The effects of differing concentrations of CO₂ and O₂ on the fermentative metabolism of the rumen fungi Neocallimastix patriciarum and Neocallimastix frontalis L2

The effects of decreasing the concentration of CO₂ and introducing up to 10% O₂ into the headspace gases on the fermentative metabolism of the rumen fungi Neocallimastix patriciarum and Neocallimastix frontalis L2 were investigated. The relative proportion of metabolites produced by both fungi depended on the concentration of CO₂ in the headspace. Under lowered CO₂ levels, both fungi produced increased acetate, lactate, and H₂, whereas the production of ethanol, formate, and (in the case of N. frontalis L2) succinate decreased. Lowered CO₂ concentrations also decreased the rate of glucose utilization and cumulative gas production by both fungal isolates. In addition, decreased CO₂ levels resulted in decreases in NAD(P)H ferredoxin oxidoreductase and hydrogenase activities, whereas malate dehydrogenase and pyruvate ferredoxin oxidoreductase activities were increased. Both N. patriciarum and N. frontalis L2 required at least 7% CO₂ in the gas phase for growth. Both isolates also showed a degree of aerotolerance as they grew when exposed to 5% O₂; they also grew in media lacking a reducing agent, providing that O₂ was initially <1% of the total headspace concentration.