Biologically Controlled Mineralization
Dennis A. Bazylinski
Department of Biochemistry, Biophysics, and Molecular Biology
Iowa State University
Ames, Iowa 50011 U.S.A.
Richard B. Frankel
Department of Physics
California Polytechnic State University
San Luis Obispo, California 93407 U.S.A.
As stated in an Chapter 4, prokaryotes of both Domains or Superkingdoms, the
Bacteria and the Archaea, mediate the formation of a large number of diverse minerals.
They are known to do this either through biologically induced mineralization (BIM)
(Lowenstam 1981) (the subject of Chapter 4 in this volume; Frankel and Bazylinski
2003) or biologically controlled mineralization (BCM). The latter has also been referred
to as organic matrix-mediated mineralization (Lowenstam 1981) and boundary-organized
biomineralization (Mann 1986). There are several important differences between BIM
and BCM that will be discussed in detail in this chapter, most notably those dealing with
aspects of the mineral crystals and the biomineralization process (see also Weiner and
Dove 2003). However, there is another significant difference between the two modes of
biomineralization and this is the aspect of functionality. Generally, in BIM there is no
function to the biomineralized particles except, perhaps, as a solid substrate for
attachment in the case of bacteria or as a form of protection against certain environmental
conditions or attack from predators. However, it is easier to recognize the primary
function of bone or shells produced by molluscs; two well-characterized examples of
BCM by higher organisms. Functionality should always be examined when dealing with
examples of biomineralization particularly in situations where the mineral product
displays some qualities of both BIM and BCM.
In BCM, the organism exerts a great degree of crystallochemical control over the
nucleation and growth of the mineral particles. For the most part, the minerals are directly
synthesized at a specific location within or on the cell and only under certain conditions.
The mineral particles produced by bacteria in BCM are characterized as well-ordered
crystals with narrow size distributions, and specific, consistent particle morphologies.
Because of these features, BCM processes are likely to be under specific
chemical/biochemical and genetic control. In the microbial world, the most characterized
example of BCM is magnetosome formation by the magnetotactic bacteria, a group of
microorganisms in which BCM-produced magnetic crystals appear to have a relatively
specific function. There are some examples of biomineralization that appear to be
intermediate between BIM and BCM; these are covered in Chapter 4 (Frankel and
Bazylinski 2003) since the mineralization product usually has more features in common
with BIM. This chapter is, in effect, a review of the magnetotactic bacteria. However, it
should be remembered that the production of magnetic minerals by the magnetotactic
bacteria is used in this volume as an example of BCM. The microbiological, geological,
chemical, biochemical, and physical approaches used to study BCM in the magnetotactic
218 Bazylinski & Frankel
bacteria are universal and will certainly be utilized again in other examples of BCM in
bacteria discovered in the future.
THE MAGNETOTACTIC BACTERIA
Classification and general features
The magnetotactic bacteria are a heterogeneous group of prokaryotes that passively
align and actively swim along the Earth’s geomagnetic field lines (Blakemore 1975,
1982). This group is morphologically, metabolically, and phylogenetically diverse and
cellular morphotypes that have been observed include coccoid (roughly spherical or
ovoid), rod-shaped, vibrioid (curved), spirilloid (helical) and even multicellular forms
(Blakemore et al. 1982; Farina et al. 1983; Rogers et al. 1990; Bazylinski 1995). The
term “magnetotactic bacteria” therefore has no taxonomic significance and should be
interpreted as a collection of diverse bacteria that possess the apparently widely
distributed trait of magnetotaxis (Bazylinski 1995).
Despite the diversity of these magnetotactic bacteria, they have several important
features in common: 1) all that have been described are gram-negative members of the
Domain Bacteria (it is possible that some members of the Domain Archaea produce
magnetosomes but none have been reported); 2) they are all motile, generally by flagella
(although this does not preclude the possibility of the existence of non-motile bacteria
that synthesize magnetosomes which, by definition, would be magnetic but not
magnetotactic); 3) all exhibit a negative tactile and/or growth response to atmospheric
concentrations of oxygen; 4) all strains in pure culture have a respiratory form of
metabolism (i.e., none are known to ferment substrates); and 5) they all possess a number
of magnetosomes, the signature feature of the group (Bazylinski 1995; Bazylinski and
Moskowitz 1997). The bacterial magnetosome is defined as an intracellular single-
magnetic-domain crystal of a magnetic iron mineral enveloped by a membrane (Balkwill
et al. 1980). The membrane is intracellular and may be connected to the cytoplasmic
membrane. There is some evidence that the magnetosome membrane comprises a vesicle
in which the magnetosome subsequently nucleates and grows (Gorby et al. 1988).
