Selenate reduction by bacteria isolated from aquatic environment free from selenium contamination

Sediment-containing water samples, collected from aquatic environments where no significant selenium pollution was observed, were examined for selenate and selenite reduction potentials. All seven samples tested exhibited selenate and selenite reduction capability to a certain degree under micro-aerobic and anaerobic conditions. There was a general tendency that the selenite reduction proceeded more rapidly and extensively than the selenate reduction. During the selenate reduction by the aquatic samples, no accumulation of selenite was observed, but a red precipitate indicating the presence of elemental selenium was formed. Enrichment of selenate-reducing bacteria was successfully performed only from two samples under a micro-aerobic condition, while the trials from the other samples resulted in failure. Three different selenate-reducing bacteria, strains FR-1, FK-2 and FK-121, were isolated from the enrichment cultures. The former two strains were identified as Pseudomonas stutzeri and the other one as Pseudomonas fluorescens. They could reductively transform up to 5 mM selenate into selenite under anaerobic conditions. During the selenate reduction, selenite was nearly stoichiometrically accumulated, and a small portion was further reduced into insoluble selenium. Presence of air delayed or inhibited their selenate reduction. Their selenite-reducing activities were not efficient, and higher concentration of selenite (>2 mM) completely or mostly inhibited their selenite-reducing activity. The selenate-reducing activities of these strains were much lower than those of previously reported selenate-reducing bacteria which were isolated from Se-polluted or -rich environment.