Electrochemical system for anaerobic oxidation of methane by DAMO microbes with nitrite as an electron acceptor

Denitrifying anaerobic methane oxidation (DAMO) is an important microbial metabolic process that simultaneously converts of methane and nitrite. In this study, electrochemical systems were investigated for DAMO with nitrite as an electron acceptor. The results showed that the auxiliary voltage enhanced anaerobic methane oxidation and nitrite reduction. The greatest methane conversion (26.61 mg L^-1 d^-1) was obtained at an auxiliary voltage of 1.6 V (EMN-1.6). Isotope tracing indicated that carbon dioxide was the oxidation product of methane, and methanol was the intermediate. The power density reached 0.60 (for EMN-0.5, the bioreactor with a voltage of 0.5 V) and 3.77 mW m^-2 (for EMN-1.6). DAMO microbes, Methylocystis sp., and Methylomonas sp. were identified as methanotrophs. Rhodococcus sp., Hyphomicrobium sp., and Thiobacillus sp. were the dominant denitrifying bacteria. The conversion pathway was speculated to be as follows: methane was oxidized to carbon dioxide and nitrite was reduced to nitrogen. The two processes were independently completed by DAMO bacteria and oxygen was simultaneously generated. For the electron transfer pathway, methanotrophs utilized the oxygen released by DAMO bacteria to convert methane into organic matter (e.g. methanol). These organic compounds were utilized by Pseudoxanthomonas sp. and Pseudomonas sp., and the generated electrons were then released to the outside of the cells and transferred to the anode. Denitrifying bacteria received electrons at the cathode, transferred them to the interior of the cell, and then converted nitrite into nitrogen. This research explored an effective consortium and a method for methane and nitrogen removal.