Trace Metal Biogeochemistry(Marine Bioinorganic Chemistry)

12.755 MIT-WHOI Joint Program Graduate Course

Lecture 1Mak Saito, Marine Chemistry and Geochemistry Department

Course website: www.whoi.edu/sites/12.755

Outline:1. Introductions, comments on course schedule, structure,

approach, assignments, and pedagogy2. Introduction to Trace Metal Biogeochemistry: an evolving

field3. Classifications of TM profiles4. Metal Speciation lecture

Class Topics

• Introduction to trace metal biogeochemistry, broad categories• Metal Speciation• Free ion model• Algal uptake kinetics • The Droop model and colimitations• Mercury Biogeochemistry (Lamborg as guest lecturer)• Iron biogeochemistry (limitation, light colimitation, redox, speciation,

uptake mechanism, colloids, and policy)• Trace elements and the ancient ocean• Metalloenzymes• Analytical approaches (in silico and proteomic/mass spec)• Specific elemental biogeochemistries (Mn, Al, Pb, Co, Zn, Cd, Cu)

Events

• Phone conference with Bill Sunda, expert trace metal phytoplankton interactions

• JGI bioinformatics module with hands on computer experiments and phone conference with JGI genomic scientist

• Anonymous review of papers• Readings on ideas in science for discussion throughout semester• Discussion of iron fertilization• Discussion of Mercury policy (Lamborg)

Iron as a limiting nutrient in HNLC regions (Review of Iron Fertilization Experiments Boyd et al., 2007, Science)

Purposeful (white crosses) and natural (red crosses) Fe enrichment studies have shown Fe limitation of phytoplankton growth.

Download PDF from

http://www.geotraces.org/

Science Plan

GEOTRACES Goal: making WOCE-like sections for Trace Elements and Isotopes

Meridional Pacific, Hiscock, Measures and Landing, GBC 2008

                         

                    

                      

                  

Summer 2007 CEBIC Undergraduate Research Fellowships: Information and Application Process

CEBIC Summer Workshop 2007Sunday, June 10 - Wednesday, June

13 Contact: Eva Groves

egroves@princeton.edu

 

 

 

 

RIP: CEBIC1999-2007

Trace metal biogeochemistrya.k.a Marine Bioinorganic Chemistry:

a field developing its own identity

Driven originally by analytical chemistry• Initial measurements of many metals far too high due to

contamination

Biological or “Bioinorganic” component has grown in:• Bioactive metals: Fe, Co, Cd, Zn, Cu, Ni, Mn, Mo etc.• Iron limitation discovered• The Role Complexation on Bioavailability• Metalloenzymes• Other limitations and colimitations• Future roles for genomics, metagenomics, proteomics

Four Categories of Trace Metal Profiles in 2D

1. Conservative distributions- Residence time greater than 100000 years - Much greater than the residence time of the oceans- Molybdenum, tungsten, antimony, rubidium: are

involved in particle cycling, but the quantities are insignificant relative to their large seawater inventory

- Concentrations of some are quite high: Mo = 105nM- Don’t increase with thermohaline circulation- Searching for the kink in Molybdenum due to

nitrogen fixation

Four Categories of Trace Metal Profiles in 2D2. Nutrient-type distributions:

– Significantly involved with internal cycles of biologically derived particulate material

– Distributions are dominated by phytoplankton uptake in surface waters followed by export of some of this material below the surface layer and subsequent remineralization and release to intermediate and deep waters

– Have a low level of scavenging in intermediate and deep waters

– (N, P, Si) Zinc, Cadmium, Barium, Silver, Nickel– Increase in concentration with thermohaline circulation– Can be used as paleoproxies for P (Cd) or Si (Zn) in

foram tests and diatom opal.

Four Categories of Trace Metal Profiles in 2D

3. Scavenged-type distributions- Strong interactions with particles- Short residence times (~100-1000y)- Increased concentration near sources - Decreased concentrations away from sources- Decreased concentrations along flow path due to

continual scavenging- Aluminum, lead, manganese

Tangent:

• Tomatoes and Tomatoes• Aluminium (British and Aussies) and Aluminum (Elsewhere)• International Union of Pure and Applied Chemistry uses Aluminium• Probably most importantly for oceanography: Chris Measures is

British

Four Categories of Trace Metal Profiles in 2D

4. Hybrid-Type Metals- Strongly influenced by both micronutrient use and

remineralization and scavenging processes. - Does not accumulate with thermohaline circulation- Can depend on geographic location: high dust input can obscure

surface drawdown signal- “Hybrid-Type” is a relatively new descriptor- Bruland and Lohan (assigned reading this week): Iron, copper- Although not included, Cobalt is undoubtedly a hybrid-type metal- Mn could be one as well, but only at high latitudes, where

nutrient-like drawdown occurs

These four geochemical categories of metals in seawater are a direct result of their chemical properties:

• Solubility• Inorganic speciation• Organic Speciation• Redox chemistry

• Biological properties is debatable as a fifth, since there appear to be non-biological elements with nutrient-like profiles

Background Aquatic Chemistry of Trace Elements:A marine water column context

Solubility Products: Example for Fe(OH)3(s)

Ksp= [Fe][OH]3 = 1042.7

Stability constants for metal complexes (where L is ligand, M is Metal):K = [ML]/[M][L]

Ligands can include inorganic chemical species:

