Effects of pH and Electron Donors on Nitrate and Nitrite Reduction in Ruminal Microbiota

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Factors affecting nitrate and nitrite reduction in the rumen were examined. Nitrate and nitrite reduction by mixed microbes harvested from the goat rumen was increased by feeding a high-roughage diet, which contained a much higher amount of nitrate than concentrate. The activities of nitrate reductase and nitrite reductase per bacterial mass appeared to be increased by feeding a high-nitrate diet, suggesting that these enzymes are induced by substrates. Nitrate and nitrite reduction was confirmed to be the fastest at neutral pH and affected by electron supply. The supplementation of electron donors, such as formate, H2, and lactate, stimulated nitrate and nitrite reduction, especially nitrite reduction, which decreased nitrite accumulation. As a result, addition of formate or lactate in the presence of nitrate alleviated the inhibitory effect of nitrite on fermentation, and greatly reduced methanogenesis.