Carotenoid-overproducing mutants of Azospirillum brasilense Sp7, which contained about 100 times more carotenoids than the wild-type, were obtained after nitrosoguanidine mutagenesis. Growth studies with one of these mutants in oxygen-controlled batch and continuous cultures revealed a slightly improved oxygen tolerance of nitrogen fixation in the mutant as compared to the wild-type. The production of carotenoids was greatly enhanced by increasing the oxygen concentration under

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... t conditions. Although nitrogen fixation was severely inhibited by increased oxygen concentrations, in both the mutant and the wild-type, the mutant showed significantly greater efficiency of nitrogen fixation at 12 PM dissolved oxygen, and it fixed five times more total nitrogen than the wild-type under these conditions. In conclusion, high levels of carotenoids slightly enhanced the oxygen tolerance of Azospirillum brasilense under conditions of oxygen stress, but did not extend the optimum p 0 2 for nitrogen fixation to higher oxygen concentrations.