Influence of CO2 and low concentrations of O2 on fermentative metabolism of the rumen ciliate Dasytricha ruminantium

The effects of ruminal concentrations of CO, and 0, on glucose-stimulated and endogenous fermentation of the rumen isotrichid ciliate Dusytrichu ruminantium were investigated. Principal metabolic products were lactic, butyric and acetic acids, H, and CO,. Traces of propionic acid were also detected; formic acid present in the incubation supernatants was found to be a fermentation product of the bacteria closely associated with this rumen ciliate. 13C NMR spectroscopy revealed alanine as a minor product of glucose fermentation by D. ruminantiurn. Glucose uptake and metabolite formation rates were influenced by the headspace gas composition during the protozoal incubations. The uptake of exogenously supplied D-ghCOSe was most rapid in the presence of 0, concentrations typical of those detected in sifu (i.e. 1-3 p~). A typical ruminal gas composition (high CO,, low 0,) led to increased butyrate and acetate formation compared to results obtained using 0,-free N,. At a partial pressure of 66 kPa CO, in N,, increased cytosolic flux to butyrate was observed. At low 0, concentrations (13 p~ dissolved in the protozoal suspension) in the absence of CO,, increased acetate and CO, formation were observed and D. ruminantium utilized lactate in the absence of extracellular glucose. The presence of both 0, and CO, in the incubation headspaces resulted in partial inhibition of H2 production by D. ruminantiurn. Results suggest that at the 0, and CO, concentrations that prevail in situ, the contribution made by D. ruminantium to the formation of ruminal volatile fatty acids is greater than previously reported, as earlier measurements were made under anaerobic conditions.