S175: Microbial Respiration of Selenium Oxyanions

Selenium is an intriguing element in that it is an essential micronutrient, but is highly toxic in large amounts. Microbes are central to the biogeochemical cycling of selenium, for example, Se oxyanions can serve as terminal electron acceptors for microbial respiration in the process of dissimilatory Se reduction in which selenate is sequentially reduced to selenite and further to insoluble elemental selenium. This process can immobilize already low levels selenium in deficient soils, or alternatively reduce the toxicity when present at high levels. The capability for dissimilatory selenate reduction occurs widely amongst diverse bacteria. Interestingly, selenate-respiring bacteria can be isolated from environments in which they have not been exposed to selenium (other than at trace concentrations). We have isolated several novel selenite-respiring bacteria, among them Desulfurispirillum indicum, an anaerobic bacterium in the deeply branching phylum of Chrysiogenetes. D. indicum is able to respire a number of different electron acceptors, including selenate, selenite, arsenate, nitrate and nitrite. Genome analysis uncovered the components of the electron transport chain, providing important insights into the ability of D. indicum to adapt to different conditions, by coupling the oxidation of various electron donors to the reduction of a wide range of electron acceptors. Our studies provide the basis for the functional analysis of metalloid oxyanions respiration and are yielding insights into how the biogeochemical cycling of selenium is affected by alternate electron acceptors. Knowledge of the microbiological and chemical interrelationships is important for environmental quality in both selenium-deficient as well as selenium-contaminated systems.