volume of the solution reactor after each sampling,(iii) per g of olivine) are represented as a function of ‘‘Total cells � Viable cells’’measured at equilibrium (for each sampling). The Freundlich model relies on thefollowing relationship: S = Kf � Cn where S = x/m. x represents the product betweenthe volume of the solution and the difference between the initial concentration(Total cells in this case) and the concentration at equilibrium of the soluteconsidered (Viable cells in this case) m represents the weight of the adsorbent (1 gof olivine in this case) C is the concentration of the solute at equilibrium (Totalcells � Viable cells in this case) Kf and n are constants.

10 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

to explain this feature. The reducing conditions induced by thepresence of bacteria also preferentially drive Fe(II) into solutionand are consistent with the observed decrease of the Fe3+/Fe2+ ratioat the surface of the olivine powder (Table 2). Consequently, thecurrent redox conditions would be expected to enhance ratherthan inhibit Mg and Si release by increasing the effective reactivesurface area exposed to the aqueous solution. It is also unlikelythat olivine dissolution is inhibited by a simple ‘mask’ effect of bac-teria coating the mineral surfaces. As a simplifying assumption, thesurface masked by a single cell is taken to be equal to the largestsection of the cell (about 1 lm2). If the cells that disappeared fromthe aqueous solution during the experiment formed a close-packedbacterial layer on the olivine surface (0.09 m2/g), the masked sur-face fractions would account for only about 2% of the mineral sur-face in the experiment with an OD600nm of 0.1, and 10% in theexperiment with an OD600nm of 1. These fractions are further re-duced for multilayer aggregates.

An implication of these data is, therefore, that the inhibition ofolivine dissolution by the presence of bacteria is of a chemicalnature. Although no organic compound is known to suppress oliv-ine dissolution in particular, some bacteria organic compoundsare known to reduce silicate dissolution in soils (Berthelin,1986) and in experiments (Welch and Vandevivere, 1994; Ullmanet al., 1996; Stone, 1997). Because organic compounds formpatches (Nugent et al., 1998) or aggregate soil particles (Walkeret al., 1989) in temperate soils, they are expected to have a coat-ing effect (Wogelius et al., 2007) decreasing accessibility to oliv-ine surface.

Given the studied bacteria concentrations in this paper, the for-mation of such an organic coat on the olivine surface is plausibleeven if bacteria are not disrupted. Indeed, the periplasmic proteinpool is expected to be released in the aqueous batch (Heppel,1967), to be resistant to aggregation (Liu et al., 2004) and to weaklybind Mg2+ (Waldron et al., 2009). In the present case, the amount ofperiplasmic proteins, considering the bacteria concentrations, is atleast three orders of magnitude higher than necessary to cover theolivine surface initially introduced in the experiment. That is whyconstrains on a coat formation process by adsorption, if organicconcentrations in the solution are mainly represented by the deadcells, are evaluated using a Freundlich adsorption isotherm model(Freundlich, 1909; Limousin et al., 2007).

So, ‘‘dead cells’’ defined as ‘‘Total cells � Viable cells’’, Table 4 inthe biotic olivine dissolution experiments, could have a coating ef-fect decreasing accessibility to olivine surface. A graphic showingthe ‘‘calculated adsorbed cells’’ (which represents the product of(i) the dead cells and (ii) the volume of the solution reactor aftereach sampling, (iii) per g of olivine) in the function of dead cells de-fined as ‘‘Total cells � Viable cells’’ measured at equilibrium (foreach sampling) is represented in Fig. 7 (see caption of Fig. 7 for de-tails of parameters of the Freundlich model). It can be seen in Fig. 7that there is a relatively good correlation between the experimen-tal values and the fit of these points with the Freundlich model. In-deed, the correlation coefficient is equal to 0.9546 for the bioticdissolution olivine experiment with 0.1 OD600nm and equal to0.9974 for the experiment performed with 1 OD600nm. Moreover,in the case of the 1 OD600nm biotic experiment, the regression fitis nearly linear, with a value of the ‘‘n’’ factor close to 1 (0.91),whereas in 0.1 OD600nm biotic experiment, ‘‘n’’ is close to 0.8(0.77). In terms of mechanism, it is difficult to identify the physi-cal/chemical phenomenon controlling adsorption, but from thosevalues two hypotheses can be advanced (Limousin et al., 2007).For 1 OD600nm biotic experiments, the experiment fits with a ‘‘C’’type isotherm: a zero origin, power close to one and no saturationplateau. For 0.1 OD600nm biotic experiments, the results fit with a‘‘L’’ type isotherm without a strict plateau: the olivine is ‘‘satu-rated’’ progressively.

