Marine sediments from littoral and sublittoral location were evaluated as inocula for methane production from anaerobic fermentation of Macrocystis pyrifera in seawater system. Littoral sediment showed the higher methanogenetic activity from acetate and resulted in a higher biomethane yield of 217.1 +/- 2.4 mL/g-VS, which was comparable with that reported in freshwater system with desalted sea-weeds. With 0.8 mM sodium molybdate added, both the maximal methane yield and concentration increased while the lag-time was greatly shortened, suggesting that sulfate was one of the major inhibitors. Microbial community analysis revealed that degradation of M. pyrifera needed cooperation of very complex microbial populations. Hydrogenotrophic methanogens had an absolute dominance in distribution compared with the acetotrophic ones, indicating syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis might play important roles in the thalassic anaerobic fermentation system. These results clearly showed that biomethane production of raw seaweeds in seawater system was feasible. (C) 2015 Elsevier Ltd. All rights reserved.