Geology Reference
In-Depth Information
oxalates and chlorides. This clearly evidences that
the mechanisms which control bacterial biominera-
lization are not mineral- or bacterium-specific.
On the contrary, they appear to be universal and dep-
end on the environment in which bacteria dwell.
This is in agreement with studies on the biominera-
lization capacity of other bacteria such as Schewa-
nella sp. that is able to induce the precipitation of
different iron oxi-hydroxides and ferric phosphates
(Hyacinthe et al. 2008) as well as arsenic sulphides
(Lee et al. 2007), Synecococcus which is able to pre-
cipitate gypsum, calcite and magnesite (Thompson
& Ferris 1990), or Halomonas sp. that is able to
induce the precipitation of calcite, Mg-calcite, ara-
gonite, struvite, monohydrocalcite and hydromag-
nesite (Rivadeneyra et al. 2006).
Regarding future research trends on bacterially
induced or mediated biomineralization, a number of
important minerals and poorly understood biomi-
neralization processes should be studied in detail.
For instance, bacteria play a crucial role deter-
mining rates, pathways and end products of the for-
mation and degradation of rocks, minerals and
organic matter. Consideration of the role of bacterial
biomineralization is thus required to gain a deeper
understanding of global biogeochemical cycles and
atmosphere-lithosphere-biosphere exchanges. Sig-
nificant progress has been made concerning well
known element cycles, such as Fe, N and S.
However, other cycles (e.g. the Ba cycle) are still
poorly understood and may also rely on bacterial
activity.
The formation of quartz in sedimentary environ-
ments has puzzled the scientific community for
decades. Bacterial activity could help explain low
temperature quartz formation. Experiments on
quartz precipitation in the presence of bacteria
would
life is possible on Mars. Furthermore, precipitation
of unusual Fe and Mg carbonates by bacteria, with
a spherulitic morphology similar to that observed
in bacterial vaterite, could help disclose whether
or
not
Martian
meteorite
spherulitic
carbonates
are, in fact, bacteriogenic.
Detailed studies should be conducted regarding
the so-called nanobacteria and their putative role
in mineralization in natural environments, human
pathological concretions and on other planets.
These studies will help corroborate the existence
of such bacteria, and if so, how they contribute,
for instance, to the mineralization of phosphates
and carbonates in kidney stones.
A critical, but poorly understood, aspect of
bacterial mineralization is the role of organics (in
bacterial cells and sheaths, as well as EPS) on the
nucleation of different solid phases. Although there
is some evidence pointing to a possible template
effect of organics in bacterial mineralization,
detailed studies of the organic-mineral interface at
the nanoscale should be performed. The combined
use of advanced techniques such as high resolution
transmission electron microscopy and near edge
X-ray absorption fine structure spectroscopy
(Benzerara et al. 2006), as wells X-ray and/or neu-
tron diffraction (e.g. using synchrotron facilities
for in-situ analysis of the early stages of mineraliz-
ation) could enable a better understanding of the role
of organics in bacterial mineralization and define
criteria for recognition of microbial biosignatures.
Another important issue, not directly dealt with
here, is how bacterial minerals grow. Banfield
et al. (2000) have shown that a self-assembled coar-
sening mechanism is responsible for the growth of
bacterial oxyhydroxides. Similar results have been
observed by Rodriguez-Navarro et al. (2007) in
the case of bacterial vaterite. During the oriented
aggregation of nanoparticles precipitated in the pre-
sence of bacteria, organics produced by the bacteria
(e.g. EPS) can be trapped within the aggregate, thus
offering a way to identify bacterial biominerals.
Research should be carried out to determine whether
or not this process is universal.
The relationship between bacterial infection in
humans and the formation of pathological precipi-
tates will also deserve further research. A causal
relationship has been found between bacteria and
phosphate mineralization in humans. However, it
has been suggested that abiotic processes could
lead to similar phosphate precipitation in human
pathological concretions. Nonetheless, pathological
concretions include other minerals such as vaterite,
typically associated to biogenic processes. There-
fore, research should also focus on vaterite formation
in the presence of bacteria. Bacteria (or other dead
cells, like osteoblasts) may play a purely passive
role in the process (i.e. providing heterogeneous
clarify
this
long
standing
geological
controversy.
Microbial growth in the deep biosphere may
have significant geobiochemical implications since
a tenth of the Earth biomass lives in deep sea-floor
layers. Future research should focus on those bio-
mineralization processes which may be associated
to this hidden microbial world.
The study of bacteria which are adapted to
extreme environments could offer new prospects
for bioremediation and pollution control. Minera-
lization experiments using extremophiles might
also further our understanding of geobiochemical
processes assumed to exist on early Earth, and
elsewhere. This research may also allow us to
more precisely establish the microbiological links
among most natural chemical and geological
processes, as well as to define constraints for life
development. For instance, halophilic bacteria and
Archaea induced mineralization in hypersaline,
low T environments could help disclose if microbial
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