CARBON METABOLISM OF AZOSPIRILLUM, AN ASSOCIATIVE NITROGEN-FIXING MICROORGANISM

The carbon metabolism of Azospirillum was studied through kinetics, Warburg respirometry and radioisotopic tracing of intermediary metabolites. In defined AZO medium, the doubling time for the heterotrophic growth of Azospirillum brasilense Sp 7 with succinate as the sole carbon source was 43 min. No growth could be measured with glucose as the sole carbon source. The doubling time for A. lipoferum Sp 59b growing on succinate and glucose was 69 min and 223 min respectively.;Radioisotopic labelling indicates that only 5% of the available glucose was assimilated into Azospirillum brasilense Sp 7, and only 1% was metabolized to carbon dioxide. The low respiratory rate measured in AZO medium containing glucose and the inability of this strain to grow on this medium may be due to a permeability barrier. The addition of an alternative energy source did not increase glucose uptake. Radiorespirometric measurements indicated the presence of the Embden-Meyerhof-Parnas pathway as the primary mode of glucose catabolism.;The distribution of radioactivity from differentially labelled TCA intermediates metabolized by A. brasilense Sp 7 indicates a functional TCA cycle and high anabolic activity. At low energy states, TCA intermediates are diverted toward lipid synthesis through either the malic enzyme or phosphoenolpyruvate carboxykinase activity. Both pyruvate carboxylase and pyruvate dehydrogenase activities were demonstrated along with the strong possibility of a glyoxylate cycle. The presence of a weak carbon fixation activity was shown by incorporation of dissolved labelled carbon dioxide.;Warburg respirometry measurements indicated that the optimal rate of oxygen consumption by A. brasilense Sp 7 was 0.14 uM per min per mg cell protein at 0.20 M succinate. This strain could respire glucose as well. The optimal rate of respiration was measured at 0.2 M glucose to be 0.034 O(,2)/min/mg. The optimal rate of oxygen consumption by A. lipoferum Sp 59b in 0.08 M succinate was 0.15 uM O(,2)/min/mg. Very rapid oxygen consumption was also measured in 0.08 M glucose, with an optimal rate of 0.13 uM O(,2)/min/mg.