References

Microorganisms in Toxic Metal-Polluted Soils 341
presence of reactive anions, e.g. sulphide, carbonate, and phosphate, some
of these arising from microbial activity. However, such biomineralization
can also occur independent of microbial activity and on dead biomass.
Mineral phases may undergo further changes in, e. g. crystallinity, with
time and in relation to physico-chemical characteristics of the environment;
some minerals may incorporate other metals into their structure
(Watson et al. 1995, 2000; Brown et al. 1999; McLean et al. 2002). It should
also be appreciated that microbial modification of their microenvironment
may result in physico-chemical conditions that promote spontaneous metal
precipitation (McLean et al. 2002).
The formation of solid mineral phases by bacterial processes may result
in mineral deposition over geological time scales by diagenesis of sediments
(Beveridge et al. 1983). While much work on microbial biomineralization
has been carried out using bacteria, it should be stressed that all major
microbial groups have roles in metal immobilization and mineral formation,
e.g. cyanobacteria, microalgae, and fungi. While cyanobacterial and
microalgal processes are generally of greater significance in aquatic environments
and in early stages of soil formation, e.g. rock colonization, fungi
also have important roles in the terrestrial environment regarding mineral
dissolution, metal and anion cycling, and also mineral formation.
Calcium oxalate is the most common form of oxalate encountered in the
environment, mostly occurring as the dihydrate (weddellite) or the more
stable monohydrate (whewellite; Gadd 1999). Calcium oxalate crystals are
commonly associated with free-living, pathogenic and plant symbiotic
fungi and are formed by the precipitation of solubilized calcium as the
oxalate (Gharieb et al. 1998; Gadd 1999). This has an important influence
on biogeochemical processes in soils, acting as a reservoir for calcium, and
also influencing phosphate availability. Fungi can also produce other metal
oxalates with a variety of different metals and metal-bearing minerals,
e.g. Cd, Co, Cu, Mn, Sr and Zn (White et al. 1997; Gadd 1999; Sayer et al.
1999).
In many arid and semi-arid regions, calcareous soils and near surface
limestones (calcretes) are often secondarily cemented with calcite (CaCO3).
This phenomenon has been partly attributed to physicochemical processes;
however, the abundance of calcified fungal filaments in weathered profiles
of chalky limestone and Quaternary calcretes indicates fungal activity
(Verrecchia and Dumont 1996; Gadd 1999). Mineralized carbonate precipitates
are also found in association with bacterial biofilms (Glasauer et al.
2003).