The differential action of metronidazole on nitrogen fixation, hydrogen metabolism, photosynthesis and respiration in Anabaena and Scenedesmus.

Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) at 1--2 mM levels has been shown to be a selective inhibitor of nitrogenase activity in Anabanena. Two constitutive hydrogenases and photosynthesis are insensitive to metronidazole at these same concentrations. At higher concentrations metronidazole inhibits photosynthesis in Anabaena while photoreduction and to a lesser extent photohydrogen production are retarded in Scenedesmus. Respiration is slightly stimulated at high metronidazole levels in both algae. The reductant source for nitrogenase in Anabaena and photohydrogen production and photoreduction electron transport in Scenedesmus are discussed. Due to the activity to metronidazole as a selective inhibitor of ferredoxin-associated processes, it should prove to be useful in N2 fixation studies and in distinguishing between ferredoxin-linked reactions of different sensitivities and other activities not associated with low reduction potential components.