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).