Control of Interspecies Electron Flow during Anaerobic Digestion: Role of Floc Formation in Syntrophic Methanogenesis

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Abstract
The flora of an anaerobic whey-processing chemostat was separated by anaerobic sedimentation techniques into a free-living bacterial fraction and a bacterial floc fraction. The floc fraction constituted a major part (i.e., 57% total protein) of the total microbial population in the digestor, and it accounted for 87% of the total CO2-dependent methanogenic activity and 76% of the total ethanol-consuming acetogenic activity. Lactose was degraded by both cellular fractions, but in the free flora fraction it was associated with higher intermediary levels of H2, ethanol, butyrate, and propionate production. Electron microscopic analysis of flocs showed bacterial diversity and juxtapositioning of tentative Desulfovibrio and Methanobacterium species without significant microcolony formation. Ethanol, an intermediary product of lactose-hydrolyzing bacteria, was converted to acetate and methane within the flocs by interspecies electron transfer. Ethanol-dependent methane formation was compartmentalized and closely coupled kinetically within the flocs but without significant formation of H2 gas. Physical disruption of flocs into fragments of 10- to 20-μm diameter initially increased the H2 partial pressure but did not change the carbon transformation kinetic patterns of ethanol metabolism or demonstrate a significant role for H2 in CO2 reduction to methane. The data demonstrate that floc formation in a whey-processing anaerobic digestor functions in juxtapositioning cells for interspecies electron transfer during syntrophic ethanol conversion into acetate and methane but by a mechanism which was independent of the available dissolved H2 gas pool in the ecosystem. Images