Summary: Carotenoid-overproducing mutants of 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 nitrogen-deficient 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 μm 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 under conditions of oxygen stress, but did not extend the optimum pO for nitrogen fixation to higher oxygen concentrations.


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