An Asymbiotic Nitrogen-Fixing Bacterium from the Root Environment of Corn

Examination of root systems and adhering soil associated with unusually green corn plants from nitrogen-deficient areas in fields has revealed higher acetylene-reducing activities than comparable root systems of chlorotic plants that appeared to be nitrogen deficient. From the root systems with acetylene-reducing activity, N(2)-fixing Enterobacter cloacae were isolated. Pure cultures of the organism grown in a medium lacking added fixed nitrogen reduced acetylene to ethylene and used N(2) as a sole source of nitrogen under anaerobic conditions. Acetylene reduction was strikingly inhibited by 0.04 atm or greater partial pressures of O(2); however, the bacteria maintained appreciable acetylene-reducing rates in medium exposed to partial pressures of O(2) ranging from 0.005 to 0.015 atm. Nitrogenase activity in cell-free extracts of the bacterium was dependent upon Na(2)S(2)O(4) and an ATP-generating system. Some environmental conditions expected to influence N(2) fixation by free-living N(2)-fixing bacteria on root surfaces of nonlegumes are discussed.