Anaerobic Degradation of Lactate by Syntrophic Associations of Methanosarcina barkeri and Desulfovibrio Species and Effect of H 2 on Acetate Degradation

Abstract

When grown in the absence of added sulfate, cocultures of Desulfovibrio desulfuricans or Desulfovibrio vulgaris with Methanobrevibacter smithii ( Methanobacterium ruminantium ), which uses H 2 and CO 2 for methanogenesis, degraded lactate, with the production of acetate and CH 4 . When D. desulfuricans or D. vulgaris was grown in the absence of added sulfate in coculture with Methanosarcina barkeri (type strain), which uses both H 2 -CO 2 and acetate for methanogenesis, lactate was stoichiometrically degraded to CH 4 and presumably to CO 2 . During the first 12 days of incubation of the D. desulfuricans-M. barkeri coculture, lactate was completely degraded, with almost stoichiometric production of acetate and CH 4 . Later, acetate was degraded to CH 4 and presumably to CO 2 . In experiments in which 20 mM acetate and 0 to 20 mM lactate were added to D. desulfuricans-M. barkeri cocultures, no detectable degradation of acetate occurred until the lactate was catabolized. The ultimate rate of acetate utilization for methanogenesis was greater for those cocultures receiving the highest levels of lactate. A small amount of H 2 was detected in cocultures which contained D. desulfuricans and M. barkeri until after all lactate was degraded. The addition of H 2 , but not of lactate, to the growth medium inhibited acetate degradation by pure cultures of M. barkeri. Pure cultures of M. barkeri produced CH 4 from acetate at a rate equivalent to that observed for cocultures containing M. barkeri. Inocula of M. barkeri grown with H 2 -CO 2 as the methanogenic substrate produced CH 4 from acetate at a rate equivalent to that observed for acetate-grown inocula when grown in a rumen fluid-vitamin-based medium but not when grown in a yeast extract-based medium. The results suggest that H 2 produced by the Desulfovibrio species during growth with lactate inhibited acetate degradation by M. barkeri.