Effects of oxygen tension on the microbial community and functional gene expression of aerobic methane oxidation coupled to denitrification systems

Aerobic CH 4 oxidation coupled to denitrification (AME-D) can not only mitigate the emission of greenhouse gas (e.g., CH 4 ) to the atmosphere, but also reduce NO 3 − and/or NO 2 − and alleviate nitrogen pollution. The effects of O 2 tension on the community and functional gene expression of methanotrophs and denitrifiers were investigated in this study. Although higher CH 4 oxidation occurred in the AME-D system with an initial O 2 concentration of 21% (i.e., the O 2 -sufficient condition), more NO 3 − -N was removed at the initial O 2 concentration of 10% (i.e., the O 2 -limited environment). Type I methanotrophs, including Methylocaldum , Methylobacter , Methylococcus , Methylomonas , and Methylomicrobium , and type II methanotrophs, including Methylocystis and Methylosinus , dominated in the AME-D systems. Compared with type II methanotrophs, type I methanotrophs were more abundant in the AME-D systems. Proteobacteria and Actinobacteria were the main denitrifiers in the AME-D systems, and their compositions varied with the O 2 tension. Quantitative PCR of the pmoA , nirS , and 16S rRNA genes showed that methanotrophs and denitrifiers were the main microorganisms in the AME-D systems, accounting for 46.4% and 24.1% in the O 2 -limited environment, respectively. However, the relative transcripts of the functional genes including pmoA , mmoX , nirK , nirS , and norZ were all less than 1%, especially the functional genes involved in denitrification under the O 2 -sufficient condition, likely due to the majority of the denitrifiers being dormant or even nonviable. These findings indicated that an optimal O 2 concentration should be used to optimize the activity and functional gene expression of aerobic methanotrophs and denitrifiers in AME-D systems.