In situ substrates for sulfidogens and methanogens in municipal anaerobic sewage digesters with different levels of sulfate

To clarify the interaction between sulfidogens and methanogens in municipal anaerobic digesters, in situ substrate utilization by the two microbial groups was characterized by the accumulation rates of substrates after the addition of specific inhibitors; 20 mM of molybdate for sulfidogens and 0.01% (v/v) of chloroform for methanogens in three anaerobic sludge digesters with different concentrations of sulfate (A, 1.2; B, 1.8; C, 2.7 mM). The rates of sulfate reduction and methane production in sludge C were the highest and the lowest, respectively among the three sludges. The major substrates for methanogens were acetate (35–58%), and propionate (17–22%) in all the sludges. Propionate should be utilized by methanogens after conversion to acetate and H2 by syntrophic hydrogen-producing acetogens, as no methanogens are known to directly utilize propionate. Detectable accumulated substrates for sulfidogens (<20%) were propionate in sludges A and B and acetate in C. Still approximately 80% of substrates for sulfidogens were unknown. These results show that acetate or propionate is a common substrate for sulfidogens and methanogens but sulfidogens can mainly grow on the other substrates in the municipal anaerobic sludges, implying nutritional segregation between sulfidogens and methanogens