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of morphologies (Fig. 6a). All are Mg-calcite of
almost similar low Mg content. Morphologies
similar to those produced by M. xanthus have been
described by various authors studying biotic and
abiotic, laboratory as well as natural systems (e.g.
Krumbein
observed barite precipitation in laboratory exper-
iments using M. xanthus (Fig. 7).
Barite production started with a phase dominated
by P and Ba that evolved to well-crystallized barite
crystals (Fig. 7a). The initial poorly crystalline
P-rich precursor phase (see the diffuse rings in
the SAED pattern in inset of Fig. 7b) suggested
that phosphoryl and carboxyl groups in the structural
polymers of the cell wall outer membrane may be
sorbent constituents which play an important role
in the precipitation process. Deprotonation of these
groups provided discrete complexation sites for
1979;
Buczynski
&
Chafetz
1991;
Fern
´
ndez-Diaz et al. 1996).
Precipitation of monohydrocalcite in the pres-
ence of M. xanthus has been reported too (Ben
Chekroun 2000). Euhedral hydrocalcite crystals
(Fig. 6b) formed, most probably at a high supersa-
turation,
as
precursors
of
more
stable
calcite
(Jimenez-Lopez et al. 2001).
When appropriate supersaturated solutions are
employed, calcium carbonate has also been found
to precipitate on M. xanthus cellular membranes
under abiotic conditions, a precipitation which can
be considered passive or indirect (Gonz´lez-Mu˜oz
et al. 1996). The precipitation of these calcite crys-
tals probably takes place on the negatively charged
points of the external side of the cellular struct-
ures. D
´
farge et al. (1996) found that microscopic
three-dimensional organic networks inherited from
sheaths of dead cyanobacteria acted as a matrix
for calcification. They reported that crystal nuclea-
tion began at acidic sites which are capable of
binding a wide range of cations.
It has been suggested that specific attributes of
certain bacteria induce and affect calcium carbonate
formation (Hammes et al. 2003). Precipitation
occurs preferentially on macromulecules such as
lipid bilayers of vesicles and glycoproteins and pro-
teoglycans that are constituents of bacterial cell
membranes. Such organics act as a nucleation tem-
plate for calcium carbonate. The nature of such an
organic matrix may determine which ion is preferen-
tially adsorbed and, consequently, which mineral
phase is formed. Thus, biomineralization could be
considered strain-specific. For instance, bacteria
that preferentially adsorb Mg
2þ
on their membranes
induce dolomite formation, whereas calcite preci-
pitation is induced by preferential adsorption of
Ca
2þ
(Van Lith et al. 2003). However, M. xanthus
appears to challenge the aforementioned hypothesis
of strain-specific biomineralization since it is able to
induce precipitation of carbonates with contrasting
structure and composition.
Production of sulphates
Myxococcus also mediates the precipitation of
barite (Gonz´lez-Mu ˜oz et al. 2003) and taylorite
[(K,NH
4
)
2
SO
4
] (Gonz´lez-Mu ˜oz et al. 1994).
Barite dissolution by sulphate reducing bacteria
has been proposed by several authors (e.g. Phillips
et al. 2001), but no bacterial contribution to barite
precipitation was considered until the finding of
Gonz´lez-Mu ˜oz et al.
Fig. 7. M. xanthus induced barite precipitation: (a) SEM
photomicrographs of barite aggregates; and (b) TEM
image on a barite precursor aggregate showing diffuse
rings in the SAED pattern (inset) thus confirming its
poorly-crystalline nature (reprinted from Gonz´lez-
Mu˜oz et al. (2003), with permission from the American
Society for Microbiology).
(2003).
These
authors
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