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Abstract
Four different carotenoids were isolated and purified from membranes of the N2-fixing bacterium Azospirillum brasilense 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 A. brasilense strains Sp 7, Sp81 and Sp51e. All of the differently pigmented Azospirillum strains tested contained similar amounts of soluble cytochrome c. In the presence of diphenylamine, which specifically inhibits carotenoid synthesis, the rate of acetylene reduction (N2 fixation) in strain Cd decreased to 50% of the control. Carotenoid synthesis was inhibited in cells grown in the presence of NH4Cl at concentrations higher than 10 mm. Carotenoid synthesis started in liquid cultures of suain Cd only after the concentration of NH4Cl in the medium decreased, and N2 fixation became evident at the same time. In comparison, strain Sp81 did not grow or fix nitrogen after NH4Cl was depleted. Carotenoids appear to protect the nitrogenase of A. brasilense strains Cd and Cd-1 from oxidative damage.
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