A Study of Mixed Continuous Cultures of Sulfate-Reducing and Methane-Producing Bacteria

Ecological relationships between sulfate-reducing and methane-producing bacteria in mud of Lake Vechten have been studied by continuous culture studies using the chemostat technique. The maximum specific growth rate (mu(max)) and saturation constant (K(delta)) were, respectively, 0.36 hr(-1) and 0.047 mM for lactate-limited growth of Desulfovibrio desulfuricans and 0,011 hr(-1) and 0.17 mM for acetate-limited growth of Methanobacterium sp. Calculated values for the true molar growth yields(Y(G)) and maintenance coefficients (m) were 30.6 g bacterial mass/mole of lactate and 0.53 g substrate/g dry wt hr for D. desulfuricans and 37.8 g bacterial mass/mole of acetate and 0.54 g substrate/g dry wt hr for Methanobacterium. No growth of Methanobacterium was observed at a pS(2-) value (the hydrogen sulfide potential) of more than 11 and there was no effect on the growth at pS(2-) values above 13. In mixed continuous culture experiments the concentration of acetate decreased in the second-stage growth vessel, whereas that of methane increased stoichiometrically. If the substrate concentration in the reservoirs (S(r)) was increased from 0.1 to 0.5 mg/ml, the population of Desulfovibrio increased and that of Methanobacterium was washed out of the culture vessel, since the concentration of hydrogen sulfide reached a pS(2-) value of 10.5. From the mixed continuous culture experiments a commensalism between the two species can be described, i.e., the acetate-fermenting Methanobacterium benefits from the acetate released by Desulfovibrio which is, in turn, not affected in the presence of the former.