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

The effects of ruminal concentrations of CO2 and O2 on glucose-stimulated and endogenous fermentation of the rumen isotrichid ciliate Dasytricha ruminantium were investigated. Principal metabolic products were lactic, butyric and acetic acids, H2 and CO2. 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. ruminantium. Glucose uptake and metabolite formation rates were influenced by the headspace gas composition during the protozoal incubations. The uptake of exogenously supplied D-glucose was most rapid in the presence of O2 concentrations typical of those detected in situ (i.e. 1-3 microM). A typical ruminal gas composition (high CO2, low O2) led to increased butyrate and acetate formation compared to results obtained using O2-free N2. At a partial pressure of 66 kPa CO2 in N2, increased cytosolic flux to butyrate was observed. At low O2 concentrations (1-3 microM dissolved in the protozoal suspension) in the absence of CO2, increased acetate and CO2 formation were observed and D. ruminantium utilized lactate in the absence of extracellular glucose. The presence of both O2 and CO2 in the incubation headspaces resulted in partial inhibition of H2 production by D. ruminantium. Results suggest that at the O2 and CO2 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.