Coexistence of sulfate reduction and methane production in an organic-rich sediment

Anaerobic mineralization processes were studied over a period of 40 d in an organic-rich sediment amended with labile organic matter (fish food pellets). Although measured sulfate reduction rates (SRR) were hlgh (up to -2400 nmol cm-3 d-l), a coexistence between sulfate-reducing bacteria (SRB) and methane-producing bacteria (MPB) was evident. Methane production rates (MPR) of the same order of magnitude as SRR (up to -1300 nmol cm-3 d-') occurred even when sulfate was present in high concentrations (5 to 60 mM). Acetate was an important substrate for the SRB initially, but its role diminished after 180 h of incubation, indicating that SRB used other substrates than acetate. There was only minor competition for acetate between SRB and MPB, as indicated by the lack of effect on MPR by inhibition of SRB with molybdate. This provides a possible explanation for the coexistence between SRB and MPB; in this organic-rich sediment the concentration of competitive substrates either exceeds the competition level or substrates are used noncompetitively.