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

Abstract

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.