Effect of Nitrate on Methane Production and Fermentation by Slurries of Human Faecal Bacteria

Summary: Most probable number counts showed that denitrifying species were the numerically predominant NO− 3 reducing bacteria in the faeces of five methanogenic individuals [about 1010 bacteria (g dry wt faeces)−1]. In faecal slurries, however, denitrification was a relatively minor route of NO− 3 dissimilation, since only about 3% of the NO− 3 was converted to gaseous products, with NO− 3 being mainly reduced to NO− 2 and NH+ 4. When KNO2 was added to the slurries, denitrification became quantitatively more significant with approximately 23% of the NO− 2 being lost as gaseous products. The addition of KNO3 (10 mM) to slurries containing either starch or casein significantly decreased H2 and CH4 production. The effect of NO− 3 on methanogenesis was twofold: firstly, H2 accumulation decreased due to diversion of electrons towards NO− 3/NO− 2 reduction, and as a result of H2 being used as an electron donor for NO− 3 reduction, resulting in the removal of the methanogenic substrate; secondly, there was direct inhibition of methane-producing bacteria by NO− 3 and NO− 2. In starch-containing slurries, acetate: butyrate molar ratios were increased when NO− 3 was added but this effect was not observed when casein replaced starch. These results show that the ability of NO− 3/NO− 2 to act as an electron sink can significantly influence the major products of the human colonic fermentation.