Nitrogen fixation and nitrogen metabolism in heliobacteria

Abstract

Three species of anoxygenic phototrophic heliobacteria, Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis, were studied for comparative nitrogen-fixing abilities and regulation of nitrogenase. Significant nitrogenase activity (acetylene reduction) was detected in all species grown photoheterotrophically on N2, although cells of H. mobilis consistently had higher nitrogenase activity than did cells of either H. chlorum or H. gestii. Nitrogen-fixing cultures of all three species of heliobacteria were subject to “switch-off” of nitrogenase activity by ammonia; glutamine also served to “switch-off” nitrogenase activity but only in cells of H. mobilis and H. gestii. Placing photosynthetically grown heliobacterial cultures in darkness also served to “switch-off” nitrogenase activity. Dark-mediated “switch-off” was complete in lactate-grown heliobacteria but in pyruvate-grown cells substantial rates of nitrogenase activity continued in darkness. In all heliobacteria examined ammonia was assimilated primarily through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway although significant levels of alanine dehydrogenase were present in extracts of cells of H. gestii, but not in the other species. The results suggest that heliobacteria, like phototrophic purple bacteria, are active N2-fixing bacteria and that despite their gram-positive phylogenetic roots, heliobacteria retain the capacity to control nitrogenase activity by a “switch-off” type of mechanism. Because of their ability to fix N2 both photosynthetically and in darkness, it is possible that heliobacteria are significant contributors of fixed nitrogen in their paddy soil habitat.