Metronidazole as a specific inhibitor of nitrogenase activity in the hetero-cystous cyanobacterium Nostoc ANTH, an isolate from Anthoceros punctatus

The effect of metronidazole on growth and nitrogen metabolism of the N₂-fixing heterocystous cyanobacterium Nostoc ANTH and its paraquat-resistant (PQ-R), chlorate-resistant (Clo-R) and Hef Nif mutants was studied. Metronidazole inhibited the diazotrophic growth of Nostoc ANTH and its PQ-R and Clo-R mutants when grown in the absence of combined nitrogen. The drug inhibited nitrogenase activity completely within 3h of treatment but did not affect the activities of nitrate reductase and glutamine synthetase, and ammonium transport. All the strains grew well in presence of metronidazole when the growth medium was supplemented with sources of combined nitrogen. The Hef Nif strain, which lacked heterocysts and was unable to fix N₂ remained unaffected by metronidazole in medium supplemented with combined nitrogen. The inhibition of diazotrophic growth by metronidazole was ascribed to the impairment of nitrogenase activity.