Summary: Nine strains of unicellular non-nitrogen-fixing cyanobacteria had high rates of hydrogen evolution and uptake compared with filamentous nitrogen-fixing strains. Maximal hydrogen uptake activities were observed in assays including oxygen under both light and dark conditions suggesting the participation of the ‘Knallgas’ reaction. Nitrogenase activity, measured as acetylene reduction, was not found in aerobic cultures of any of the unicellular strains tested, and the capacity to synthesize nitrogenase under anaerobic conditions was absent in all five of the strains tested under these conditions. Thus the capacity to synthesize nitrogenase is not a prerequisite for hydrogenase synthesis and hydrogen evolution by these unicellular cyanobacteria. Rates of hydrogen evolution and uptake by six of the unicellular strains were increased following adaptation with CO/H gas mixtures.


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