SUMMARY: Four different carotenoids were isolated and purified from membranes of the N-fixing bacterium strain Cd, grown under aerobic conditions, and one from strain Cd-1. Carotenoid synthesis did not occur under microaerobic conditions. Cells of these strains formed aggregates (>0-1 mm in diameter) when subjected to aerobic conditions. Neither carotenoids nor cell aggregation were observed in strains Sp 7, Sp81 and Sp51e. All of the differently pigmented strains tested contained similar amounts of soluble cytochrome In the presence of diphenylamine, which specifically inhibits carotenoid synthesis, the rate of acetylene reduction (N fixation) in strain Cd decreased to 50% of the control. Carotenoid synthesis was inhibited in cells grown in the presence of NHCl at concentrations higher than 10 mM. Carotenoid synthesis started in liquid cultures of suain Cd only after the concentration of NHCl in the medium decreased, and N fixation became evident at the same time. In comparison, strain Sp81 did not grow or fix nitrogen after NHCl was depleted. Carotenoids appear to protect the nitrogenase of strains Cd and Cd-1 from oxidative damage.


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