Chemistry Reference
In-Depth Information
Proteobacteria (Emerson 2000). Phylotypes of these organisms have been identified from
the Loihi Seamount near Hawaii (Moyer et al. 1995) where there is extensive, low-
temperature, hydrothermal, venting and very large mats of hydrous Fe(III) oxides. These
organisms appear to be very abundant at the site and a pure culture of a related organism
has been obtained (Emerson and Moyer 2002). These organisms have also been
associated with, and isolated from, the rhizosphere and Fe(III) hydroxide plaques on the
roots of wetland plants (Emerson et al. 1999; Neubauer et al. 2002).
The anaerobic Fe(II) oxidizers that grow at or around neutral pH include several
strains of phototrophic bacteria and some nitrate-respiring bacteria. Several freshwater
strains of phototrophic bacteria are known to oxidize Fe(II) in iron sulfides, or in a
mixture of ferrous carbonate and ferrous phosphate, to insoluble, rust-colored Fe(III)
oxyhydroxides whose precise composition was not determined (Ehrenreich and Widdel
1994). These strains belong to the
α
- and
γ
-subgroups of the Proteobacteria. Two marine
phototrophic strains of
Rhodovulum
(
-subgroup of Proteobacteria) growing on the same
substrates produced the iron oxides ferrihydrite (~98%) and magnetite (trace amounts)
(Straub et al. 1999). Both the freshwater and marine strains grow photoautotrophically
and photoheterotrophically. The discovery of this novel type of microbial metabolism
received much attention because it provided an alternative explanation for the
development of the massive banded iron formations which formed in the absence of free
dioxygen (Widdel et al. 1993; Ehrenreich and Widdel 1994).
An anaerobic group of Fe(II) oxidizers that uses nitrate as a terminal electron
acceptor (Straub et al. 1996) includes a number of mesophilic strains belonging to the
α
β
-
and
-subgroups of the Proteobacteria (Buchholz-Cleven et al. 1997). All form rust-
colored ferric oxyhydroxides from Fe(II) which probably contain considerable carbonate
(Straub et al. 1996). A study using 16S rRNA-targeted probes designed from several
strains showed that these organisms are quite widespread in diverse European sediments
(Straub and Buchholz-Cleven 1998). This finding together with the fact that other known
nitrate-reducing bacteria, including
Thiobacillus denitrificans
and
Pseudomonas stutzeri,
are also capable of Fe(II) oxidation suggests that this form of metabolism is widespread
in anoxic habitats containing sufficient Fe(II) and nitrate (Emerson 2000). Chaudhuri et
al. (2001) reported the isolation of
Dechlorosoma suillum
strain PS, a bacterium that is
capable of oxidizing Fe(II) anaerobically with nitrate as the terminal electron acceptor.
After the initiation of Fe(III) formation, the Fe(III), unreacted Fe(II), and carbonate in the
medium were found to combine to form green rust which transformed into magnetite
after prolonged incubation.
A hyperthermophilic member of the Archaea,
Ferroglobus placidus
, isolated from a
shallow marine hydrothermal vent in Italy, is known to grow lithotrophically with Fe(II)
as ferrous carbonate (Hafenbrandle et al. 1996). The optimum growth temperature of this
organism is 85°C although the products of Fe(II) were not discussed. This organism can
also reduce thiosulfate using hydrogen as the electron donor, and in the presence of
Fe(II), produces iron sulfide minerals.
Some chemoheterotrophic bacteria also oxidize Fe(II). Two of the most well-
described are the filamentous, sheathed bacteria
Sphaerotilus
and
Leptothrix
. Proteins in
their sheaths catalyze the oxidation of Fe(II) and Mn(II) and nucleate the precipitation of
Fe and Mn oxides, with which they are often encrusted. Members of the family
Siderocapsaceae, which contains the genera
Siderocapsa
,
Naumanniella
,
Siderococcus
,
and
Ochrobium
, seem to oxidize Fe(II) but the evidence is circumstantial in that most of
the information about them is derived from environmental studies and enrichment
cultures rather than studies with pure cultures (Hanert 2000b). In fact, it is seems
questionable whether true strains of these genera actually exist (Emerson 2000).
γ
