Microbial Selenate Reduction Driven by a Denitrifying Anaerobic Methane Oxidation Biofilm

Anaerobic oxidation of methane (AOM) plays a crucial role in controlling the flux of methane from anoxic environments. Sulfate-, nitrite-, nitrate-, and iron-dependent methane oxidation processes have been considered to be responsible for the AOM activities in anoxic niches. We report that nitrate-reducing AOM microorganisms, enriched in a membrane biofilm bioreactor, are able to couple selenate reduction to AOM. According to ion chromatography, X-ray photoelectron spectroscopy, and long-term bioreactor performance, we reveal that soluble selenate was reduced to nanoparticle elemental selenium. High-throughput 16S rRNA gene sequencing indicates that Candidatus Methanoperedens and Candidatus Methylomirabilis remained the only known methane-oxidizing microorganisms after nitrate was switched to selenate, suggesting that these organisms could couple anaerobic methane oxidation to selenate reduction. Our findings suggest a possible link between the biogeochemical selenium and methane cycles.