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

Selenate (SeO4 2–) 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 SeO4 2– bioreduction in a membrane biofilm reactor (MBfR) using CH4 as the sole added electron donor. The introduction of nitrate (NO3 –) slightly inhibited SeO4 2– reduction, but the two oxyanions were simultaneously reduced, even when the supply rate of CH4 was limited. The main SeO4 2–-reduction product was nanospherical Se0, which was identified by scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDS). Community analysis provided evidence for two mechanisms for SeO4 2– bioreduction in the CH4-based MBfR: a single methanotrophic genus, such as Methylomonas, performed CH4 oxidation directly coupled to SeO4 2– reduction, and a methanotroph oxidized CH4 to form organic metabolites that were electron donors for a synergistic SeO4 2–-reducing bacterium.