Growth of Spirillum Lipoferum At Constant Partial Pressures of Oxygen, and the Properties of its Nitrogenase in Cell-free Extracts

Abstract

SUMMARY: Spirillum lipoferum, an N₂-fixing organism, was grown at constant concentrations of dissolved O_2. When supplied with NH₄+ aerobically, its doubling time was 1 h; when it fixed N₂ microaerophilically, its doubling time was 5.5 to 7 h and the optimal P_o2 for growth was 0.005 to 0.007 atm. At its optimal P^o2 for growth on N₂, S. lipofernum assimilated 8 to 10 mg nitrogen/g carbon substrate used; its efficiency was less at higher P_o2 levels. Nitrogenase in cell-free extracts required Mg²⁺ and Mn²⁺, and the Fe-protein was activated by Rhodospirillum rubrum activating factor. The nitrogenase had an optimal pH of 7.1 to 7.4 and an apparent K_m for acetylene of 0.0036 atm. Extracts of S. lipoferum lost their nitrogenase activity on storage at -18 °, and activity was restored by adding purified Feprotein from other N₂-fixing bacteria.