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

Recent experiments of olivine dissolution in the presence ofaerobic gram-negative bacteria have been performed in acidic-CO2 bearing solutions in nutrient-rich media (Shirokova et al.,2012). A biofilm-like surface coverage of reacted grains was ob-served, only in nutrient-bearing solutions. In terms of mechanism,this biofilm was likely to be formed via concerted extraction of EPSmaterial by many cells, attached together on the mineral surface.Nevertheless, no secondary mineral formation within these EPS-covered surface layers was in evidence and biofilm did not appre-ciably affect the rate of Mg and Si release from the olivine surface.The substantial modifications of the original physical–chemicalproperties of the solid surfaces that are observed during the accu-mulation of the organic molecules preceding the bacterial cellattachment (Palmer et al., 2007) allow suggesting that the impactof organics cannot be restricted to an inert coating. However, thein situ monitoring of the reaction mechanism of the bacterial or-ganic compounds on dissolving crystalline silicate surfaces is iden-tified as one of the most difficult remaining tasks not yet addressedin geochemical kinetics (Kump et al., 2000; Morris and Wogelius,2008). Formation of complexes with olivine surface Mg2+ is consid-ered to explain that numerous organic acids and ligands that arepreferentially excreted by bacteria in organic-rich and nutrient-poor media (Ullman et al., 1996), have a small but measurableenhancing effect on the olivine dissolution rate at pH and concen-tration comparable to those found in soils (Morris and Wogelius,2008; Olsen and Rimstidt, 2008).

In the present study, carried out at neutral pH, olivine dissolu-tion involves the exchange of two H+ ions per surface Mg2+

(Pokrovsky and Schott, 2000). Dissolution inhibition could be dueto the formation of H-bonds of organics with surface hydroxylgroups as reported for ‘‘core sugars’’ of the bacteria lipopolysaccha-rides (LPS) on oxide surfaces (Jucker et al., 1997). Such a reactionmechanism of inhibition of olivine dissolution likely occursconcomitantly to an inert surface coating and possibly to othermechanisms related to major cellular compounds released in thecase of cell disruption or to some unidentified ‘‘stress’’ componentsynthesized specifically under the studied extreme oligotrophicconditions.

5. Conclusions

Assessing the mass balance of Mg in experimental open-air sus-pensions of olivine powders with a stable population of E. coli in

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx 11

water at neutral pH demonstrates that the presence of bacteriainhibits Mg release during weathering of olivine. Although Mgand oligotroph data in watersheds with abundant mafic and ultra-mafic bedrocks are critically missing, this study suggests that free-living Proteobacteria, a prevalent group of subsurface bacteria,should decrease the amount of riverine Mg released by chemicalweathering of silicates. Results obtained in this study suggest thatProteobacteria could play an inhibitor role in the drawdown ofatmospheric CO2.

Acknowledgements

B.G. is grateful to C. Douchet for precious technical assistance,to E. Albalat for the analyses of the E. coli Mg content, and to Ph.Telouk for ICP-AES and ICP-MS instrumental assistance. LL grate-fully acknowledges Dr. Fabien Mongelard, for his constructivecomments. He helped to significantly improve the presentationof this work. This work was supported by the CNRS, CNES and anANR Jeune Chercheur Fellowship.

References

Amro, N.A., Kotra, L.P., Wadu-Mesthrige, K., Bulychev, A., Mobashery, S., Liu, G.-Y.,2000. High resolution atomic force microscopy studies of the Escherichia coliouter membrane: structural basis for permeability. Langmuir 16, 2789–2796.

Banfield, J.F., Hamers, R.J., 1997. Processes at minerals and surfaces with relevanceto microorganisms and prebiotic synthesis. In: Banfield, J.F., Nealson, Z.K.H.(Eds.), Geomicrobiology: Interactions between Microbes and Minerals. Rev.Mineral., vol. 35, pp. 81–122.

