Lecture series in Ecology and Environmental Research WS 2015/16

Bacterial metabolism of selenium. Survival or profit?

Bacteria are involved in the redox transformations of selenium (Se) contributing to the biogeochemical cycle of this chalcogen element. The metabolism of Se by bacteria follows several basic strategies which include (1) cellular energy production coupled to oxidation/reduction reactions, (2) satisfying a trace element requirement for nutrition, and (3) detoxification processes.
Se has four oxidation states, (+VI), (+IV), (0) and (-II), that are commonly observed in biology. Se oxyanions, selenate (Se[VI], SeO₄^2-) and selenite (Se[IV], SeO₃^2-) are water soluble and toxic but a number of bacteria can reduce them to elemental Se, Se(0), either as a detoxification response or to generate energy by using the oxyanions as final electron acceptors. Se(0) is insoluble and shows a lower toxicological potential, however it exhibits colloidal properties that contribute to its environmental persistence. The microbial-mediated reduction of Se can proceed down to selenides, Se(-II), both for the production of Se-containing molecules (e.g. selenoproteins) in an assimilatory fashion and to detoxify Se in the form of methylated chemical species. Bacterial oxidation of lower valance states of Se has also been documented but the reaction rates are several orders of magnitude lower than the reductive side of the cycle.
To achieve the Se metabolic activities, numerous unique enzymes are employed. While some enzymes from the selenium cycle have been isolated and are markedly specific for Se, many of the Se enzymes remain to be isolated. An understanding of selenium metabolism is complicated by the observations that enzymes from sulfur or nitrogen metabolism may also recognize selenium compounds. From a biotechnological perspective, Se resistant bacteria can be employed in a variety of applications ranging from Se-laden wastewater treatment (i.e. bioremediation) to the production of semiconductor quantum dots.