Selenate and Nitrate Bioreductions Using Methane as the Electron Donor in a Membrane Biofilm Reactor

Selenate (SeO42-) bioreduction is possible with oxidation of a range of organic or inorganic electron donors, but it never has been reported with methane gas (CH4) as the electron donor. In this study, we achieved complete SeO42- bioreduction in a membrane biofilm reactor (MBfR) using CH4 as the sole added electron donor. The introduction of nitrate (NO3-) slightly inhibited SeO42- reduction, but the two oxyanions were simultaneously reduced, even when the supply rate of CH4 was limited. The main SeO42- reduction product was nanospherical Se, which was identified by scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDS). Community analysis provided evidence for two mechanisms for SeO42- bioreduction in the CH4-based MBfR: a single methanotrophic genus, such as Methylomonas, performed CH4 oxidation directly coupled to SeO42- reduction, and a methanotroph oxidized CH4 to form organic metabolites that were electron donors for a synergistic SeO42--reducing bacterium.