Fig. 7
A conceptual model of depth-related microbially driven CH4, N and S cycling and their coupling mechanisms in the mangrove sediment. First, physicochemical properties differed vertically by depths and these changes, especially AVS and pH, had a great effect on the distribution of functional genes/pathways involved in CH4, N and S cycling. Second, possible coupling mechanisms were proposed: a SOB-denitrifier/DNRA; b SRB-methanogen; c Methanogen-methanogen. The relative abundance of functional genes involved in denitrification and S oxidation was enriched in the surface sediment and they could be coupled by S-driven autotrophic denitrifiers, such as Burkholderiaceae and Sulfurifustis, whereas those involved in dissimilatory sulphate reduction and methanogenesis were enriched in the middle and deep sediments. SRB could co-exist with methanogens in the middle sediment, which could be pulled forward upon the demand of ANME and SOB or directly transfer H+ or electrons. Third, the depth-related CH4, N and S cycling microbiomes and their coupling processes may have a great effect on mangrove ecosystem functions and services, such as greenhouse gas emissions, N supply for plant growth, and detoxification. The red and blue colours of words and arrows represent increase or decrease of environmental variables or key genes, respectively along the sediment depth. Black-coloured genes indicate no momentous differences along the sediment depth. Yellow arrows represent H+/e− transfer between microbes. TC: total carbon; TN: total nitrogen; AVS: acid volatile sulphide; SOB: sulphur oxidizer; SRB: sulphate reducer; ANME: anaerobic methane oxidizer; Met: methanogen