Abstract
Experiments in vitro were carried out to study the effects of protozoal fraction and lactate on nitrate reduction by rumen microorganisms. Rumen fluid from fistulated sheep fed hay and concentrates supplemented with potassium nitrate was divided into bacterial and protozoal fractions by centrifugation. Reduction rates of nitrate and nitrite and production rates of VFA and lactate by bacterial fraction with or without protozoal one were determined. Both nitrate and nitrite reduction rates were increased with the addition of protozoal fraction irrespective of nitrate adaptation of microorganisms. The appearing rate of nitrite increased markedly in bacterial fraction alone, whereas the further reduction rate of nitrite increased in bacterial fraction with protozoal one, especially after achieving nitrate adaptation, and the same tendency was observed in case the concentrates consumption of the animal increased. The protozoal fraction heated with boiling water did not activate the nitrate and nitrite reduction. With bacterial fraction alone lactate was accumulated and a small amount of VFA was produced. On the contrary, lactate was hardly accumulated and a large amount of VFA was produced without a marked decline of pH in bacterial fraction with protozoal one. The addition of sodium lactate stimulated slightly the reduction of nitrate to nitrite with bacterial fraction alone. In the mixed system of bacterial and protozoal fractions, however, both of nitrate and nitrite reduction rates were accelerated markedly by adding lactate, and rapid lactate decrease and VFA production were observed simultaneously. These results suggested that protozoal fraction played a very important role in nitrite reduction in the rumen and that lactate might be useful as hydrogen donor for such nitrate and nitrite reduction.
