Summary: The effects of ruminal concentrations of CO and O on glucose-stimulated and endogenous fermentation of the rumen isotrichid ciliate 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. C NMR spectroscopy revealed alanine as a minor product of glucose fermentation by . 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 O concentrations typical of those detected (i.e. 1-3 μM). A typical ruminal gas composition (high CO, low O) led to increased butyrate and acetate formation compared to results obtained using O-free N. At a partial pressure of 66 kPa CO in N, increased cytosolic flux to butyrate was observed. At low O concentrations (1-3 μM dissolved in the protozoal suspension) in the absence of CO, increased acetate and CO formation were observed and utilized lactate in the absence of extracellular glucose. The presence of both O and CO in the incubation headspaces resulted in partial inhibition of H production by . Results suggest that at the O and CO concentrations that prevail , the contribution made by to the formation of ruminal volatile fatty acids is greater than previously reported, as earlier measurements were made under anaerobic conditions.


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