Methane (CH₄) production or methanogenesis in wetland ecosystems is a key process in the CH₄ emission, one of the strongest greenhouse gases. Methanogens—the microorganism involved in methanogenesis—in mangrove wetlands are commonly thought to be out-competed by sulphate-reducing bacteria (SRB) in saline environments, which are rich in sulphate (SO₄^2-) content. This is due to the common group of methanogen (i.e. acetotrophic and hydrogenotrophic methanogens) use similar substrates with SRB. However, there are methylotrophic methanogens that may co-exist with SRB because these microorganisms use different substrates. Our study aims to investigate methanogenic pathways in overwash mangrove forests, which have much less freshwater inputs. We combined analysis of sediment geochemistry, microbial functional groups and potential CH₄ production to elucidate which pathways of methanogenesis were occurring in the mangrove sediment or dominant. The result of sediment geochemistry showed that SO₄^2- was abundance in the sediment pore-water but it apparently has not affected to CH₄ production in that sediment. According to qPCR analysis, the abundance of methanogens and SRB in the sediment samples ranged within the similar order (i.e. 72 - 6 × 10⁵ CFU g^-1 sediment and 2 × 10² - 2 × 10⁵ CFU g^-1 sediment, respectively). Further analysis indicated that methylotrophic methanogen is the most abundance among the methanogen groups. Moreover, the analysis of potential CH₄ production showed that methanogenesis through methylotrophic pathway was more favourable than the others. Therefore, this study confirmed the importance of the methylotrophic pathway for CH₄ production in overwash mangrove environments. Furthermore, our study may change the view on mangrove ecosystems as a minor source of CH₄ emission.

methane production, methanogenesis, methanogens, methylotrophic