Geology Reference
In-Depth Information
BE1 and BE2, respectively), isolated from the
uppermost part of the microbial mat in Brejo do
Espinho. Under low temperature (30°C), hyper-
saline conditions in the presence of oxygen, it
was possible to precipitate dolomite in semi-
solid cultures. A signifi cant rise in pH occurred
in the growing cultures from the original pH 7.0
of the D-1 medium up to ~ pH 7.9. In the control
experiments using non-inoculated medium,
neither mineral precipitation nor changes in pH
were detected.
Figure 7a gives an overview of the biologic-
ally mediated crystals formed in aerobic cul-
ture experiments with
V. marismortui
. All of
the observed crystals have the same spherical
morphology but with variable diameters. The
larger spheres (diameter
200 μm) are hydro-
magnesite (hydrated magnesium carbonate) and
the smaller spheres (diameter
10 μm) are dolo-
mite (Fig. 7b). At higher magnifi cation, the
dolomite spheres formed in the aerobic culture
experiments with
Marinobacter
sp. are com-
posed of multitudinous nanocrystals of dolomite,
and the mineral microspheres and dumbbells
are enveloped in a biofi lm or EPS (Fig. 7c and d
respectively). These spherical and dumbbell mor-
phologies are typical for microbially induced
carbonates (Buczynski & Chafetz, 1991; Knorre &
Krumbein, 2000; Warthmann
et al
., 2000).
The co-presence of spheroidal dolomite and
hydromagnesite in the experiments suggests that
spheroidal morphology and hydromagnesite
both may be indicators for microbial dolomite.
Indeed, the same co-precipitation with similar
morphology has been found in other modern
environments, such as Lake Walyungup in South
Australia (Coshell
et al
., 1998; Rosen & Mcnamara,
1998). These results link bacterial precipitation of
dolomite and hydromagnesite, both of which are
found in natural habitats (Renaut, 1990; Warren,
1990; Russell
et al
., 1999; Last & Ginn, 2005).
10
m
DISCUSSION
Fig. 6.
SEM photomicrograph of the 100% dolomite
sediment from 21 cm depth in Brejo do Espinho showing
an early diagenetic transition from spheroidal clusters of
nanocrystals to rhombohedra with well-defi ned crystal
faces.
Mechanism of carbonate precipitation
The two heterotrophic microorganisms,
V. maris-
mortui
and
Marinobacter
sp., which were isolated
(a)
(b)
Hydromagnesite
dolomite
100
μ
m
Fig. 7.
Spherical carbonate minerals formed in
pure culture experiments. (a) Large hydromagne-
site spheres (200-300 μm) and smaller dolomite
spherulites (5-25 μm), formed in
Bacillus maris-
mortui
culture. (b) Detailed view of the dolomitic
spherulites on the surface of a hydromagnesite
sphere. (c) Dolomite, with spheroidal morpho-
logy covered by organic fi lm (arrow), formed in
Marinobacter
sp. culture. (d) Small nucleating
dumbbell, with well-defi ned crystallized surface
texture, attached to extracellular organic matter
(arrow), formed in
Marinobacter
sp. culture.
20
μ
m
(c)
(d)
10
m
1
μ
m
μ
