Metal-microbe interaction
This mechanism of metal mobilization has been used for many centuries to
leach metals from low-grade ores, and currently supports a lucrative
global market in mineral extraction. It can also be harnessed to remove
metals from contaminated soils and sediments, and can be combined with a
second ex situ step to remove the metals as insoluble sulfides
using sulphate-reducing bacteria, which reverses the metal mobilizing
step (Lloyd, 2002). Microorganisms
are ubiquitous and offer a potentially enormous gene pool to select
specific enzymes that help treat metal contaminations. Indeed,
microorganisms have evolved a wide range of biochemical tricks to
protect themselves from potentially toxic metals and these activities
can be useful for bioremediation applications. Many microbial
detoxification processes involve efflux or exclusion of metal ions from
the cell, which in some cases can result in high local concentrations of
metals at the cell surface where they may react with biogenic ligands
and precipitate (Gadd, 1994). Alternative mechanisms involve redox
transformations, for example the enzyme-catalyzed reduction of the toxic
mercuric ion (Hg2+) to non-toxic elemental mercury
[Hg (0)]. This approach has been used recently to treat chloralkali
wastewaters contaminated with Hg2+ ions
(Deckweret al., 2004).