Geology Reference
In-Depth Information
results in pore-waters being undersaturated with
respect to (magnesium) calcite while remaining
supersaturated with respect to dolomite. In the
natural environment, aerobic and anaerobic car-
bonate-forming processes may alternate in seasonal
driven cycles. In summary, dolomite formation in
the Brejo do Espinho lagoon undoubtedly occurs
in a dynamic environment, strongly infl uenced
by seasonal changes (wet vs. dry, oxic vs. anoxic),
resulting in different microbial processes being
involved in the formation of dolomite.
Finally, the demonstration that moderately
halophilic aerobic bacteria can induce dolomite
precipitation suggests that this process may be
active in other modern carbonate-forming envir-
onments. For example, the modern dolomitic
crusts, studied by Robert N. Ginsburg and his
colleagues, are reported to occur at or near the
surface in an area covering hundreds of square
kilometres on Andros Island, Bahamas (Shinn
et al
., 1965, 1969). Revisiting this supratidal
environment to test for the mediation of dolo-
mite precipitation through the activity of aerobic
bacteria mediating in this important cementation
process would surely add a new prospective to
Bahamian carbonate sedimentology.
CONCLUSIONS
This study of natural samples from Brejo do
Espinho lagoon and laboratory experiments using
discrete, moderately halophilic aerobic bacteria
isolated from the uppermost oxic sediments pro-
vides new evidence of the microbial processes
associated with carbonate mineral formation. It has
been demonstrated that isolated aerobic strains
(
V. marismortui
and
Marinobacter
sp.) are able
to mediate dolomite precipitation in a synthetic
hypersaline medium in a manner consistent with
dolomite formation under ambient Earth surface
conditions in the presence of oxygen. Thus, a
new metabolic process, aerobic respiration at the
water-sediment interface, can now be included
in the list of microbial factors that contribute
to the precipitation of dolomite, in addition to
bacterial sulphate reduction (Vasconcelos
et al
.,
1995; Warthmann
et al
., 2000) and methano-
genesis (Roberts
et al
., 2004). The proposed
mechanism is based on a metabolically medi-
ated pH shift and associated increase in alkalinity
with precipitation occurring within an EPS
matrix.
The similarity of crystal habit (shape) between
the experimentally and naturally produced dolo-
mite is strong evidence for the involvement of
microorganisms in the precipitation processes
in the latter. The morphologies of the bacterial
precipitates may have important implications for
the recognition and interpretation of carbonate
sediments and microcrystalline cements in marine
carbonates. Furthermore, the crystal habit is obvi-
ously not specifi c to a particular bacterial genus
and is also common to bacterial Mg-calcite, cal-
cite and aragonite precipitates. The recognition
of bacterial carbonate morphology will contribute
substantially to better understanding of recent
carbonate formation and to the identifi cation of
past bacterial activity in the rock record.
ACKNOWLEDGEMENTS
We would like to thank Martin Müller for
guidance with SEM imaging, Michael Plötze
for help with the XRD analyses and Stefano
Bernasconi for his assistance with interpreting the
stable isotope data. The Swiss Science National
Foundation (SNF) is gratefully acknowledged for
generous fi nancial support through Grant nos.
20-067620 and 20-105149.
REFERENCES
Allen, C.C, Albert, F.G., Chafetz, H.S., Combie, J.,
Graham, C.R., Kieft, T.L., Kivett, S.J., McKay, D.S.,
Steele, A., Taunton, A.E., Taylor, M.R., Thomas-
Keprta K.L.
and
Westall, F.
(2000) Microscopic
physical biomarkers in carbonate hot springs: implica-
tions in the search of life on Mars.
Icarus
,
147
, 49-67.
Arahal, D.R., Marquez, M.C., Volcani, B.E., Schleifer, K.H.
and
Ventosa, A.
(1999)
Bacillus marismortui
sp. nov.,
a new moderately halophilic species from the Dead
Sea.
Int. J. Syst. Bacteriol.,
49
, 521-530.
Barbiére to Primo, P.B.S.
and
Bizerril, C.R.S.F.
(2002)
Lagoa de Araruama
, 1a edição, SEMADS, Rio de
Janeiro, 160 pp.
Braissant, O., Callileau, G., Dupraz, C.
and
Verecchia, E.P.
(2003) Bacterially induced mineralization of calcium car-
bonate in terrestrial environments: the role of expolysac-
charides and amino acids.
J. Sediment. Res.,
73
, 485-490.
Buczynski, C.
and
Chafetz, H.S.
(1991) Habit of bacteri-
ally induced precipitates of calcium carbonate and the
infl uence of medium viscosity on mineralogy.
J. Sediment. Petrol.
,
61
, 226-233.
Castanier, S., Le Metayer-Levrel, G.
and
Perthuisot, J.P.
(1999) Ca-carbonates precipitation and limestone
genesis - the microbiologist point of view.
Sediment.
Geol.,
126
, 9-23.
Chafetz, H.S.
(1986) Marine peloids: a product of bac-
terially induced precipitation of calcite.
J. Sediment.
Petrol.,
56
, 812-817.
