Effects of sulfate on lactate and C2-, C3- volatile fatty acid anaerobic degradation by a mixed microbial culture.

The effects of sulfate on the anaerobic degradation of lactate, propionate, and acetate by a mixed bacterial culture from an anaerobic fermenter fed with wine distillery waste water were investigated. Without sulfate and with both sulfate and molybdate, lactate was rapidly consumed, and propionate and acetate were produced; whereas with sulfate alone, only acetate accumulated. Propionate oxidation was strongly accelerated by the presence of sulfate, but sulfate had no effect on acetate consumption even when methanogenesis was inhibited by chloroform. The methane production was not affected by the presence of sulfate. Counts of lactate- and propionate-oxidizing sulfate-reducing bacteria in the mixed culture gave 4.5 X 10(8) and 1.5 X 10(6) viable cells per ml, respectively. The number of lactate-oxidizing fermentative bacteria was 2.2 X 10(7) viable cells per ml, showing that sulfate-reducing bacteria outcompete fermentative bacteria for lactate in the ecosystem studied. The number of acetoclastic methanogens was 3.5 X 10(8) viable cells per ml, but only 2.5 X 10(4) sulfate reducers were counted on acetate, showing that acetotrophic methanogens completely predominated over acetate-oxidizing sulfate-reducing bacteria. The contribution of acetate as electron donor for sulfate reduction in the ecosystem studied was found to be minor.