Geology Reference
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in eastern Belgium, was bacterially induced or
mediated precipitates. Furthermore, Mansfi eld
(1980) reported urolithic biogenic dolomite with
spheroidal structures. Mastandrea
et al
. (2006)
found nanoscale spheroids associated with micro-
bial primary dolomite within a microbiolitic facies
in Late Triassic carbonate platforms of southern
Italy. Lee & Golubic (1999) interpreted spherulitic
structures in the Mesoproterozoic Gaoyuzhuang
Formation, China (1.2-1.5 Ga), as being microbial
in origin. Buczynski & Chafetz (1991) suggested
that 'sheaf-of-wheat' or dumbbell morphologies
are unique to bacterially induced precipitates and
can be used to identify such precipitates in the
rock record. Apparently, carbonate spherulites are
the fi nal stage of dumbbell growth (Buczynski &
Chafetz, 1991; Warthmann
et al
., 2000). However,
previous inorganic experiments result in the pre-
cipitation of spherical morphology (Malone
et al
.,
1996; Golden
et al
., 2001), which implies that the
morphology alone is not necessarily indicative of
a biological process. However, in this study, the
association of microbes with mineral precipitation
in both the laboratory experiments and environ-
mental samples does support biological involve-
ment associated with the spherical morphology.
medium D-1 simulates biofi lms and/or microbial
mats and provides an organic matrix in which
to experimentally study biomineralization. The
control experiments consisting of non-inoculated
culture media do not produce carbonate precipi-
tates and, thus, emphasize the role of these
bacteria in mineral precipitation. Indeed, microbes
can provide nucleation sites and may actively
contribute to the alkalinization of the medium.
Dolomite formation in Brejo do Espinho lagoon
Our laboratory culture experiments demon-
strate that the two strains isolated from Brejo do
Espinho (
V. marismortui
and
Marinobacter
sp.)
can mediate the precipitation of dolomite under
aerobic, low-temperature conditions within a
solid organic substrate. In natural systems, the
process of dolomite precipitation actually occurs,
however, within a more complex microbial com-
munity. Based on the laboratory culture experi-
ments and studies of both Brejo do Espinho and
Lago Vermelha, it is proposed that microbially
mediated dolomite precipitation occurs in a series
of changing microenvironments with a combina-
tion of metabolic processes acting separately or
in conjunction, as follows: (1) In the dry season,
high-Mg calcite and dolomite precipitation is
mediated by aerobic bacteria in the uppermost
oxic sediment near the sediment-water interface
and is subsequently buried to depths where early
diagenetic processes occur under anoxic condi-
tions. (2) Additionally, in the wet season, anoxy-
genic photosynthetic sulphur bacteria mediate
the precipitation of Ca-Mg carbonates directly
below the sediment-water interface, as illustrated
in the microbial mat forming in the neighbouring
hypersaline lagoon, Lagoa Vermelha (Vasconcelos
et al
., 2006). (3) With burial below the level of oxy-
gen diffusion, early diagenetic processes driven
by bacterial sulphate-reducing activity occur
under anoxic conditions leading to an ageing
process where high-Mg calcite and Ca dolomite
(20-40% Mg), which are metastable with respect
to stoichiometric dolomite, would be expected to
become stoichiometric dolomite (Malone
et al
.,
1994). The occurrence of 100% dolomite concre-
tions at shallow depths of < 25 cm, together with
XRD and SEM studies (Fig. 7), in both Brejo do
Espinho and Lagoa Vermelha (Vasconcelos &
McKenzie, 1997), provide evidence for this
process. (4) As hypothesized by Moreira
et al
.
(2004), the bacterial sulphate reduction is com-
plemented by
in situ
sulphide oxidation which
Spherulite internal structure
In general, spherulites are composed of fi brous
radiate crystals, an observation consistent with
spherulite surface features shown in Fig. 7d.
Apparently, the presence of polymeric organic
molecules can promote the spherulitic growth of
calcium carbonate by effectively minimizing the
surface area (Yu
et al
., 2002; Coelfen & Qi, 2001).
In contrast, the present work shows that the pres-
ence of
V. marismortui
and
Marinobacter
sp
.
cells
are important in mediating calcium-magnesium
carbonate (dolomite) spherulitic growth under
oxic Earth surface conditions. In addition, our
results imply that the agar gel medium, which
is often used in biomineralization experiments
(Buczynski & Chaftez, 1991; Chaftez & Buczynski,
1992), plays a crucial role in the development
of these specifi c morphologies and carbonates
(e.g. by providing the organic polymeric com-
pounds, as mentioned above).
Whereas the spheroidal and dumbbell shapes
serve as evidence for bacterially induced carbon-
ate precipitation, the organic molecules originally
in the gel medium or produced by the bacteria
appear to be crucial for the development of these
morphologies. In other words, the gel-like culture
