Rumen microbial responses to supplemental nitrate. I. Yeast growth and protozoal chemotaxis in vitro as affected by nitrate and nitrite concentrations

Nitrates have been fed to ruminants, including dairy cows, as an electron sink to mitigate CH₄ emissions. In the NO₃^? reduction process, NO₂^? can accumulate, which could directly inhibit methanogens and some bacteria. However, little information is available on eukaryotic microbes in the rumen. Protozoa were hypothesized to enhance nitrate reductase but also have more circling swimming behavior, and the yeast Saccharomyces cerevisiae was hypothesized to lessen NO₂^? accumulation. In the first experiment, a culture of S. cerevisiae strain 1026 was evaluated under 3 growth phases: aerobic, anoxic, or transition to anoxic culture. Each phase was evaluated with a control or 1 of 3 isonitrogenous doses, including NO₃^?, NO₂^?, or NH₄⁺ replacing peptone in the medium. Gas head phase, NO₃^?, or NH₄⁺ did not influence culture growth, but increasing NO₂^? concentration increasingly inhibited yeast growth. In experiment 2, rumen fluid was harvested and incubated for 3 h in 2 concentrations of NO₃^?, NO₂^?, or sodium nitroprusside before assessing chemotaxis of protozoa toward glucose or peptides. Increasing NO₂^? concentration decreased chemotaxis by isotrichids toward glucose or peptides and decreased chemotaxis by entodiniomorphids but only toward peptides. Live yeast culture was inhibited dose-responsively by NO₂^? and does not seem to be a viable mechanism to prevent NO₂^? accumulation in the rumen, whereas a role for protozoal nitrate reductase and NO₂^? influencing signal transduction requires further research.