The Number of Hydrogenases in Cyanobacteria

Cyanobacteria consume H2 by two different pathways: the oxyhydrogen reaction and anaerobic, light-dependent H2 utilization. The two pathways are shown here to be induced differently by incubating cyanobacteria anaerobically under H2.Inthe unicellular Anacystis nidulans and in N2 − and NH+ 4-grown Anabaena cylindrica and Nostoc muscorum,such treatment greatly enhances the activity of the oxyhydrogen reaction in all cell types. In contrast, the light-dependent pathway, determined by the H2-dependent photoreduction of NADP+, is demonstrable with higher activity only in heterocysts. Whereas the activity of the oxyhydrogen reaction isdirectly correlated to the structural integrity of membranes, there is an inverse correlation between membrane integrity and H2 formation catalysed by hydrogenase. These findings, together withphysiological 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.