Diversity of selenate-reducing bacteria in the sediment of theOlentangy River experimental wetlands: Potential reduction underanaerobic and aerobic conditions

During the past decade selenium has received considerable attention from biologists and toxicologists as a result of its dual role as both an essential trace nutrient at low levels and a toxin at higher concentration (Measures and Burton, 1980). Oxidized forms are readily available for uptake by a range of marine organisms and can accumulate in the food web and generate toxicosis (Foda et al., 1983). Rivers and estuaries are susceptible to selenium contamination resulting from anthropogenic activities. These activities can cause abnormally high concentrations of trace elements to be deposited in estuarine systems and wetlands (Velinsky, 1987). Among the various sources, the effluents from agricultural drainage waters have been in focus in the mid-1980s in the western USA. Naturally high concentrations of selenium are associated with soils of marine sedimentary parent material (Losi and Frankenberger, 1997a). Agricultural irrigation of these kinds of soils has resulted in the death and deformities of waterfowl in evaporation ponds especially in San Joaquin Valley, CA (Oremland et al., 1989). A number of other anthropogenic activities, such as petroleum refining, mining and fossil fuel combustion generate Se-laden wastes (Losi and Frankenberger, 1997a). For that reason, the development of efficient, costeffective remediation technology for removing or immobilizing selenium from contaminated waters and sediments is of great interest. In San Francisco Bay, manmade marshes have been used for removal of selenium. Sediments can act as a sink for selenium via dissimilatory reduction in anoxic conditions. Selenate and selenite have been substantially removed from surface and interstitial waters by precipitation as elemental selenium (Oremland et al., 1989). The microbial transformation of selenium oxyanions could be a strategy for bioremediation (White et al., 1995). These biological processes include anaerobic dissimilatory reduction of selenate (SeO 4 ) and selenite (SeO 3 ) into insoluble elemental selenium (SeO), which is nontoxic. Reduction and methylation of oxyanions under aerobic conditions, which result in production of volatile organoselenium forms, can also achieve removal (Losi and Frankenberger, 1997). A wide variety of microorganisms (pure and mixed cultures) have been studied that have the ability to reduce Se oxyanions, primarily to elemental form but also to volatile methylated forms. Until now only few bacterial strains able to grow using selenate as the terminal acceptor of electrons have been isolated and studied (White et al., 1995). These organisms, which require anaerobic conditions for growth on selenate, are difficult to isolate and culture. Moreover, the ecological relevance and representativity of these strains are unknown. The aim of this study was to compare the activity and the community structure of natural bacteria performing selenateremoval under anaerobic or aerobic conditions in wetland sediments. Sediment slurries were amended with selenate and the removal of selenate was followed by measuring total selenium in soluble phase. Community structure of selenate-removing populations was assessed using DGGE profiles of 16S RDNA.