The effect of glyoxylate on nitrogenase-catalyzed hydrogen formation in Anabaena cylindrica.

Photosynthetic bacteria, such as Rhodopseudomonas capsulata, when incubated in the light under conditions of nitrogen starvation, produce copious amounts of hydrogen gas via nitrogenase, presumably as a means of regulating their energy charge and/or redox balance and thereby preventing photooxidative damage until growth can begin again [1,2]. A similar role has been suggested for the hydrogenase of the green alga Chlorella [3]. Higher plants are now thought to use the reactions of the photorespiratory pathway for a similar purpose in conditions of stress [4,5]. Heterocystous cyanobacteria are capable of both hydrogen gas formation [6] and photorespiration [7,8], although they differ from higher plants in the nature of their further metabolism of glyoxylate [9]. Although there has been considerable controversy concerning the significance of photorespiratory reactions in cyanobacteria [ 10,l l] it now appears that they are not significant except in special circumstances, such as when the organisms are shifted from high to low CO2, when carbonic anhydrase levels are still low [12]; only under these conditions are significant amounts of glycolate excreted. Hence, reports of glyoxylate functioning to stimulate nitrogenase by inhibiting photorespiration [ 13,141 were of considerable interest. It seemed to us that if nitrogenase-mediated H2 formation and photorespiration were alternative or concomitant processes for relieving photoinhibitory effects under conditions of stress due to nitrogen limitation, then glyoxylate might well have an even more substantial effect on nitrogenase-mediated hydrogen gas formation in Here, we report that glyoxylate markedly stimulates hydrogen formation by A. cylindrica but that the stimulation is substantial under conditions where photorespiration would be negligible. The data suggest that glyoxylate stimulates nitrogenase, not as an inhibitor of photorespiration, but rather as an electron supply for nitrogenase in the heterocysts.