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Spirillum lipoferum, an N2-fixing organism, was grown at constant concentrations of dissolved O2. 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 PO2 for growth was 0-005 to 0-007 atm. At its optimal PO2 for growth on N2, S. lipoferum assimilated 8 to 10 mg nitrogen/g carbon substrate used; its efficiency was less at higher PO2 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 Km for acetylene of 0-0036 atm. Extracts of S. lipoferum lost their nitrogenase activity on storage at -18 degrees C, and activity was restored by adding purified Fe-protein from other N2-fixing bacteria.

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