| Selenium contamination of natural aquatic habitats, due to agricultural run-off from seleniferous soils, has become a matter of great public concern since the early 1980s poisoning of wild birds at the Kesterson Reservoir and Wildlife Refuge in the San Joaquin Valley of Central California. Selenate (SeO42−) and selenite (SeO3 2−) are two of the most bioavailable and mobile forms of selenium, owing to their water solubility over a wide range of pH.; The Algal-Bacterial Selenium Removal (ABSR) system is a demonstration scale facility in the Panoche District of the San Joaquin Valley that provides cost effective treatment of agricultural wastewaters containing these two ions as well as nitrate. Indigenous bacteria use algae and/or molasses as electron donors to reduce nitrates to nitrogen gas and selenate or selenite to elemental selenium, which is environmentally stable and less toxic.; Microbial analysis of ABSR treatment systems demonstrated temporal variations in the bacterial populations. Isolates were able to reduce selenite under aerobic as well as anoxic conditions (in the presence and absence of nitrate) to varying degrees. Sulfate affected the rate of selenite reduction in several cases, suggesting a mechanism similar to assimilatory sulfate reduction. In comparing the selenate and selenite reducing abilities of various bacteria, selenite was more bioactive in aerobic and facultative bacteria. Higher concentrations of selenite (>2 mg/L) resulted in morphological abnormalities, especially in Vibrio cholerae and Aeromonas trota . In Bacillus megaterium and Shewanella putrefaciens intracellular inclusions of elemental selenium were observed.; Reduction of selenium oxides has often been associated with denitrification. In comparing the selenite reducing ability of metabolically distinct Se-acclimated and naïve wastewater treatment communities (denitrifying, nitrifying and neither), it was realized that the aerobic selenite reduction mechanism was not unique to Se-acclimated communities; instead it was a function of the addition of appropriate C-substrates, such as glucose or glycerol. |