Banfield, J.F., Barker, W.W., Welch, S.A., Taunton, A., 1999. Biological impact onmineral dissolution: application of the lichen model to understanding mineralweathering in the rhizosphere. Proc. Natl. Acad. Sci. 96, 3404–3411.

Barker, W.W., Welch, S.A., Banfield, J.F., 1997. Biogeochemical weathering of silicateminerals. In: Banfield, J.F., Nealson, Z.K.H. (Eds.), Geomicrobiology: Interactionsbetween Microbes and Minerals. Rev. Mineral., vol. 35, pp. 391–428.

Basu, B.K., Pick, F.R., 1997. Factors related to heterotrophic bacterial and flagellateabundance in temperate rivers. Aquat. Microb. Ecol. 12, 123–129.

Benedetti, M.F., Dia, A., Riotte, J., Chabaux, F., Gérard, M., Boulègue, J., Fritz, B.,Chauvel, C., Bulourde, M., Déruelle, B., Ildefonse, Ph., 2003. Chemical weatheringof basaltic lava flows undergoing extreme climatic conditions: the watergeochemistry record. Chem. Geol. 201, 1–17.

Berthelin, J., 1986. Microbial weathering processes in natural environments. In:Lerman, A., Meybeck, M. (Eds.), Physical and Chemical Weathering inGeochemical Cycles. Kluwer, Dordrecht, pp. 33–60.

Beveridge, T.J., 1999. Structures of gram-negative cell walls and their derivedmembrane vesicles. J. Bacteriol. 181, 4725–4733.

Beveridge, T.J., Koval, S.F., 1981. Binding of metals to cell envelopes of Escherichiacoli – K12. Appl. Environ. Microbiol. 42, 325–335.

Boone, T.L., Balkwill, D.L., 1988. Morphological and cultural comparison ofmicroorganisms in surface soil and subsurface sediment at a pristine studysite in Oklahoma. Microb. Ecol. 16, 49–64.

Bowen, H.J.M., 1966. Trace Elements in Biochemistry. Academic Press Inc., NewYork.

Bratbak, G., Dundas, I., 1984. Bacterial dry matter content and biomass estimation.Appl. Environ. Microbiol. 48, 755–757.

Briggs, D., 1983. Practical surface analysis. In: Briggs, D., Seah, M.P. (Eds.), Auger andX-ray Photoelectron Spectroscopy. John Wiley & Son, New York.

Broecker, W.S., Peng, T.H., 1982. Tracers in the Sea. Eldigio Press, Lamont DohertyGeological Observatory.

Chang, C.-F., Shuman, H., Somlyo, A.P., 1986. Electron probe analysis, X-raymapping, and electron energy-loss spectroscopy of calcium, magnesium, andmonovalent ions in log-phase and in dividing Escherichia coli cells. J. Bacteriol.167, 935–939.

De Médicis, E., 1986. Magnesium, manganese and mutual depletion systems inhalophilic bacteria. FEMS Microbiol. Lett. 39, 137–143.

Delvigne, J., Bisdom, E.B.A., Sleeman, J., Stoops, G., 1979. Olivines, theirpseudomorphs and secondary products. Pedologie 29, 247–309.

Dessert, C., Dupré, B., Gaillardet, J., François, L.M., Allègre, C.J., 2003. Basaltweathering laws and the impact of basalt weathering on the global carboncycle. Chem. Geol. 202, 257–273.

Einen, J., Kruber, C., Ovreas, L., Thorseth, I.H., Torsvlk, T., 2006. Microbialcolonization and alteration of basaltic glass. Biogeosci. Discuss. 3, 273–307.

Epstein, W., Schultz, S.G., 1965. Cation transport in Escherichia coli. V. Regulation ofcation content. J. Gen. Physiol. 49, 221–234.

Fisk, M.R., Popa, R., Storrie-Lombardi, M.C., Vicenzi, E.P., 2006. Iron–magnesiumbioweathering on Earth and Mars? Astrobiology 6, 48–68.

Freundlich, H., 1909. Kapillarchemie: eine Darstellung der Chemie der Kolloide undVerwandter Gebiete. Akademische Verlagsgesellschaft, Leipzig.

