Geology Reference
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vaterite crystals, as well as the presence of vaterite
spherical aggregates (Fig. 5), is in agreement with
their formation under high supersaturated conditi-
ons, while large, rhombohedral calcite crystals are
formed at relatively low supersaturation. Mullin
(1992) previously described the details of supersa-
turation influence on nucleation density, crystal
size and crystal morphology. Rodriguez-Navarro
et al. (2007) found precipitation of vaterite spheru-
lites in the microenvironment around M. xanthus
cells, and directly onto the surface of bacterial cells.
In the latter case, fossilization of bacteria occurred.
Vaterite crystals formed by aggregation of oriented
nanocrystals with c-axes normal to the bacterial cell-
wall, or to the core of the spherulite when bacteria
were not encapsulated. The preferred orientation of
vaterite c-axes appeared to be determined by electro-
static affinity (ionotropic effect) between vaterite
crystal (0001) planes and the negatively-charged
functional groups of organic molecules on the bac-
terium cell-wall or on EPS. According to analyses
of the changes in the culture medium chemistry as
well as high resolution transmission electron micro-
scopy (HRTEM) observations, polymorph selection
was associated to both physicochemical (kinetic)
factors (high supersaturation) and to stabilization
by organics, both connected with bacterial activity.
Precipitation of Mg-calcite induced by M.
xanthus was reported by Gonz´lez-Mu ˜oz et al.
(2000). This work describes the results of exper-
iments designed to study the crystallization of
CaCO
3
by a biogenic process in the presence of
Mg
2þ
. For this purpose, a culture of M. xanthus
was developed in solidified agar - agar nutritive
solution. This work makes some interesting obser-
vations, regarding both the location of crystals in
M. xanthus colonies and the diversity of the crystal
morphologies. As a consequence of M. xanthus
metabolic activity, the various metabolites produced
(CO
2
and NH
3
) have to diffuse from the zones of
higher density of microbial growth to the rest of the
culture medium. Ammonia production also led to
rises in pH. The consumption of nutrients from the
culture media brought about the diffusion of different
ions in the opposite direction. Counter-diffusion
of microbial metabolites and culture ions produces
spatio-temporal concentration gradients which lead
to supersaturation conditions. Such conditions
permit the onset of precipitation at certain moments
and at certain points. As a consequence, the crystal
precipitation zone is a band located on the periphery
of the M. xanthus colonies with a width of c. 1 mm.
Appropriate supersaturation and pH conditions
occur in this zone. In addition, the pattern in
which counterdiffusion evolves influences the
development of different crystal morphologies
produced by Mg-calcite (Fig. 6a). On the other
hand, a striking relationship was found between
Fig. 6. SEM photomicrographs of: (a) dumbbells and
spheres of magnesium calcite crystal aggregates; and
(b) monohydrocalcite dipyramidal crystal formed in the
presence of M. xanthus.
viscosity of the medium and the type of precipitate.
The precipitation of the Mg-calcite invariably took
place in gelled media. The viscosity of the medium,
and hence the rate of ion diffusion and precipitation,
seem to be the overriding control. These results are
consistent with the observations of Buczynski &
Chafetz (1991) showing bacterially induced precipi-
tation of calcium carbonate in the gel-like slime
(biofilm) present in stromatolites. On the other hand,
Mg probably plays a key role in the development of
the morphologies of the precipitates since these
morphologies had never been observed with
M. xanthus in the absence of Mg. In general, when
a particular form of a crystal or a crystal bundle is
found in a culture, most of the morphologies are
similar (Buczynski & Chafetz 1991). In contrast, it
is noteworthy that M. xanthus gives rise, both simul-
taneously and in the same proportion, to a diversity
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