Geology Reference
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Kajander & ΒΈ iftcioglu 1998). However, the possible
role of bacteria, as well as 'nanobacteria', in the
development of such pathologies is a matter of
strong controversy (Cisar et al. 2000; Aloisi 2008).
Chlorides such as halite (NaCl) form in natural
habitats in the presence of halophilic bacteria
(Castanier et al. 1999). Despite the previous
example, works linking halide precipitation with
microbial activity are inconclusive. There is,
however, evidence for the development of bacteria
and archaea in hypersaline environment associated
to halite precipitation. For instance, Vreeland et al.
(2000) report the presence of viable bacteria
included in 250 million-year-old halite crystals,
although it is difficult to link past bacterial activity
and the formation of such crystals. On the other
hand, cyanobacteria have been found in halite
rocks in the hyperarid core region of the Atacama
Desert where no other life form has been detected
(Wierzchos et al. 2006), while halite biomineraliza-
tion by halophilic archaea has been suggested as a
means for these microbes preservation (Adamski
et al. 2006). As a result, much attention is paid
to the study of brines and evaporites as analogs
for microbial life in salt-rich deposits on Mars
(Rothschild 1990; Mancinelli et al. 2004). Interest-
ingly, halophylic bacteria appear to be highly
versatile from a biomineralization perspective: they
have been shown to induce the precipitation of a
range of minerals including carbonates such as
calcite, magnesium calcite, aragonite, hydromagne-
site [Mg
5
(CO
3
)
4
(OH)
2
.
2H
2
O] and monohydrocal-
cite (CaCO
3
.
H
2
O), as well as phosphates such as
struvite (Rivadeneyra et al. 2006).
Economic deposits of native sulphur have been
associated to bacterial activity (e.g. Sebastian-Pardo
et al. 1983). In this respect, the bacterial origin of
sulphur deposits in several caves was corroborated
by
34
S values showing enrichment in light sulphur
isotopes (Northup & Lavoie 2001). Other native
metals such as gold have also been found to
precipitate in the presence of bacteria (Ferris
1997). Karthikeyan & Beveridge (2002) reported
toxic soluble gold precipitation by Pseudomonas
aeruginosa.
orientation of the precipitates (Mann 2001). This
biomineralization process is typical of metazoan
(e.g. shells of bivalves), but quite uncommon in bac-
teria. On the other hand, biologically induced (or
mediated) mineralization is the result of microbial
metabolism, but the microorganisms do not directly
control how and where the precipitates form.
Two stages can be identified in the biominerali-
zation process induced or mediated by bacteria. A
first stage includes the active modification of the
physical-chemistry in the interior or surroundings
of the bacteria (i.e. pH, Eh, ion concentration,
P
CO2
). These changes ultimately lead to an increase
in ion concentration (i.e. supersaturation) which
is a prerequisite for mineral precipitation. The bac-
teria can directly contribute to ion concentration inc-
rease by producing inorganic metabolic by-products.
Otherwise, bacteria can indirectly contribute to
mineral precipitation by gathering and concentrating
different ions both within and on/around the cells
(Fortin et al. 1997). Nucleation of a mineral phase
occurs in a second stage. Nucleation is a crucial
moment for mineral precipitation. It can occur either
homogeneously or heterogeneously (Mullin 1992).
Homogeneous nucleation requires a significantly
high supersaturation. The activation energy (i.e.
supersaturation) can be drastically reduced by the
presence of a foreign surface on which hetero-
geneous nucleation occurs. Heterogeneous nuclea-
tion, the most plausible nucleation process in
nature, is promoted by the coupling between func-
tional (macro)molecules in the bacterial cell wall
and the new mineral phase. Heterogeneous nuclea-
tion may also occur on bacterial exopolymeric sub-
stances (EPS) (Braissant et al. 2003, 2007; Dupraz
& Visscher 2005). This complex process is explained
by the so-called theory of template-directed (or
organic-matrix mediated) biomineralization (Mann
2001). This theory states that organic molecules
can (self )assemble into a template so that there can
be electrostatic (ionotropic effect), geometric, or
stereochemical affinity/matching between the
template and the inorganic precipitate (biomineral).
Organic macromolecules and
biomineralization
How does bacterial mineralization
take place? Current models
Mineralization concepts based on inorganic solid
phase precipitation aided by organic macromol-
ecules have been developed over the last few
decades (Mann 2001). These concepts have been
applied to the understanding of how biomineraliza-
tion occurs (Lowenstam & Weiner 1989; Mann
2001; Rodriguez-Navarro et al. 2007).
Organic macromolecules are normally acidic
polyanionic polymers (e.g. proteins, glycoproteins,
proteoglycans)
Biogenic mineral formation is thought to occur
through either biologically controlled or induced
processes (Lowenstam & Weiner 1989). Biologi-
cally controlled mineralization occurs in an isolated
compartment within a living organism (e.g. mag-
netosomes in magnetotactic bacteria). Minerals
formed in this way display a highly ordered struc-
ture and the organisms can control size, texture and
which
include
carboxylic
or
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