REDUCTION OF SELENIUM OXYANIONS BY ENTEROBACTER-CLOACAE SLD1A-1 - ISOLATION AND GROWTH OF THE BACTERIUM AND ITS EXPULSION OF SELENIUM PARTICLES

A facultative bacterium capable of removing the selenium (Se) oxyanion s selenate (SeO42-) and selenite (SeO32-) from solution culture in fla sks open to the atmosphere was isolated and studied with the goal of a ssessing its potential for use in bioremediation of seleniferous agric ultural drainage water. Elemental Se (Se-0) was confirmed as a product of the reaction. The organism, identified as Enterobacter cloacae and designated strain SLD1a-1 (ATCC 700258), removed from 61.5 to 94.5% o f added SeO42- (the primary species present in agricultural drainage w ater) at concentrations from 13 to 1,266 mu M. Equimolar amounts of ni trate (NO3-), which interferes with SeO42- reduction in some organisms , did not influence the reaction in growth experiments but had a sligh t inhibitory effect in a washed-cell suspension. Washed-cell suspensio n experiments also showed that (i) SeO32- is a transitory intermediate in reduction of SeO42-, being produced and rapidly reduced concomitan tly; (ii) NO3- is also reduced concomitantly and at a much higher rate than SeO42-; and (iii) although enzymatic, reduction of either oxyani on does not appear to be an inducible process. Transmission electron m icroscopy revealed that precipitate particles are <0.1 mu m in diamete r, and these particles were observed free in the medium. Evidence indi cates that SLD1a-1 uses SeO42- as an alternate electron acceptor and t hat the reaction occurs via a membrane-associated reductase(s) followe d by rapid expulsion of the Se particles.