Characterisation of Methane Production Pathways in Sediment of Overwashed Mangrove Forests

Methane (CH4) emissions in mangrove ecosystems may complicate the ecosystem’s potential carbon offset for climate change mitigation. Microbial processes and the mass balance of CH4 in mangrove sediment are responsible for the emissions from the ecosystems. This is the follow up of our previous research which found the super saturation of CH4 in the pore water of mangrove sediment compared to atmospheric CH4 and the lack of a correlation between pore water sulphate and CH4 concentrations. This study is going to investigate methane production pathways in the sediment of overwashed mangrove forests. Two approaches were used to study methanogens here: (1) the spread plate count method and the quantitative polymerase chain reaction (qPCR) method, and (2) laboratory experiments with additional methanogenic substrates (methanol, acetate, and hydrogen) to determine which substrates are more conducive to methane production. According to the qPCR method, methanogen abundance ranged from 72 to 6 × 105 CFU g−1 sediment, while SRB abundance ranged from 2 × 102 to 2 × 105 CFU g−1 sediment. According to the plate count method, the abundance of methylotrophic methanogens (the only group of methanogens capable of competing with SRBs) ranged from 8.3 × 102 to 5.1 × 104 CFU g−1, which is higher than the abundance of the other group of methanogens (0 to 7.7 × 102 CFU g−1). The addition of methanol to the sediment slurry, a substrate for methylotropic methanogens, resulted in a massive production of CH4 (up to 9 × 104 ppm) and intriguingly the control treatments with autoclaving did not kill methanogens. These findings suggested that mangrove ecosystems in the marine environment provide favourable conditions for methanogens and further characterisation of the methanogen involved in the process is required. As a result, future research in this ecosystem should include methane production in carbon offset calculations, particularly due to methylotropic methanogenesis.