Evaluation of marine sediments as microbial sources for methane production from brown algae under high salinity

•Marine sediments effectively converted brown algae to methane at seawater salinity.•All activities needed for methane production were detected under saline conditions.•The marine sediments possessed higher activity than sources of non-marine origin.•Marine sediments were microbial sources suitable for the methane production. Various marine sediments were evaluated as promising microbial sources for methane fermentation of Saccharina japonica, a brown alga, at seawater salinity. All marine sediments tested produced mainly acetate among volatile fatty acids. One marine sediment completely converted the produced volatile fatty acids to methane in a short period. Archaeal community analysis revealed that acetoclastic methanogens belonging to the Methanosarcina genus dominated after cultivation. Measurement of the specific conversion rate at each step of methane production under saline conditions demonstrated that the marine sediments had higher conversion rates of butyrate and acetate than mesophilic methanogenic granules. These results clearly show that marine sediments can be used as microbial sources for methane production from algae under high-salt conditions without dilution.