Methylotrophic substrates stimulated higher methane production than competitive substrates in mangrove sediments

Although mangrove forests can uptake atmospheric CO₂ and store carbon as organic matter called "blue carbon", it is also an important natural source of greenhouse gas methane. Methanogens are major contributors to methane and play important roles in the global carbon cycle. However, our understanding of the key microbes and metabolic pathways responsible for methanogenesis under specific substrates in mangrove sediments is still very limited. Here, we set an anaerobic incubation to evaluate the responses of methanogens in mangrove sediments from South China to the addition of diverse methanogenic substrates (H₂/CO₂, acetate, trimethylamine (TMA), and methanethiol (MT)) and further investigated the dynamics of the whole microbial community. Our results showed that diverse substrates stimulated methanogenic activities at different times. The stimulation of methanogenesis was more pronounced at early and late periods by the addition of methylotrophic substrates TMA and MT, respectively. The amplicon sequencing analysis showed that genus Methanococcoides was mainly responsible for TMA-utilized methanogenesis in mangrove sediment, while the multitrophic Methanococcus was most abundant in H₂/CO₂ and MT treatments. Apart from that, the bacteria enrichments of Syntrophotalea, Clostridium_sensu_stricto_12, Fusibacter in MT treatments might also be associated with the stimulation of methane production. In addition, the metagenomic analysis suggested that Methanosarcinaceae was also one of the key methanogens in MT treatments with different genomic information compared to that in TMA treatments. Finally, the total relative abundances of methanogenesis-related genes were also highest in TMA and MT treatments. These results will help advance our understanding of the contributions of different methanogenesis pathways and methanogens to methane emissions in mangrove sediments.