Sea wrack (dislodged sea grasses and seaweeds) was used in biogas production. Fresh water scarcity in island communities where sea wrack could accumulate led to seawater utilization as liquid substrate. Three microbial seeds cow manure (CM), marine sediment (MS), and sea wrack-associated microflora (SWA) were explored for biogas production. The average biogas produced were 2172 +/- 156 mL (MS), 1223 +/- 308 mL (SWA) and 551 +/- 126 mL (CM). Though methane potential (396.9 mL(CH4) g(-1) volatile solid) computed from sea wrack proximate values was comparable to other feedstocks, highest methane yield was low (MS = 94.33 mL(CH4) g(-1) VS). Among the microbial seeds, MS proved the best microbial source in utilizing sea wrack biomass and seawater. However, salinity (MS = 42 parts per thousand) observed exceeded average seawater salinity (34 parts per thousand). Hence, methanogenic activity could have been inhibited. This is the first report on sea wrack biomass utilization for thalassic biogas production. (C) 2013 Elsevier Ltd. All rights reserved.