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

Gaillardet, J., Dupre, B., Louvat, P., Allegre, C., 1999. Global silicate weathering andCO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol.159, 3–30.

Garrels, R.M., MacKenzie, F.T., 1971. Evolution of Sedimentary Rocks: AGeochemical Approach. Norton, New York.

Grandstaff, D.E., 1986. The dissolution rate of forsteritic olivine from Hawaiianbeach sand. In: Colman, S.M., Dethier, D.P. (Eds.), Rates of Chemical Weatheringof Rocks and Minerals. Academic Press, NY, pp. 41–60.

Gunnarson, I., Arnorsson, S., 2000. Amorphous silica solubility and thethermodynamic properties of H4SiO4 in the range of 0� to 350 �C at Psat.Geochim. Cosmochim. Acta 64 (13), 2295–2307.

Guskov, A., Nordin, N., Reynaud, A., Engman, H., Lundback, A.K., Jin Oi Jong, A.,Cornvik, T., Phua, T., Eshaghi, S., 2012. Structural insights into the mechanismsof Mg2+ uptake, transport, and gating by CorA. PNAS 109, 18459–18464.

Heppel, L.A., 1967. Selective release of enzymes from bacteria. Science 156, 1451–1455.

Hilley, G.E., Porder, S., 2008. A framework for predicting global silicate weatheringand CO2 drawdown rates over geologic time-scales. Proc. Natl. Acad. Sci. 105,16855–16859.

Hoyle, B., Beveridge, T.J., 1983. Binding of metallic ions to the outer membrane ofEscherichia coli. Appl. Environ. Microbiol. 46, 749–752.

Jacobson, G.R., Takacs, B.J., Rosenbusch, J.P., 1976. Properties of major proteinsreleased from Escherichia coli by osmotic shocks. Biochemistry-US 15, 2297–2303.

Jasper, P., Silver, S., 1977. Magnesium transport in microorganisms. Ann. Rev.Microbiol. 29, 61–77.

Jernelov, A., Martin, A.-L., 1975. Ecological implications of metal metabolism bymicroorganisms. Ann. Rev. Microbiol. 29, 61–77.

Jones, G.E., Royle, L.G., Murray, L., 1979. Cationic composition of 22 species ofbacteria grown in seawater medium. Appl. Environ. Microb. 38, 800–805.

Jucker, B.A., Harms, H., Hug, S.J., Zehnder, A.J.B., 1997. Adsorption of bacterialsurface polysaccharides on mineral oxides is mediated by hydrogen bonds.Colloids Surf., B: Biointerface 9, 331–343.

Kalinowski, B.E., Liermann, L.J., Givens, S., Brantley, S.L., 2000a. Rates of bacteria-promoted solubilization of Fe from minerals: a review of problems andapproaches. Chem. Geol. 169, 357–370.

Kalinowski, B.E., Liermann, L.J., Brantley, S.L., Barnes, A., Pantano, C.G., 2000b. X-rayphotoelectron evidence for bacteria-enhanced dissolution of hornblende.Geochim. Cosmochim. Acta 64, 1331–1343.

Kasting, J.F., 1992. Proterozoic climates: the effect of changing atmospheric carbondioxide concentrations. In: Schopf, J.W., Klein, C. (Eds.), The ProterozoïcBiosphere: A Multidisciplinary Study. Cambridge University Press, pp. 53–57,165–169.

Kasting, J.F., 1993. Earth’s early atmosphere. Science 259, 920–926.Kasting, J.F., Holland, H.D., Kump, L.R., 1992. Atmospheric evolution: the rise of

oxygen. In: Schopf, J.W., Klein, C. (Eds.), The Proterozoïc Biosphere: AMultidisciplinary Study. Cambridge University Press, pp. 53–57, 159–165.

Kump, L.R., Brantley, S.L., Arthur, M., 2000. Chemical weathering, atmospheric CO2,and climate. Ann. Rev. Earth Planet. Sci. 28, 611–667.

Kung, F.-C., Raymond, J., Glaser, D.A., 1976. Metal ion content of Escherichia coliversus cell age. J. Bacteriol. 126, 1089–1095.

Liermann, L.J., Kalinowski, B.E., Brantley, S.L., Ferry, J.G., 2000. Role of bacterialsiderophores in dissolution of hornblende. Geochim. Cosmochim. Acta 64, 587–602.

