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morphogenesis and differentiation involve a phase
in which 80 - 90% of the cells undergo lysis
(Dworkin & Kaiser 1993). Lysis contributes to the
production of heterogeneous nuclei for crystalliza-
tion. Since myxobacteria are abundant in organically
enriched soils, the hypothesis that they participate
in struvite precipitation in nature (Ben Omar et al.
1995) would appear to deserve more attention.
Another interesting finding was that M. xanthus
membranes (total membrane fraction: cytoplasmic
and outer membranes) supply heterogeneous
nuclei in the production of struvite (Gonz
´
lez-
Mu˜oz et al. 1996).
While some authors have found that each bac-
terial species displays a very narrow range of stru-
vite crystalline habits (P´rez-Garc´a et al. 1989),
others have found that struvite production by M.
xanthus and M. coralloides evidence a large
number of diverse crystal morphologies (Fig. 2)
(Ben Omar et al. 1996).
Other phosphates produced by myxobac-
teria, newberyite (MgHPO
4
.
3H
2
O) and schertelite
[(NH
4
)
2
MgH
2
(PO
4
)
2
.
4H
2
O], were reported by
Gonz
´
lez-Mu˜oz et al. (1994). These minerals,
which have been considered syngenetic with stru-
vite (Dana 1966), were produced by M. coralloides
D as minor mineral phases when struvite was found
in certain liquid cultures under static conditions
(Gonz
´
lez-Mu˜oz et al. 1994). The production of
struvite syngenetic phosphates by a myxobacterium
is of relevance: before the latter publication it had
not been reported that bacteria could produce such
minerals, and also because newberyite is associated
with struvite in kidney calculi.
Additionally M. xanthus shows a significant
capacity for biosorption of heavy metals and lantha-
nides (Gonz
´
lez-Mu˜oz et al. 1997; Merroun et al.
1998, 2001, 2003). The metals biosorbed frequently
appear as metal-phosphate (Merroun et al. 2003).
These metals are linked mainly to extracellular
polysaccharide and to cell wall (Fig. 3a). However,
in some cases, they appear as intracellular polypho-
sphate granules (Fig. 3b). In this latter case highly
crystalline phases are commonly observed, as
shown by selected area electron diffraction (SAED)
pattern of barium phosphate nodules formed within
M. xanthus cells (inset in Fig. 3b). Extended X-ray
absorption fine structure spectroscopy (EXAFS)
analysis has shown that at pH 4.5, M. xanthus
cells are able to precipitate uranium phosphate, a
mineral phase belonging to the meta-autunite group
(Jroundi et al. 2007). U-phosphate precipitates
were localized mainly within EPS and on the cell
surface, although some poorly-crystalline intra-
cellular precipitates were also observed (Fig. 3c).
The biomineralization of U(VI) may be associated
with the activity of indigenous acidic phosphatase.
The activity of this enzyme has been shown to play
Fig. 2. SEM photomicrographs of struvite crystals
precipitated in the presence of M. xanthus:(a) twined;
and (b) bipyramidal struvite crystals.
of the bacteria is also necessary. The production of
struvite using both living and dead (entire or dis-
rupted) cells, as well as with and without EPS, indi-
cate that M. xanthus cells may act as, or supply,
heterogeneous nuclei for struvite crystallization
when suitable media and bacterium culture ages
are chosen (Ben Omar et al. 1995). Working with
different strains of Pseudomonas and Azotobacter,
Rivadeneyra et al. (1992) also found that, depending
on culture age, heat-killed cells trigger struvite
formation.
Dead cells and cell debris may contribute to the
formation of struvite deposits in nature. The crystal-
lization processes of very different genera of bacteria
previously reported (Myxococcus, Pseudomonas
and Azotobacter) could indicate that this contri-
bution is a widespread phenomenon. In the case of
myxobacteria in general, their social behaviour,
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