Geology Reference
In-Depth Information
metals in solution. Sorption is enhanced as pH incre-
ases and as surface groups deprotonate. Sorption of
large cations such as Ba
2þ
is particularly favoured
by PO
32
ligands and this process enables the for-
mation of large coordination polyhedra (a coordi-
nation of 10 or higher). As the SO
22
content of the
culture medium increases (likely due to degradation
and oxidation of amino acids), the ions are captured
by the Ba ions, thus giving rise to a barite growth
nucleus. Because mineral production only occurred
in the living bacterial colonies, favourable condi-
tions for crystallization relate to bacterium presence
and metabolism. Although these results cannot be
directly extrapolated to natural environments, this
represents a significant step in our understanding of
the biogeochemical cycle of Ba, which has attracted
considerable research due to its close relationship
with marine biological productivity (e.g. Paytan
et al. 1996).
Taylorite, a mineral that frequently appears asso-
ciated with struvite in guano deposits (Dana 1966),
was produced by M. corolloides D (Gonz
´
lez-
Mu˜oz et al. 1994), as in nature, when this bacter-
ium produces struvite in certain conditions. This
work was the first to report bacterial production of
this mineral. Although myxobacteria have not to
date been isolated from guano deposits, they are
commonly found in soils rich in decomposing
organic material, and in fact, dungs of various
animals are some of the preferred isolation sour-
ces (Dworkin & Kaiser 1993). On the other hand,
heterotrophic, ammonifying microorganisms are
present in pigeon dung (Strzelczyk 1981). Taylorite
production by M. corolloides may therefore provide
a basis from which a model for the biogenesis of this
mineral can be developed.
Fig. 8. SEM photomicrographs of weddellite
aggregates: (a) fibrous-radiate crystal aggregate formed
in solid medium inoculated with M. xanthus; and (b)
prismatic-shaped aggregate formed on calcarenite stone
following a M. xanthus conservation treatment.
Other minerals produced by M. xanthus
M. xanthus can produce other types of minerals
depending on the culture medium composition
and/or culture conditions. Among the phosphates:
apatite, hydroxylapatite and natrophosphate
[Na
7
(PO
4
)
2
F
.
19(H
2
O)] have been obtained in
small amounts under different conditions (Ben
Chekroun 2000; Fern´ndez-Luque 2002). Pro-
duction of a chloride, chlorargyrite (AgCl), has
been reported by Merroun et al. (2001) in silver bio-
sorption experiments in which non-proliferative M.
xanthus biomass was used. The silver was located as
electron-dense deposits in the EPS, on the cell wall
and within the cytoplasm. Also, the production of
the oxalate weddellite (CaC
2
O
4
.
2H
2
O) occurs
(Fig. 8a) when culture medium contains calcium-
acetate (Ben Chekroun 2000; Fern´ndez-Luque
2002) or in the heavy metal presence. Small amounts
of weddellite have been observed to develop during
M. xanthus-based
carbonate stone (Fig. 8b). Finally, data exist regard-
ing the production of silica minerals; for instance,
opal CT has been found using a culture medium
with silica (Ben Chekroun 2000).
Concluding remarks
Bacteria are able to precipitate a large number of
minerals, playing a direct and/or an indirect role
in this process. Their role is direct when metabolic
activity leads to supersaturation with respect to a
particular mineral phase. Bacterial dead cells and
their membranes, as well as EPS, play an indirect
role when acting as substrates for heterogeneous
nucleation.
A single bacterial genus, Myxococcus, can pre-
cipitate phosphates, carbonates, sulphates, silicates,
consolidation
of
ornamental
Search WWH ::
Custom Search