Limousin, G., Gaudet, J.P., Charlet, L., Szenknect, S., Barthès, V., Krimissa, M., 2007.Sorption isotherms: a review on physical bases, modelling and measurement.Appl. Geochem. 22, 249–275.

Liu, Y., Fu, X., Shen, J., Zhang, H., Hong, W., Chang, Y., 2004. Periplasmic proteinsof E. coli are highly resistant to aggregation: reappraisal for roles ofmolecular chaperones in periplasm. Biochem. Biophys. Res. Commun. 316,795–801.

Louvat, P., Gislason, S.R., Allegre, J.-C., 2008. Chemical and mechanical rates inIceland as deduced from river dissolved and solid material. Am. J. Sci. 308, 679–726.

Lucas, Y., 2001. The role of plants in controlling rates and products of weathering:importance of biological pumping. Ann. Rev. Earth Planet. Sci. 29, 135–163.

Luce, R.W., Bartlett, R.W., Parks, G.A., 1972. Dissolution kinetics of magnesiumsilicates. Geochim. Cosmochim. Acta 36, 35–50.

Lusk, J.E., Williams, R.J.P., Kennedy, E.P., 1968. Magnesium and the growth ofEscherichia coli. J. Biol. Chem. 243, 2618–2624.

Madigan, M.T., Martinko, J.M., Dunlap, P.V., Clark, D.P., 2008. Measuring microbialgrowth. In: Brock Biology of Microorganisms, International Edition, 12th ed.Pearson Higher Education. Pearson Prentice Hall, Upper Saddle River, NewJersey (Chapter 6).

Mailloux, B.J., Fuller, M.E., 2002. Determination of in situ bacterial growth rates inaquifers and aquifer sediments. Appl. Environ. Microb. 69, 3798–3808.

Merli, M., Oberti, R., Caucia, F., Ungaretti, L., 2000. Determination of site populationin olivine: warnings on X-ray data treatment and refinement. Am. Mineral. 85,55–65.

Meybeck, M., 1987. Global chemical weathering of surficial rocks estimated fromriver dissolved load. Am. J. Sci. 287, 401–428.

Miller, W.G., Lindow, S.E., 1997. An improved GFP cloning cassette designed forprokaryotic transcriptional fusions. Genes 191, 149–153.

Morris, P.M., Wogelius, R.A., 2008. Phthalic acid complexation and the dissolution offorsteritic glass studies via in situ FTIR and X-ray scattering. Geochim.Cosmochim. Acta 72, 1970–1985.

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/

12 B. Garcia et al. / Applied Geochemistry xxx (2013) xxx–xxx

Navarre-Sitchler, A., Brantley, S., 2007. Basalt weathering across scales. Earth Planet.Sci. Lett. 261, 321–334.

Neidhardt, F.C., Umbarger, E.H., 1996. Chemical composition of Escherichia coli. In:Neidhardt, F.C. (Ed.), Escherichia coli and Salmonella Cellular and MolecularBiology. American Society for Microbiology, Washington, pp. 13–16.

Neu, H.C., Heppel, L.A., 1965. The release of enzymes from Escherichia coli byosmotic shock and during the formation of spheroplasts. J. Biol. Chem. 240,3685–3692.

Norde, W., Lyklema, J., 1989. Protein adsorption and bacterial adhesion to solidsurfaces: a colloid chemical approach. Colloids Surf. 38, 1–13.

Nugent, M.A., Brantley, S.L., Pantano, C.G., Maurice, P.A., 1998. The influence ofnatural mineral coatings on feldspar weathering. Nature 395, 588–591.

Olsen, A.A., Rimstidt, J.D., 2008. Oxalate-promoted forsterite dissolution at low pH.Geochim. Cosmochim. Acta 72, 1758–1766.

Onstott, T., Phelps, T.J., Kieft, T.L., Colwell, F.S., Balkwill, D., Fredrickson, J., Brockman,F.J., 1999. A global perspective on the microbial abundance and activity in thedeep subsurface. In: Seckbach, J. (Ed.), Enigmatic Microorganisms and Life inExtreme Environments. Kluwer Academic Publishers, Dordrecht, TheNetherlands, pp. 487–500.

