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Sphaerotilus, Crenothrix and Clonothrix (Little
et al. 1997). Environmental scanning electron
microscopy (ESEM) has enabled the study of iron
oxides and hydroxides production by Gallionella,
in situ, at high magnification (Hallberg & Ferris
2004). This work demonstrates the enormous poten-
tial of such a tool for studying bacterial biomin-
eralization processes. It has recently been found
that Pyrolobus fumarii, an Fe(III)-reducing bacter-
ium which is responsible for magnetite precipitation
in hydrothermal vents, is able to survive at tempera-
tures of around 120 8C (Kashefi & Lovley 2003).
This study extends the upper temperature limit for
life. Note that mineralization induced by extremo-
philes, that is, microbes that live in extreme environ-
ments (e.g. with very high or very low pH, high
pressure, high temperature, high radiation, high
salinity), may help us to understand how and
where life appeared on Earth, or if life is possible
on other planets (Rothschild & Mancinelli 2001).
Iron sulphides such as pyrrhotite (Fe
12x
S),
greigite (Fe
3
S
4
) and mackinawite (Fe
9
S
8
) also form
in magnetosomes of magnetotactic bacteria (see
review by Bazylinski & Moskowitz 1997, and refer-
ences therein). Extracellular precipitation of iron
and nickel sulphides has also been reported (Ferris
et al. 1987). A striking case of bacterial sulphide
mineralization is the occurrence of framboidal pyrite
(FeS
2
) in antique topics (Garc
´
a-Guinea et al. 1997).
It has been observed that sulphate-reducing bacteria
can cause sphalerite (ZnS) deposition in nature
(Labrenz et al. 2000). Sillitoe et al. (1996) reported
chalcocite (Cu
2
S) enrichment during weathering.
This study showed that chalcocite precipitated
following Cu-binding by bacteria, thus resulting
in a sevenfold enrichment of copper content in
sulphide deposits. This work is a clear example of
the important economic implications of bacterial
mineralization. Microbially induced precipitation
of sulphides around oceanic hydrothermal vents
has recently drawn much attention, since this
process could be evidence of the most primitive
life on Earth (Banfield et al. 1998). These environ-
ments could have provided compounds such as
(Ni,Fe)S which might be crucial for the origins of
life (Huber & W¨chtersh¨user 1998). Examples of
sulphate-reducing bacteria responsible for sulphide
mineralization are Desulfovibrio and Desulfotoma-
culum (Gould et al. 1997). For additional infor-
mation, see reviews by Gould et al. (1997) and
Nordstrom & Southam (1997) on microbiological
formation of sulphides.
Bacterial activity may result in the formation
of silicate phases (Ferris et al. 1986; Urrutia &
Beveridge 1994). Fe - Al silicate, assumed to be
the clay mineral chamosite [(Fe,Mg)
5
Al(Si
3
Al)
O
10
(OH,H)
8
], was found to precipitate in the pres-
ence of bacteria (Ferris et al. 1987). Phoenix et al.
(2001) have described a remarkable case of silica
precipitation on bacterial cells. The authors obse-
rved that an Fe-rich silica crust formed around cya-
nobacteria. They suggested that this mineral shell
could act as a shield against UV radiation. However,
these bacteria face a dilemma, namely, how to
escape from the silica coating when it surrounds
the entire microorganism. Cyanobacteria have sol-
ved this problem by restricting silica deposition on
both ends, thus allowing the cell to exit the shell
following cellular division. This strategy could
have allowed these bacteria to endure the high
UV irradiation conditions of early Earth.
Sulphates were demonstrated to precipitate
on bacterial cells, both in nature and in the labo-
ratory (Thompson & Ferris 1990). Precipitation of
common sulphates such as gypsum (CaSO
4
.
H
2
O)
(Thompson & Ferris 1990) and barite (BaSO
4
)
(Gonz
´
lez-Mu˜oz et al. 2003) has been reported.
Uncommon sulphates such as jarosite [KFe
3
(SO
4
)
2
(OH)
6
] and schwertmannite [Fe
8
O
8
(OH)
6
SO
4
] pre-
cipitated in spring waters in the presence of bacteria
(Kawano & Tomita 2001). Jarosite presence in
Martian Gusev Crater soils has been reported
(Morris et al. 2004), which raises the question:
Did bacteria play a role in its formation? Bacterial
oxidation of sulphides results in the precipitation
of sulphates, which causes soil and shale heave
and significant geotechnical problems (Gillot et al.
1974). Acidothiobacillus ferrooxidans transform
pyrite into sulphates. Following pyrite oxidation,
jarosite and gypsum precipitate within shales when
potassium and calcium ions are available (Quigley
et al. 1973). Acidic mine drainage has been also
directly connected with iron sulphide oxidation by
microorganisms (Davis 1997).
Nitrates such as nitrocalcite [Ca(NO
3
)
2
.
4(H
2
O)],
also known as 'saltpetre', are commonly found as in
cave deposits and in soils (Northup & Lavoie 2001).
The formation of calcium nitrate has been associ-
ated to the activity of nitrifying bacteria such as
Nitrosomonas and Nitrobacter. They convert
ammonium ions into nitrite and then nitrate (Focht
& Verstraete 1977), which in the presence of Ca
ions leads to the precipitation of nitrocalcite.
Phosphates, particularly those including Ca and
Mg, such as apatite [Ca
5
(PO
4
)
3
(F,Cl,OH)] and stru-
vite (NH
4
MgPO
4
.
6H
2
O) have been shown to grow
on the outer membrane of Gram-negative bacteria.
Phosphate precipitation induced by bacteria is par-
ticularly relevant because of its health-related impli-
cations. For instance, formation of human kidney
stones has been related to bacterial infection. The
calcium phosphates (apatite and hydroxylapatite)
which form the calculi systematically enclose
bacterial bodies. This observation suggests that bac-
terial infection is directly related to the formation of
these pathological concretions (Hess et al. 1994;
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