There are two general types of magnetotactic bacteria based upon the minerals they
biomineralize: there are the iron-oxide types that mineralize crystals of magnetite (Fe3O4)
and the iron-sulfide types that mineralize crystals of greigite (Fe3S4) (Bazylinski and
Frankel 2000b). The iron-oxide type are obligate microaerophiles, facultative anaerobes
(that are microaerophilic when growing with O2), or obligate anaerobes while the iron-
sulfide type appear to be obligate anaerobes.
Ecology of magnetotactic bacteria
Magnetotactic bacteria are ubiquitous in aquatic environments containing water with
pH values close to neutrality and are not thermal, strongly polluted, or well-oxygenated
(Moench and Konetzka 1978; Blakemore 1982). They are cosmopolitan in distribution
(Blakemore 1982) and because magnetotactic bacterial cells are easy to observe and
separate from mud and water by exploiting their magnetic behavior using simple
laboratory magnets (Moench and Konetzka 1978), there are frequent reports of their
occurrence in various freshwater and marine locations (e.g., Matitashvili et al. 1992; Iida
and Akai 1996).
On a local basis, magnetotactic bacteria are generally present in the highest numbers
at the microaerobic oxic-anoxic transition zone (OATZ) and just below it in the anaerobic
zone (Bazylinski 1995). In many freshwater habitats, the OATZ is located at the
sediment-water interface or just below it. However, in some brackish-to-marine systems,
Biologically Controlled Mineralization in Prokaryotes 219
the OATZ is found, or is seasonally located, in the water column. The Pettaquamscutt
Estuary (Narragansett Bay, RI, USA) (Bazylinski et al. 1995) and Salt Pond (Woods
Hole, MA, USA) (Wakeham et al. 1984, 1987) are typical examples of this situation.
Hydrogen sulfide, produced by sulfate-reducing bacteria in the anaerobic zone and
sediment, diffuses upward while oxygen diffuses downward from the surface resulting in
a double, vertical, chemical concentration gradient with a co-existing redox gradient.
Strong pycnoclines and other physical factors, probably including the microorganisms
themselves, stabilize the vertical chemical gradients and the resulting OATZ.
Several morphotypes of iron-oxide and iron-sulfide magnetotactic bacteria are found
at sites like the Pettaquamscutt Estuary and Salt Pond. The magnetite-producing
magnetotactic bacteria are generally present at the OATZ proper and appear to behave as
microaerophiles. Experiments with pure cultures support this observation. Two strains of
magnetotactic bacteria have been isolated from the Pettaquamscutt and are now in pure
culture. One is a vibrio designated strain MV-2 (Meldrum et al. 1993b; Dean and
Bazylinski 1999) and the other is a coccus designated strain MC-1 (DeLong et al. 1993;
Meldrum et al. 1993a; Frankel et al. 1997; Dean and Bazylinski 1999). Both strains grow
as microaerophiles although strain MV-2 can also grow anaerobically with nitrous oxide
(N2O) as a terminal electron acceptor. Other cultured magnetotactic bacterial strains,
including spirilla (e.g., Blakemore et al. 1979; Schleifer et al. 1991; Schüler et al. 1999)
and rods (Sakaguchi et al. 1993a), are microaerophiles or anaerobes or both. The greigite-
producers appear to be strict anaerobes positioned in the more sulfidic waters just below
the OATZ where oxygen is barely or not detectable (Bazylinski et al. 1990, 1992;
Bazylinski 1995). To date, no greigite-producing magnetotactic bacterium has been
grown in pure culture.
Phylogeny of the magnetotactic bacteria
Because magnetite particles resembling those of magnetotactic bacteria in ancient
and recent sediments have been interpreted as magnetofossils (discussed later in the
chapter), it is important to understand the phylogeny of the magnetotactic bacteria.
Phylogenetic analysis, based on the sequences of 16S ribosomal RNA genes of many
cultured and uncultured magnetotactic bacteria, initially showed that the magnetite-
producing strains are associated with the α-subdivision of the Proteobacteria (Burgess et
al. 1993; DeLong et al. 1993; Eden et al. 1991; Schleifer et al. 1991; Spring and Schleifer
1995; Spring et al. 1992, 1994, 1998), a vast assemblage of Gram-negative prokaryotes in