Outten, C.E., O’Halloran, T.V., 2001. Femtomolar sensitivity of metalloregulatoryproteins controlling zinc homeostasis. Science 292, 2488–2492.

Outten, F.W., Huffman, D.L., Hale, J.A., O’Halloran, T.V., 2001. The independent cueand cus systems confer copper tolerance during aerobic and anaerobic growthin Escherichia coli. J. Biol. Chem. 276, 30670–30677.

Palijan, G., Fuks, D., 2006. Alternation of factors affecting bacterioplanktonabundance in the Danube River floodplain (Kopacki Rit, Croatia).Hydrobiologia 560, 405–415.

Palmer, J., Flint, S., Brooks, J., 2007. Bacterial cell attachment, the beginning of abiofilm. J. Ind. Microbiol. Biotechnol. 34, 577–588.

Pembrey, R.S., Marshall, K.C., Schneider, R.P., 1999. Cell surface analysis techniques:what do cell preparation protocols do to cell surface properties? Appl. Environ.Microbiol. 65, 2877–2894.

Pogge von Strandmann, P.A.E., Burton, K.W., James, R.H., van Calsteren, P., Gislason,S.R., Sigfusson, B., 2008. The influence of weathering processes on riverinemagnesium isotopes in a basaltic terrain. Earth Planet. Sci. Lett. 276, 187–197.

Pokrovsky, O.S., Schott, J., 2000. Kinetics and mechanism of forsterite dissolution at25 �C and pH from 1 to 12. Geochim. Cosmochim. Acta 64, 3313–3325.

Regnier, P., Dale, A., Pallud, C., van Lith, Y., Bonneville, S., Hyacinthe, C., Thullner, M.,Laverman, A.M., Van Cappellen, P., 2005. Incorporating geomicrobial processesin subsurface reactive transports models. In: Aargaard, P., Viotti, P., Nuetzmann,G. (Eds.), Reactive Transport in Soil and Groundwater: Processes and Models.Springer, Berlin.

Rosso, J.J., Rimstidt, J.D., 2000. A high resolution study of forsterite dissolution rates.Geochim. Cosmochim. Acta 64, 797–811.

Santelli, C.M., Welch, S.A., Westrich, H.R., Banfield, J.F., 2001. The effect of Fe-oxidizing bacteria on Fe-silicate mineral dissolution. Chem. Geol. 180, 99–115.

Schott, J., Berner, R.A., 1983. X-ray photoelectron studies of the mechanism of ironsilicate dissolution during weathering. Geochim. Cosmochim. Acta 47, 2233–2240.

Scofield, J.H., 1976. Hartree–Slater subshell photoionization cross-sections at 1254and 1487 eV. J. Electron Spectrosc. Rel. Phen. 8, 129–137.

Seah, M.P., 1980. The quantitative analysis of surfaces by XPS: a review. Surf.Interface Anal. 2, 222–239.

Seah, M.P., Dench, W.A., 1979. Quantitative electron spectroscopy of surfaces: astandard data base for electron inelastic mean free paths in solids. Surf.Interface Anal. 1, 1–11.

Seyama, H., Soma, M., Tanaka, A., 1996. Surface characterization of acid-leachedolivines by X-ray photoelectron spectroscopy. Chem. Geol. 129, 209–216.

Shirley, D.A., 1972. High-resolution X-ray photoemission spectrum of the valencebands of gold. Phys. Rev. B 5, 4709–4714.

Please cite this article in press as: Garcia, B., et al. An experimental model approcoli – Impact on chemical weathering of mafic rocks and atmospherij.apgeochem.2013.01.007

Shirokova, L.S., Bénézeth, P., Pokrovsky, O.S., Gerard, E., Ménez, B., Alfredsson, H.,2012. Effect of the heterotrophic bacterium Pseudomonas reactans on olivinedissolution kinetics and implications for CO2 storage in basalts. Geochim.Cosmochim. Acta 80, 30–50.

Sigee, D.C., 2005. Freshwater Microbiology: Biodiversity and Dynamic Interactionsof Microorganisms in the Aquatic Environment. Wiley, Chichester, UK.

Silver, S., Clark, D., 1971. Magnesium transport in Escherichia coli. Interference bymanganese with magnesium metabolism. J. Biol. Chem. 246, 569–576.

