Growth patterns and hydrogen sulfide production by mixed cultures in a cellulose-based microcosm.

Abstract

Studies of the trophic relationships in microbial reduction of sulfate established the need for three physiological types of organisms: cellulosedigesters (Cellulomonas, Cytophaga or Micromonospora), a lactic acid fermenter (Enterobacter), and a sulfate reducer (Desulfovibrio). Microcosms based on cellulose mineral salts medium were inoculated with varying combinations of mixed cultures and incubated at 30°C for 18 days. The extent of sulfate reduction varied in the microcosms. A 5-member culture which contained the three cellulolytic organisms reduced the most sulfate (52%). Cellulomonas was the most efficient cellulolytic partner. Unless all three physiological types were present, sulfate was not reduced.In nature, the formation of sulfides through sulfate reduction is enhanced by low redox potential (Eh), availability of organic matter and a suitable temperature (1). When sulfate is reduced, the hydrogen sulfide (H2S) evolved may combine with ferrous salts to form black sulfide deposits which appear in the profiles of boggy soils and aquatic mulls. Sulfate-reducing bacteria may use organic substrates such as lactate (2), choline and acetate (3, 4) as electron donors for sulfate reduction. In some strains, a reductive pathway through hydrogen oxidation has been observed (5, 6). Species of Desulfovibrio have limited substrate range so trophic interactions with other organisms is necessary to release nutrients from decomposing organic matter and to produce a reducing microenvironment for sulfate reduction. The degradation of plant detritus is through the activities of cellulolytic organisms. It is therefore clear that the generation and maintenance of various nutrient cycles in nature necessitate complex syntrophic relationships among microbial species. TEZUKA et al. (7) in an ecological study of the Sumida River ' Present Address: