Microbial oxidation of organic and elemental selenium to selenite

Selenium (Se) is an essential trace element for life. Se reduction has attracted much attention in the microbial Se cycle, but there is less evidence for Se oxidation. In particular, it is unknown whether microorganisms oxidise organic Se(-II). In this study, four strains of bacteria, namely Dyella spp. LX-1 and LX-66, and Rhodanobacter spp. LX-99 and LX-100, isolated from seleniferous soil, were involved in the oxidation of selenomethionine (SeMet), selenocystine (SeCys2), selenourea and Se(0) to selenite (Se(IV)) in pure cultures. The oxidation rates of organic Se were more rapidly than those of Se(0) in liquid media. Then Se(0) and SeMet were used as examples, microbial oxidation was the predominant process for both additional Se(0) and SeMet in sterilised alkaline or acidic soils. The Se(IV) concentrations were significantly higher at pH 8.56 than at pH 5.25. In addition, water-soluble Se (SOLSe) and exchangeable and carbonate-bound Se (EXC-Se) fractions increased dramatically with these four Se-oxidising bacteria in unsterilised seleniferous soil. To our knowledge, this is the first study to find that various bacteria are involved in the oxidation of organic Se to Se oxyanions, bridging the gap of Se redox in the Se biogeochemical cycle.