In oxic systems: OH-, CO32-,SO4

2-, Cl-, PO43-,

In anoxic systems add: HS-,, S2-

Ligands can also include organic chemical species:EDTA, DTPA, NTA, Citrate, Tris, siderophores, cobalophores,

DFB, TETA, and the famous unknown ligand(s) “L”

Detailed balancing: Principle of Microscopic Reversibility

kf

Mn+ + L- ML

kb

d[ML]/dt = kf [M+] [L-]

-d[M+]/dt = -d[L-]/dt = kb [ML]

At steady state:

kf [M+] [L-] = kb [ML]

kf / kb = [ML]/([M+][L-]) = K

Background Aquatic Chemistry of Trace Elements:A marine water column context

However, there can be Non-Ideal effects (Morel and Hering 76-82):

- The effects of other solutes on the free energy of ion(s) of interest

- Solubility product and stability constants need to be corrected, or better, determined to/at the appropriate ionic strength.

- The activity of the metal is: {Mn+} = [Mn+]Mn+

- The activity coefficient, Mn+, can be estimated by the Debye-Huckel correction or

the Davies expression (modified Debye-Huckel)

- I = ionic strength

- Z=charge, A = 1.17 M-1/2, B=0.3M-1/2

- Thermodynamic databases (Martell and Smith) will provide the ionic strength experimental conditions for each constant (e.g. 0.1M)

Background Aquatic Chemistry of Trace Elements:A marine water column context

From Morel and Hering, 1993, p77

Quasi constant value between I=0.3-0.7

Definitions

• Ligand – an atom, ion, or molecule that donates/shares electrons with one or more central atoms or ions. Metal-ligand bonds (inner sphere) are covalent.

• Chelate – (from Greek chelos = crab, with two binding claws) two or more donor atoms from a single ligand to the central metal atom

• Coordination environment or chemistry: number of ligands that a metal can have. Most metals have a # of 6, forming octahedral complexes

Vraspir and Butler 2009

Characteristics of Metal Ion Binding to Ligands

• Soft vs Hard

– Soft: Ions are large and easily polarizable

– Hard: Small and less easily polarizable

• Soft metals tend to “like” soft ligands

• Hard metals tend to “like” hard ligands

• Examples:

• Hard: Fe3+, Co3+ and OH-

• Soft: Cd2+, Cu+, Hg2+ and sulfide groups

Valence

• Metal chemistry strongly influenced by the removal of electrons from a neutral atom

• Main group: outer electron shells consist of s and p orbitals (Li, Na, K)

– React violently with water (e.g. pure sodium to NaOH, +1 ions)

• Transition metals have incomplete d electron shell

• Most transition metals have variable valence, a major component of their chemistry

– Fe: +2, +3

– Mn: +2, +3, +4, +6, +7

Ionic radii of Cd2+ > Co2+ > Fe3+

Characteristics of Metal Ion Binding to Ligands

• Soft vs Hard

– Soft: Ions are large and easily polarizable

– Hard: Small and less easily polarizable

• Soft metals tend to “like” soft ligands

• Hard metals tend to “like” hard ligands

• Examples:

• Hard: Fe3+, Co3+ and OH-

• Soft: Cd2+, Cu+, Hg2+ and sulfide groups

Average Major Seawater Ions (mM)

(Morel and Hering, p291)

HCO3- 2.38

SO42- 28.2

Cl- 0.545

Ca2+ 0.0102

Mg2+ 0.0532

Na+ 0.0468

K+ 0.0102

Average Major Seawater Ions (mM)

(Morel and Hering, p291)

HCO3- 2.38

SO42- 28.2

Cl-

Ca2+

Mg2+ 0.0532

Na+

K+ 0.0102

The Irving-Willliams Series

• Observations that complex stability for each ligand have a tendancy to rank:

Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+

• Caused by increases in ionic radius and ligand field stabilization effects

• Many implications both for ligands “L’s” in seawater and for protein binding of metals inside cells, area for much future research

– For example it is hard to find any cobalt(II) ligand that is stronger than a nickel(II) ligand

Complexation Environment

• “Free ions” is really a misnomer

• Cu2+ is actually Cu(H2O)62+

, if not bound by other inorganic species

• Water is a ligand, ligand-exchange rxn constants indicative of rate of reactivity, or the kinetics

• Dissociation of water molecules dependent on size and inversely to the size of the metal cation

Water loss exchange rates

Abundance (or lack there of) is our friend

Seawater constituents:• Major ions (the salt) – millimolar and higher

– Na+

– Cl-

– Mg2+

– Ca2+

– HCO3-

• Organic ligands/chelators - nanomolar– “L”

• Trace metals/elements – picomolar to nanomolar– Mn+

• With major ions, everything depends on everything (and must be considered simultaneously

• With trace elements, we can consider one element at a time, independently of other constituents

Preview: Software for Metal Speciation

• Mineql – Westall et al. a program made for calculating aqueous speciation and solubility at low temperature geochemical conditions

• Critical.exe – Smith and Martell volumes built into a DOS based-database.

• But need to know how to do it by hand well in order to use software effectively. I usually use both hand calculations and computer assisted calculations to cross-check assumptions.

Readings – available on websitewww.whoi.edu/sites/12.755

• Bruland and Lohan -Treatise on Geochemistry Chapter• Morel and Hering, Principles of Aquatic Chemistry

Chapter 6• Background: Lippard and Berg Bioinorganic Chemistry

chapter 2• Goldberg Biography