References

Microorganisms in Toxic Metal-Polluted Soils 339
extremely reducing conditions which can chemically reduce species such
as U(VI) (White and Gadd 1998b).
Bacterial Fe oxidation is ubiquitous in environments with sufficient Fe 2+
and conditions to support bacterial growth such as drainage waters and
tailings piles in mined areas, pyritic and hydric soils (bogs and sediments),
drain pipes and irrigation ditches, and plant rhizospheres. Iron-oxidizers
foundinacidicsoilenvironmentsareacidophilicchemolithotrophs,such
as Thiobacillus ferrooxidans, significant for its role in generating acid mine
drainage (Ewart and Hughes 1991). Fungi also oxidize metals in their environment.
Desert varnish is an oxidized metal layer (patina) a few millimetres
thick found on rocks and in soils of arid and semi-arid regions, and is
believed to be of fungal and bacterial origin.
6
Metalloid Transformations
Main microbial transformations carried out in the soil are reduction and
methylation which can lead to alterations in bioavailability and toxicity.
For selenium, some bacteria can use SeO 2−
4 as a terminal e− acceptor in
dissimilatory reduction and also reduce and incorporate Se into organic
components,e.g.selenoproteins(assimilatoryreduction).Methylationand
subsequent volatilization of methylated selenium derivatives is also a widely
found property of soil bacteria and fungi and may be an important pro-
cess in Se transport from terrestrial to aquatic environments (Dungan and
Frankenberger 1999). Selenate (SeO2− 4 ) and selenite (SeO2− 3 ) can be reduced
to Seo ,withSeO2− 3 reduction appearing more ubiquitous than SeO2− 4 reduction.
However, only SeO2− 4 can support bacterial growth under anaerobic
conditions: SeO2− 4 reduction to Seo is a major sink for Se oxyanions in anoxic
sediments (Oremland et al. 1989; Stolz and Oremland 1999; Oremland and
Stolz 2000). Anaerobic sulphate-reducing bacteria like Desulfovibrio desulfuricans
can reduce selenate/selenite to Seo , but neither oxyanion could be
used for respiratory growth (Tomei et al. 1995). Reduction to Seo can be
considered a detoxification mechanism (Dungan and Frankenberger 1999).
The opposite process of Seo oxidation can occur in soils and sediments
(Dowdle and Oremland 1998; Losi and Frankenberger 1998). It is possible
that Seo oxidation is a similar process to S oxidation, and may be mediated
by heterotrophs and autotrophs (Losi and Frankenberger 1998). In aerobic
soil slurries, Se4+ was the main product with lower amounts of Se6+ being
produced: heterotrophic and autotrophic thiobacilli were believed to be the
active organisms (Dowdle and Oremland 1998).
Methylation of Se is a ubiquitous property of microorganisms and can
occur in soils, sediments and water (Gadd 1993b). Bacteria and fungi