Roles of Oxygen in Methane-dependent Selenate Reduction in a Membrane Biofilm Reactor: Stimulation or Suppression

Abstract Although methane (CH4) has been proven to be able to serve as an electron donor for bio-reducing various contaminants (e.g., selenate (SeO42−)), little is known regarding the roles of oxygen in methane-based reduction processes. Here, a methane-based membrane biofilm reactor (MBfR) was established for evaluating the effects of oxygen supply rates on selenate reduction performance and microbial communities. The oxygen supply rate played a dual role (stimulatory or suppressive effect) in selenate reduction rates, depending on the presence or absence of dissolved oxygen (DO). Specifically, selenate reduction rate was substantially enhanced when an appropriate oxygen rate (e.g., 12 to 184 mg/L.d in this study) was supplied but with negligible DO. The highest selenate reduction rate (up to 34 mg-Se/L.d) was obtained under oxygen supply rate of 184 mg/L.d. In contrast, excessive oxygen supply rate (626 mg/L.d) would significantly suppress selenate reduction rate under DO level of 3 mg/L. Accordingly, although the high oxygen supply rate (626 mg/L.d) would promote the expression of pmoA (5.9 × 109 copies g−1), the expression level of narG (a recognized gene to mediate selenate reduction) would be significantly downregulated (6.1 × 109 copies g−1), thus suppressing selenate reduction. In contrast, the expression of narG gene significantly increased to 2.8 × 1010 copies g−1, and the expression of pmoA gene could still maintain at 1.1 × 109 copies g−1 under oxygen supply rate of 184 mg/L.d. High-throughput sequencing targeting 16S rRNA gene, pmoA, and narG collectively suggested Methylocystis acts as the major aerobic methanotroph, in synergy with Arthrobacter and Variovorax which likely jointly reduce selenate to selenite (SeO32−) and further elemental selenium (Se0). Methylocystis was predominant in the biofilm regardless of variations of oxygen supply rates, while Arthrobacter and Variovorax were sensitive to oxygen fluctuation. These findings provide insights into the effects of oxygen on methane-dependent selenate reduction and suggest that it is feasible to achieve a higher selenate removal by regulating oxygen supply rates.