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Methane emissions from wetlands contribute to the currently rising levels of methane in the atmosphere. Nonetheless, the assessments of methane-producing Archaea (methanogens) and their environmental controls are still rare particularly in soils from estuarine tidal marshes. In estuarine tidal marshes, the salinity gradient is commonly observed, and its effects on the methanogenic community are hardly known. In this study, in the Min River estuary, southeast China, we analyzed two freshwater and two brackish (oligohaline) marshes in a subtropical estuary, where we assessed methanogenic community structure and abundance by molecular cloning, sequencing, and quantitative PCR methods, as well as by means of soil physicochemical characteristics and concentration of methane dissolved in pore water. The common and dominant genera of these methanogens include Methanoregula , Methanobacterium , Methanosarcina , and Methanolinea in the four marshes. Phylotypes of the methanogens using H 2 /CO 2 (Methanomicrobiales) or acetate (Methanosarcinales) were identified in the mcrA gene clone libraries and by restriction fragment length polymorphism (RFLP) analyses. The abundance levels of methanogens in the two freshwater marshes were significantly higher than those in the two brackish marshes. The value of the Shannon Index ( H ′) of diversity and Simpson index ( D ) of evenness of methanogen mcrA clone libraries decreased along the salinity gradient from freshwater to brackish water. Methanogen community structure varied with soil depth in the four wetlands. The concentrations of methane in pore water also decreased from the freshwater sites to the brackish sites; this pattern is consistent with a change in the abundance of methanogens. Our results showed that in a low-level salinity gradient of a subtropical estuary, salinity is still a major factor controlling the overall community structure and abundance of methanogens. [ABSTRACT FROM AUTHOR]

Assessing the diverse communities of methanogenic Archaea and sulfate-reducing bacteria (SRB) is important to understand methane (CH 4 ) production in wetland ecosystems. However, the vertical distribution of composition and diversity, and the effects of environmental factors on the methanogen and SRB communities in the sediments of subtropical estuarine brackish marshes have been poorly characterized. To assess the effects of variable environmental conditions on methanogenic and SRB communities in marshes, we studied three brackish marsh zones dominated by Phragmites australis , Cyperus malaccensis and Spartina alterniflora , respectively, in the Min River estuary, southeastern China. Methanogens of the Methanomicrobiales order was the dominant group at sediment depths of 0–30 cm, which indicated that the main pathway of methane production was H 2 /CO 2 in this zone. In general, methanogens of the genus Methanoregula were dominant in the three marsh zones. For SRB, Desulfobacterales was the dominant group, and Desulfobacterium and Desulfosarcina were the predominant genera at the depth of 0–30 cm. The community composition of methanogens and SRB changed with vegetation type and soil depth. Compared with SRB, vegetation type demonstrated a stronger influence on the community composition of methanogens. Canonical correspondence analysis (CCA) analysis further revealed that the main factors affecting the methanogens community composition were EC (electric conductivity) and pH, and the main factors affecting SRB community composition were pH, SOC and TN, suggesting that pH is a common factor influencing the community compositions of both methanogen and SRB in the sediments of brackish marshes. [ABSTRACT FROM AUTHOR]