An ecological substrate relationship between sulfate-reducing and methane-producing bacteria in mud of Lake Vechten has been studied in experiments using 14C-labeled acetate and lactate as substrates. Fluoroacetate strongly inhibited the formation of 14CO2 from [U-14C]-acetate and β-fluorolactate gave an inhibition of similar magnitude of the breakdown of [U-14C]-l-lactate to 14CO2 thus confirming earlier results on the specific action of these inhibitors.
The turnover-rate constant of l-lactate was 2.37 hr-1 and the average l-lactate pool size was 12.2 μg per gram of wet mud, giving a turnover rate of 28.9 μg of lactate/gram of mud per hr. The turnover-rate constant of acetate was 0.35 hr-1 and the average pool size was 5.7 μg per gram of wet mud, giving a rate of disappearance of 1.99 μg of acetate/gram of mud per hr. Estimations of the acetate turnover rate based upon the formation of 14CO2 from [U-14C]-acetate or [1-14C]-acetate yielded figures of the same magnitude (range 0.45 to 1.74). These and other results suggest that only a portion of the lactate dissimilated is turned over through the acetate pool.
The ratio of 14CO2/14CH4 produced from [U-14C]-acetate by mud was 1.32; indicating that 0.862 moles of CH4 and 1.138 moles of CO2 are formed per mole of acetate. From the rate of disappearance of acetate (0.027 μmoles/gram wet mud per hr) and the rate of methane production (0.034 μmoles/gram wet mud per hr), it may be concluded that acetate is an important precursor of methanogenesis in mud (approximately 70%). A substrate relationship between the two groups of bacteria is likely since 14CH4 was formed from [U-14C]-l-lactate.