Growth Kinetics and Competition Between Methanosarcina and Methanosaeta in Mesophilic Anaerobic Digestion

Methanosarcina species with a high maximum specific growth rate (μ_max) and high half-saturation coefficient (K _S) and Methanosaeta species with a low μ_max and low K _S are the only known aceticlastic methanogens. Because of Methanosaeta's low K _S, the low acetate concentrations in conventional, mesophilic anaerobic digestion yield Methanosaeta dominance. However, Methanosarcina absorbs increases in acetate more efficiently and thus promotes more stable digestion. This paper tests the hypothesis that decreasing digester feeding frequencies can increase Methanosarcina predominance. Two acetate-fed reactors were established at a 17-day solids retention time. One reactor was fed hourly, and one was fed once daily. Microscopic and molecular methods were used to verify that the hourly fed reactor enriched for Methanosaeta, while the daily fed reactor enriched for Methanosarcina. Growth and substrate-use kinetics were measured for each reactor. A digester overload condition was simulated, and the Methanosarcina-enriched reactor was found to perform better than the Methanosaeta-enriched reactor. These findings indicate that Methanosarcina dominance can be achieved with infrequent feedings, leading to more stable digestion.