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 (μmax) and saturation constant (Ks) were, respectively, 0.36 hr−1 and 0.047 mM for lactate-limited growth ofDesulfovibrio desulfuricans and 0,011 hr−1 and 0.17 mM for acetate-limited growth ofMethanobacterium sp. Calculated values for the true molar growth yieldsYG) and maintenance coefficients (m) were 30.6 g bacterial mass/mole of lactate and 0.53 g substrate/g dry wt hr forD. desulfuricans and 37.8 g bacterial mass/mole of acetate and 0.54 g substrate/g dry wt hr forMethanobacterium.
No growth ofMethanobacterium was observed at apS2− value (the hydrogen sulfide potential) of more than 11 and there was no effect on the growth atpS2− values above 13. In mixed continuous culture experiments the concentration of acetate decreased in the secondstage growth vessel, whereas that of methane increased stoichiometrically. If the substrate concentration in the reservoirs (Sr) was increased from 0.1 to 0.5 mg/ml, the population ofDesulfovibrio increased and that ofMethanobacterium was washed out of the culture vessel, since the concentration of hydrogen sulfide reached apS2− value of 10.5. From the mixed continuous culture experiments a commensalism between the two species can be described, i.e., the acetate-fermentingMethanobacterium benefits from the acetate released byDesulfovibrio which is, in turn, not affected in the presence of the former.