Fungal and Abiotic Controls on Selenium Redox and Implications for Remediation of Contaminated Soils.

Worldwide, selenium (Se) is proving to be a significant environmental concern, with many anthropogenic activities (e.g., coal mining and combustion, phosphate mining and agricultural irrigation) releasing potentially hazardous concentrations into soil and natural water ecosystems. Various abiotic and biological processes coupled to Mn(III,IV)(hydr)oxides and microorganisms mediate Se oxidation/reduction (redox) transformations in soils, thus influencing its solubility and bioavailability. Several common soil fungi aerobically reduce soluble Se(IV,VI) anions to Se(0). In this research, we assess the relative contribution of biotic and abiotic pathways for these aerobic Se transformations.

To study biotic and abiotic contributions to Se transformations, soluble Se(IV) or Se(VI) was combined with Mn(III,IV)(hydr)oxides, a Se-transforming fungus (Alternaria alternata), and/or oxalic acid to mimic Se biogeochemistry at plant-soil-water interfaces. Despite slightly increasing the redox potential of the solution, the presence of Mn (III,IV) (hydr)oxides did not inhibit aerobic fungal-mediated Se(IV) reduction to red precipitates of biogenic Se(0), although no biogenic Se(0) production was observed in any Se(VI) treatments. Without fungi, Se(IV) loss from solution was negligible over the course of the experiment, while the presence of fungi resulted in removal of 60-80% of Se(IV) from solution after 28 days. The presence of Mn (III,IV) (hydr)oxides did reduce the overall fungal-mediated Se(IV) removal from solution, with 10-20% more Se(IV) removed from treatments without Mn (III,IV) (hydr)oxides. Conversely, removal of Se(VI) was more limited, with little difference between any treatments. Bioreduction of Se(IV) to Se(0) did occur in all A. alternata treatments, though it is unknown whether alternative processes such as volatilization or incorporation into fungal biomass also play a role in removing Se(IV) from solution.

Results from this study are key for understanding Se-transforming reactions in soils, and the interaction between abiotic and biotic soil components play in influencing Se bioavailability and mobility in contaminated ecosystems.