Anaerobic methane oxidation coupled to denitrification is an important potential methane sink in deep-sea cold seeps

Microbes play a crucial role in mediating the methane flux in deep-sea cold seep ecosystems, where only methane-related microbes have been well studied, while the whole microbial community and their ecological functions were still largely unknown. Here, we utilized metagenomic data to investigate how the structure and metabolism of microbial community shift in the reduced sediment habitats along the spatial scales. Microbial communities in cold seeps and troughs formed two distinct clades likely driven by environmental factors, such as total sulfur, total phosphate and NO₃^-, rather than geographical proximity. The predominance of Methanosarcinales reflected a high potential for methane production. In addition to the already well-reported ANME-1/SRB consortia, prevalence of bacterial Methylomirabilis and archaeal Methanoperedens as important performers in the n-damo process with respective of nitrite and nitrate as respective electron acceptor was observed in deep-sea hydrate-bearing regions as well. Aerobic methane oxidization was conducted mainly by type I methanotrophs at Site F (Formosa Ridge), but also via the n-damo process by Methanoperedens and Methylomirabilis in the Haima seep and Xisha Trough, respectively. Based on the high abundance of those denitrifying-dependent methane oxidizers and their related functional genes, we concluded that the previously overlooked n-damo process might be a major methane sink in cold seeps or in gas hydrate-bearing sediments if nitrate is available in the anoxic zones. The signature of isotopic labeling would be essential to confirm the contribution of different anaerobic methane oxidizing pathways in deep-sea cold seep ecosystems.