Microbial ecophysiology of whey biomethanation: characterization of bacterial trophic populations and prevalent species in continuous culture.

The organization and species composition of bacterial trophic groups associated with lactose biomethanation were investigated in a whey-processing chemostat by enumeration, isolation, and general characterization studies. The bacteria were spatially organized as free-living forms and as self-immobilized forms appearing in flocs. Three dominant bacterial trophic group populations were present (in most probable number per milliliter) whose species numbers varied with the substrate consumed: hydrolytic, 10; acetogenic, 10 to 10; and methanogenic, 10 to 10. The three prevalent species utilizing lactose were identified as Leuconostoc mesenteroides, Klebsiella oxytoca, and Clostridium butyricum. Clostridium propionicum and Desulfovibrio vulgaris were the dominant lactate-consuming, hydrogen-producing acetogenic bacteria, while D. vulgaris was the only significant ethanol-degrading species. Methanosarcina barkeri and Methanothrix soehngenii were identified as the dominant acetate-utilizing methanogens, and Methanobacterium formicicum was the prevalent hydrogen-utilizing methanogen. A microbial food chain is proposed for lactose biomethanation that comprises multiple species in three different groups, with the major hydrogen-producing acetogen being a sulfate-reducing species, D. vulgaris, which functioned in the absence of significant levels of environmental sulfate.