Dinitrogen fixation by a thermophilic methanogenic bacterium

Abstract

Methanogenic bacteria are known to use NH⁺₄ as a nitrogen source for growth¹. Previous work with an impure methanogenic culture suggested that a methanogen might fix atmospheric dinitrogen as a nitrogen source², but no further work on this phenomenon has been documented. We have now examined the use of N₂ by Methanococcus thermolithotrophicus and find that the organism can grow well, with multiple transfers, in medium having N₂ as the source of nitrogen. Control cultures without N₂ and containing less than 0.1 mM NH⁺₄ do not grow. Growth yields with N₂ are on the average one-third those with NH⁺₄, suggesting that, as in other nitrogen-fixing organisms, this bacterium requires a large amount of ATP for the reduction to occur. After growing in NH⁺₄-containing medium, a long lag is observed before growth begins with N₂ as the nitrogen source; the NH⁺₄ levels must be very low for growth to begin. Cells grown in N₂-fixing conditions reduce acetylene to ethylene. The discovery of a nitrogen-fixing archaebacterium has important implications for studies on the evolution of nitrogenase, and the fact that M. thermolithotrophicus nitrogenase is active at 64 °C suggests that a novel enzyme is involved.