Methane-metabolizing microbial communities in sediments of the Haima cold seep area, northwest slope of the South China Sea

Abstract

Cold seeps are widespread chemosynthetic ecosystems in the deep-sea environment, and cold seep microbial communities of the South China Sea (SCS) are poorly constrained. Here, we report on the archaeal communities, particularly those involved in methane metabolization, in sediments of a newly discovered cold seep (named 'Haima') in the northwest slope of the SCS. Archaeal diversity, abundance, and distribution were investigated in two piston cores collected from the seep area of ROV1 (QDN-14B) and a non-seep control site (QDN-31B). Geochemical investigation of QDN-14B identified an estimated sulphate-methane transition zone (Estimated SMTZ) at 300–400 cm, where a high abundance of anaerobic methane-oxidizing archaea (ANME) occurred, as revealed by analysis of the 16S rRNA gene and the gene (mcrA) encoding the alpha subunit of the key enzyme methyl-coenzyme M reductase. ANME-2a/b was predominant in the upper and middle layers of the Estimated SMTZ, whereas ANME-1b outcompeted ANME-2 in the sulphate-depleted bottom layers of the Estimated SMTZ and the methanogenic zone. Fine-scale phylogenetic analysis further divided the ANME-1b group into three subgroups with different distribution patterns: ANME-1bI, ANME-1bII, and ANME-1bIII. Multivariate analyses indicated that dissolved inorganic carbon and sulphate may be important factors controlling the composition of the methane-metabolizing community. Our study on ANME niche separation and interactions with other archaeal groups improves our understanding of the metabolic diversity and flexibility of ANME, and the findings further suggest that ANME subgroups may have evolved diversified/specified metabolic capabilities other than syntrophic anaerobic oxidation of methane coupled with sulphate reduction in marine sediments.

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