References

326 G.M. Gadd
cases, microbial activities can result in remobilization of metals from other
wastes and transfer into aquatic systems. It is commonly accepted that toxic
metals, and their chemical derivatives, metalloids, and organometals can
have significant effects on microbial populations and almost every index
of microbial activity can be affected, depending on the particular situation.
However, metal toxicity is greatly affected by the physico-chemical
nature of the environment and the chemical behaviour of the particular
metal species in question (Gadd and Griffiths 1978). The degree of metal
toxicity reduction can be quantitatively related to adsorption capacity and
selectivity of adsorbents for example (Malakul et al. 1998). Further, unlike
certain xenobiotics perhaps, not always is there a link between bioavailability
and toxicity, a good example being the frequent amelioration of
metal fungitoxicity by acidic pH (Gadd 1993a). Despite apparent toxicity,
many microorganisms survive, grow and even flourish in apparently
metal-polluted locations and a variety of mechanisms, both active and incidental,
contribute to tolerance. All mechanisms depend on some change
in metal speciation leading to decreased or increased mobility. Such metal
transformations between soluble and insoluble phases are also at the heart
of metal biogeochemistry, thus providing a direct link between microbial
responses and element cycles. Thus, interactions of microorganisms with
metals are extremely important and underpin many aspects of soil biology.
The objective of this chapter is to detail major interactions of microorganisms
with metals with particular reference to effects on speciation,
toxicity, and biogeochemical cycling. In addition, attention is drawn to
their significance in plant nutrition and in environmental biotechnology
where certain microbial processes have application in metal recovery or
detoxification.
2
Metals in Soils
All soils contain metals, the concentrations in uncontaminated soil being related
to the geology of the parental material. ‘Normal’ total concentrations
may be high with speciation and bioavailability determining any effects
on microbial populations. Metal release by environmental disturbance,
weathering processes and biological activities may result in toxic effects
on indigenous microbial populations, as well as metal transfer to aquatic
habitats. However, effects on microbial populations are more commonly
associated with pollution events, where concentrations of introduced toxic
metals may greatly exceed indigenous levels. To appreciate and understand
possible effects on microbial populations, it is necessary to consider some
aspects of metal chemistry in soil since this determines speciation, bioavail-