PNNL

Abstract

Selenium is an essential trace element with a narrow range between dietary deficiency and toxicity. The toxicity of Se depends on its bioavailability, which is directly related to its oxidation state, of which four occur in the environment (SeVI, SeIV, Se0 and Se-II). Microbial communities drive the cycling of Se between these oxidation states. In order to investigate the effect of microbial activity on Se cycling in the environment, a field site in County Meath, Ireland, was identified with anomalously high levels of Se as a result of weathering of black shales within the Lucan formation, leading to cases of Se toxicity in farm animals. Soil cores were extracted from the site for Se speciation and microbial community analysis prior to microcosm experiments to assess Se stability and microbial Se transformations. Se was present as a recalcitrant, reduced organic phase that was strongly coordinated with carbon, concordant with suggested hypotheses of Se phytoconcentration within a clay-lined, postglacial marshland. Se was not mobilised in microcosm experiments, and supplementation with SeVI resulted in rapid reduction and removal from solution as Se0. Additional electron donors did not affect Se stability or removal from solution, although nitrate did hinder SeVI reduction. TRFLP analysis indicated a significant shift in microbial community following amendment with SeVI. This work extends the current knowledge of Se cycling in the environment, and provides information on the bioavailability of Se in the soil, which determines Se content of foodstuffs.