METHANOGENIC DEGRADATION OF POLYSACCHARIDES AND THE CHARACTERIZATION OF POLYSACCHAROLYTIC CLOSTRIDIA FROM ANOXIC RICE FIELD SOIL

The anaerobic degradation of polysaccharides to methane via intermedia ry metabolites and the characteristics of some polysaccharolytic bacte ria were investigated in Italian rice field soil. The addition of xyla n, pectin or cellulose to slurries of anoxic rice field soil enhanced the production rates of CH, and CO, both at 15 and 30 degrees C (cellu lose only after prolonged incubation), and resulted in the transient a ccumulation of H-2, acetate, propionate and lactate. Counting of bacte ria using the MPN technique indicated that xylan- and pectin-fermentin g bacteria were more abundant than cellulolytic bacteria. Three gram-p ositive, strictly anaerobic, rod-shaped clostridia strains: RCel1, RXy l1 and RPec1 were isolated by enrichment cultures using as substrates cellulose, xylan and pectin, respectively, and characterized phylogene tically (16S rDNA sequences) and physiologically. The fermentation of cellulose by the cellulolytic strain RCel1, closely related to Clostri dium papyrosolvens, was investigated in more derail both at 30 degrees C and 15 degrees C, both in monoculture and in coculture with either Methanospirillum hungatei or Sporomusa ovata. The cocultures generally resulted in higher growth yields of strain RCel1 than in monoculture suggesting a better ATP yield per mol of anhydroglucose fermented. In monoculture, strain RCel1 fermented cellulose primarily to H-2, acetat e and ethanol and to smaller amounts of succinate, lactate and formate . Propionate was produced only at 15 degrees C. In coculture with M. h ungatei, an increase in H-2 and acetate production and a decrease in l actate, ethanol, and formate production was observed. The H-2 partial pressures in the cocultures were lower than in the monoculture. In coc ulture with S. ovata, cellulose was fermented to acetate as main produ ct. The steady stare Hz partial pressure in coculture with S. ovata wa s higher than with M. hungatei, and was lower at 15 degrees C than at 30 degrees C in both cocultures as expected from the theory The levels of H-2 in coculture with M. hungatei were similar to those observed i n rice field soil slurries. The other fermentation products of cellulo se, xylan or pectin observed in bacterial cultures were also mostly th e same as observed in slurries of anoxic rice field soil, but importan t quantitative differences remained.