Utilization of aromatic compounds as carbon and energy sources during growth and N2-fixation by free-living nitrogen fixing bacteria

Six species of free-living nitrogen fixing bacteria, Azomonas agilis, Azospirillum brasilense, Azospirillum lipoferum, Azotobacter chroococcum, Azotobacter vinelandii, and Beijerinckia mobilis, were surveyed for their ability to grow and fix N₂ using aromatic compounds as sole carbon and energy source. All six species grew and expressed nitrogenase activity on benzoate, catechol, 4-hydroxybenzoate, naphthalene, protocatechuate, and 4-toluate. In many cases, growth rates on one or more aromatic compounds were comparable to or greater than those on the non-aromatic substrates routinely used for cultivation of the organisms. Specific activity of nitrogenase in extracts of aromatic-grown cells often exceeded that in cells grown on non-aromatic substrates. All six species growing on substrates typically converted to catechol expressed inducible catechol 1,2-dioxygenase and/or catechol 2,3-dioxygenase. When grown on substrates typically converted to protocatechuate, inducible protocatechuate 3,4-dioxygenase and/or protocatechuate 4,5-dioxygenase was expressed. A. chroococcum expressed only ortho cleavage dioxygenases during growth on naphthalene and 4-toluate and only meta cleavage dioxygenases on the other aromatics. B. mobilis expressed only ortho cleavage dioxygenases. The other four species examined expressed both ortho and meta cleavage enzymes.A preliminary account of this work was presented at the 91st General Meeting of the American Society for Microbiology, Dallas, TX, 1991