Effect of ethanol on nitrate and nitrite reduction and methanogenesis in the ruminal microbiota

Abstract

ABSTRACTThe effect of ethanol on nitrate and nitrite reduction was examined by conducting in vitro experiments with mixed ruminal microbes. The addition of ethanol to cultures of mixed ruminal microbes stimulated nitrate reduction, and, to a greater extent, nitrite reduction, which resulted in a decrease in nitrite accumulation. However, known nitrate‐reducing ruminal bacteria, such as Selenomonas ruminantium, Veillonella parvula and Wolinella succinogenes, were unable to utilize ethanol directly as an electron donor for nitrate reduction. No nitrate‐reducing bacterium capable of utilizing ethanol was found in the rumen of goats. However, when mixed ethanol‐utilizing, hydrogen gas (H2)‐producing bacteria (Ruminococcus albus and Ruminococcus flavefaciens) were added to the culture of the mixed nitrate‐reducing bacteria described above, nitrate and nitrite reduction was observed. These results suggest that the nitrate‐reducing bacteria utilized the H2 that was produced from ethanol oxidation by the ethanol‐utilizing bacteria as an electron donor. It is conceivable that the stimulation of nitrate and nitrite reduction by ethanol, observed in the culture of mixed ruminal microbes, was a result of electron transfer from ethanol to nitrate, and nitrite through H2, that is, ‘interspecies hydrogen transfer’ from ethanol‐metabolizing bacteria to nitrate‐reducing bacteria. Thus, the addition of ethanol to high‐nitrate diets may be effective for preventing nitrate poisoning. Furthermore, methane production was reduced to less than one‐third by the addition of mixed nitrate‐reducing bacteria to the co‐culture of mixed methanogens with mixed ethanol‐utilizing bacteria incubated in a medium containing ethanol and nitrate. Therefore, the addition of ethanol and nitrate may decrease methanogenesis without suppressing overall fermentation in the rumen.