SELENITE REDUCING BACTERIA OF PANOCHE ALGAL BACTERIAL SELENIUM REMOVAL (ABSR) FACILITY, CA

摘要

The agricultural drainage water in the Panoche Water District (western San Joaquin Valley, central California) is polluted with selenium in the form of selenate and selenite (50 - 1200 μg/ L), and nitrate (No{sub}3{sup}-)(20 - 120 mg/L as N). Selenate and selenite are easily bio-absorbed and tend to bio-accumulate. Nitrate promotes excessive photosynthetic growth. The ABSR (algal bacterial selenium removal) plant is a demonstration facility designed, constructed, and operated by Applied Algae Research Group under the supervision of Dr. W. J. Oswald of the University of California, Berkeley. It consists of a series of anaerobic and aerobic ponds designed to grow indigenous bacteria and algae that remove selenium and nitrate. Routine chemical analysis of ABSR samples has shown that seasonal variations play an important role in the efficiency of the 'selenium detoxification-process'. Since 1995, Leighton laboratory researchers, in collaboration with Oswald's group, have been identifying bacterial members of the microbial communities of the ABSR treatment plant. The objective is to study their seasonal variation and selenium reduction potential. Common species that have been found are Acinetobactor calcoaceticus, Aeromonas sp., Bacillus megaterium, Bacillus subtilis, Pseudomonas stutzeri, Shewanella putrefaciens, Thauera merchernichii and Vibrio cholerae. Amongst these, Vibrio have been isolated only during peak summer seasons, whereas Bacillus species are present year-round. Almost 90% of the isolates are able to reduce selenite to selenium metal on enriched agar media. A large number of the Panoche bacteria, including Aeromonas sp, B. subtilis, P. stutzeri, S. putrefaciens and V. cholerae partially or completely denitrify and reduce selenite. Moreover, in some species such as S. putrefaciens, denitrification precedes selenite reduction. Transmission and scanning electron microscopy on selected species has been conducted in order to understand whether the fate of reduced selenium is intracellular or extracellular. It is hoped that this will shed light on the selenite reduction mechanism.