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) oxyanions selenate (SeO42-) and selenite (SeO32-) from solution culture in flasks open to the atmosphere was isolated and studied with the goal of assessing its potential for use in bioremediation of seleniferous agricultural 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% of added SeO42- (the primary species present in agricultural drainage water) at concentrations from 13 to 1,266 mu M. Equimolar amounts of nitrate (NO3-), which interferes with SeO42- reduction in some organisms, did not influence the reaction in growth experiments but had a slight inhibitory effect in a washed-cell suspension. Washed-cell suspension experiments also showed that (i) SeO32- is a transitory intermediate in reduction of SeO42-, being produced and rapidly reduced concomitantly; (ii) NO3- is also reduced concomitantly and at a much higher rate than SeO42-; and (iii) although enzymatic, reduction of either oxyanion does not appear to be an inducible process. Transmission electron microscopy revealed that precipitate particles are <0.1 mu m in diameter, and these particles were observed free in the medium. Evidence indicates that SLD1a-1 uses SeO42- as an alternate electron acceptor and that the reaction occurs via a membrane-associated reductase(s) followed by rapid expulsion of the Se particles.