Cyanobacteria consume H by two different pathways: the oxyhydrogen reaction and anaerobic, light-dependent H utilization. The two pathways are shown here to be induced differently by incubating cyanobacteria anaerobically under H. In the unicellular and in N and NH grown and , such treatment greatly enhances the activity of the oxyhydrogen reaction in all cell types. In contrast, the light-dependent pathway, determined by the Hdependent photoreduction of NADP, is demonstrable with higher activity only in heterocysts.

Whereas the activity of the oxyhydrogen reaction is directly correlated to the structural integrity of membranes, there is an inverse correlation between membrane integrity and H formation catalysed by hydrogenase. These findings, together with physiological considerations, suggest that a ‘reversible’ soluble hydrogenase does not exist in photoautotrophic cyanobacteria. No definite conclusions about the existence of two membrane-bound uptake hydrogenases are possible at present.


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