References

344 G.M. Gadd
tant removal from soil into shoots and leaves), phytodegradation (pollutant
degradation by plant–microbe systems), rhizofiltration (absorption of pollutants
by plant roots), phytostabilization (plant-mediated reduction of
pollutant bioavailability), phytovolatilization (plant-mediated volatilization
of pollutants) and phytoscrubbing (plant removal of atmospheric pollutants).
Most attention has focused on metals to date. Two basic strategies
are chelate-assisted and continuous phytoextraction. Chelate-assisted
phytoextraction has been invoked since plants do not naturally accumulate
important toxic elements, e.g. Pb, Cd, As, and many radionuclides, to levels
that would be significant in a remediative context. Application of various
synthetic chelates can enhance plant metal accumulation (Huang et al. 1997;
Salt et al. 1998). Continuous phytoextraction of metals relies on intrinsic
properties of plants that lead to accumulation in aerial plant tissues. However,
many natural ‘hyperaccumulators’ often exhibit low biomass, slow
growth rates and none are known for important elements like Pb, Cd, As
and U (Salt et al. 1998). Ni, Zn and Se appear to be the elements accumulated
to the highest levels (Salt et al. 1998).
Plants possess analogous metal resistance mechanisms as microorganisms,
i.e. chelation, intracellular compartmentation, transformations etc.,
although plants may be relatively metal-sensitive compared to microorganisms.
Manipulation of metal tolerance may provide a means for phytoremediation:
bacterial Hg 2+ -reductase has been expressed in Arabidopsis
thaliana (Rugh et al. 1996).
Biodegradation of organic pollutants is generally accelerated in vegetated
soils confirming plant roles and that of rhizosphere-inhabiting microorganisms
(Schnoor et al. 1995; Cunningham and Ow 1996). There is often
uncertainty in the relative roles of plants and microorganisms in the overall
process. It seems clear that microorganisms will often be most important
in this context, their soil populations being markedly stimulated by the rhizosphere
effect (Salt et al. 1998). Organic compounds in root exudates can
also serve as co-metabolites for bacterial xenobiotic degradation (Donnelly
et al. 1994).
11
Conclusions
This chapter has outlined the importance of metal–microbe interactions in
several soil contexts, not least the fundamental position in the biogeochemical
cycling of metals and associated elements and nutrients, and in plant
productivity. The real application and unrealised potential of many natural
microbial and microbe/plant processes are also growing topics in the area of
bioremediation. However, analysis and understanding of the effects of toxic