Silver, S., Levine, E., 1968. Reversible alterations in membrane permeability ofEscherichia coli induced by a steroidal diamine, irehdiamine A. Biochem.Biophys. Res. Commun. 31, 743–748.

Smith, K.L., Milnes, A.R., Eggleton, R.A., 1987. Weathering of basalt formation ofiddingsite. Clays Clay Miner. 6, 418–428.

Stallard, R.F., Edmond, J.M., 1983. The influence of geology and weatheringenvironment on the dissolved load. J. Geophys. Res. 88, 9671–9688.

Stone, A.T., 1997. Reactions of extracellular organic ligands with dissolved metalions and mineral surfaces. In: Banfield, J.F., Nealson, Z.K.H. (Eds.),Geomicrobiology: Interactions between Microbes and Minerals. Rev. Mineral.,vol. 35, pp. 309–344.

Stopar, J.D., Taylor, G.J., Hamilton, V.E., Browning, L., 2006. Kinetic model of olivinedissolution and extent of aqueous alteration on Mars. Geochim. Cosmochim.Acta 70, 6136–6152.

Thorseth, I.H., Fumes, H., Tumyr, O., 1995. Textural and chemical effects of bacterial-activity on basaltic glass – an experimental approach. Chem. Geol. 119, 139–160.

Ullman, W.J., Kirchman, D.L., Welch, S.A., Vandevivere, Ph., 1996. Laboratoryevidence for microbially mediated silicate mineral dissolution in nature. Chem.Geol. 132, 11–17.

Van Herk, J., Pietersen, H.S., Schuiling, R.D., 1989. Neutralization of industrial wasteacids with olivine – the dissolution of forsteritic olivine at 40–70 �C. Chem.Geol. 76, 341–352.

Waldron, K.J., Rutherford, J.C., Ford, D., Robinson, N.J., 2009. Metalloproteins andmetal sensing. Nature 460, 823–830.

Walker, S.G., Flemming, C.A., Ferris, F.G., Beveridge, T.J., Bailey, G.W., 1989.Physicochemical interaction of Escherichia coli cell envelopes and Bacillussubtilis cell walls with two clays and ability of the composite to immobilizeheavy metals from solution. Appl. Environ. Microb. 55, 2976–2984.

Welch, S.A., Banfield, J.F., 2002. Modification of olivine surface morphology andreactivity during natural and experimental chemical weathering. Geochim.Cosmochim. Acta 66, 213–221.

Welch, S.A., Ullman, W.J., 1999. The effect of microbial glucose metabolism onbytownite feldspar dissolution rates between 5� and 35 �C. Geochim.Cosmochim. Acta 63, 3247–3259.

Welch, S.A., Vandevivere, P., 1994. Effect of microbial and other naturally occurringpolymers on mineral dissolution. Geomicrobiol J. 12, 227–238.

White, A.F., Brantley, S.L., 1995. Chemical weathering rates of silicate minerals. Rev.Min. 31.

White, A.F., Brantley, S.L., 2003. The effect of time on the weathering of silicateminerals: why do weathering rates differ in the laboratory and field? Chem.Geol. 202, 479–506.

Wogelius, R.A., Walther, J.V., 1991. Olivine dissolution at 25 �C: Effects of pH, CO2,and organic acids. Geochim. Cosmochim. Acta 55, 943–954.

Wogelius, R.A., Walther, J.V., 1992. Olivine dissolution kinetics at near-surfaceconditions. Chem. Geol. 97, 101–112.

Wogelius, R.A., Morris, P.M., Kertesz, M.A., Chardon, E., Stark, A.I.R., Warren, M.,Brydie, J.R., 2007. Mineral surface reactivity and mass transfer in environmentalmineralogy. Eur. J. Mineral. 19, 297–307.

Wu, L., Jacobson, A.D., Chen, H.-S., Hausner, M., 2007. Characterization of elementalrelease during microbe-basalt interactions at T = 28 �C. Geochim. Cosmochim.Acta 71, 2224–2239.

ach of biologically-assisted silicate dissolution with olivine and Escherichiac CO2 drawdown. Appl. Geochem. (2013), http://dx.doi.org/10.1016/