Geology Reference
In-Depth Information
from microbial mats growing in Brejo do Espinho
lagoon, mediate the precipitation of dolomite
under oxic conditions in laboratory experiments.
Although the experimental results reported here
were produced outside of the complex microbial
community structure found in the Brejo do Espinho
sediments, it is postulated that the experimental
process may be similar to the one inducing dolo-
mite precipitation in the uppermost sediment lay-
ers of Brejo do Espinho, as observed at 5 cm depth.
A mechanism for carbonate precipitation has been
previously proposed for both natural environ-
ments (Ehrlich, 2002) and aerobic culture experi-
ments (Rivadeneyra
et al
., 1998, 2004).
Bacillus
sp.
and
Marinobacter
sp. can metabolize nitrogenated
organic material (proteins, amino acids, nucleic
acids) releasing CO
2
and NH
4
+
, which lead to the
observed increase of pH (and alkalinity). However,
by using proteinaceous substrate, as the 'peptone'
medium for this study, there is more acid pro-
duced by CO
2
than base by ammonia, according to
the following equation, showing, as an example,
the degradation of the amino acid serine:
contains abundant organic matter (mean value of
TOC = 6.2%) and has a pH close to 9.
Carbonate morphologies
Bacterial metabolism and calcium-magnesium
carbonate precipitation lead to spatial-temporal
changes in pH and ion concentration. These bio-
chemical factors infl uence the morphology of the
carbonate minerals, as well as the organic fi lm cov-
ering minerals (Fig. 7c) and extracellular organic
matter attached to the mineral (Fig. 7d). The extra-
cellular organic matter appears to play an important
role in determining the morphology as indicated
by a number of
in vitro
studies, which used dif-
ferent marine and freshwater bacterium (Castanier
et al
., 1999; Fujita
et al
., 2000; Warren
et al
., 2001).
Similar morphological features, such as spheres
and dumbbells, have been found in the geological
record (Chafetz, 1986; Pedley, 1992; Folk, 1993;
Kazmierczak & Altermann, 2002). Interestingly,
rod-like carbonate features were found in Martian
meteorite ALH84001 and have been presumed to
be of bacterial origin (McKay
et al
., 1996, 1997).
Van Lith
et al
. (2003) and Rivadeneyra
et al
.
(2004) discussed the fact that bacteria have the
capacity to adsorb ions, mainly Ca
2+
and Mg
2+
,
on their cell envelope, creating a microenviron-
ment which induces the precipitation of minerals.
Each bacterium has particular activities and cell
envelope characteristics that create its own chem-
ical microenvironments leading to the precipita-
tion of different minerals, which would not form
in the absence of bacterial activity. Dolomite crys-
tals formed in the aerobic culture experiments
and in Brejo do Espinho lagoon show similar
morphology, particularly the internal structure
of multitudinous nanocrystals constructing
macroscale spheroidal structures (Figs 6 and 7).
3HCO
3
+ 2H
2
O +
1NH
4
+
+ 2H
+
C
3
O
3
H
7
N + 2.5O
2
To clarify the observed carbonate precipitation,
the expected pH change was simulated using the
PHREEQC program, provided by the US Geological
Survey including the parameters pH, alkalin-
ity, CO
2
, NH
4
+
, Mg
2+
and Ca
2+
. The calculated pH
change by microbial degradation of 10 mM amino
acids indicates that a decrease from pH 7.0 to 6.3
would occur with aerobic respiration. Considering
that bicarbonate is already present in the natural
sediment, which would buffer the pH, the oxidation
of biomass would decrease the pH to a lesser extent
than the calculated value. However, to explain the
observed increase of pH in the laboratory culture
experiments from 7 to ~7.9, a massive outgassing of
CO
2
must have occurred (probably more than 70%
of the CO
2
produced). In the culture medium, the
calculated saturation index of dolomite is +1.7 and
calcite +0.39. In the microenvironment adjacent to
the bacterium, the saturation may be even higher,
inducing a local supersaturation with subsequent
precipitation of carbonate. In the presence of Ca
2+
and Mg
2+
, the bicarbonate ion will precipitate as
Ca- and Mg carbonates, such as dolomite
CaMg(CO
3
)
2
and hydromagnesite Mg
5
(CO
3
)
4(OH)
2
ยท4(H
2
O), as observed in the aerobic culture
experiments. Similar conditions may exist in the
dolomitic sediment of Brejo do Espinho, which
Spherulites in the geological record as
evidence for microbial involvement
Numerous examples of spheroidal dolomite,
similar to our microbial dolomite, have been
reported in the literature. Modern spheroidal
dolomite was reported from the Coorong lakes,
South Australia (von der Borch & Jones, 1976) and
in ancient rocks from Kuwait, ranging in age from
Eocene to Quaternary, which are characterized by
unique spheroidal dolomite cement (Guanatilaka
et al
., 1987; Guanatilaka, 1989). Nielsen
et al
.
(1997) proposed that spheroidal dolomite in the
karstifi ed top of a Dinantian dolomite sequence,
