Hydrogen metabolism in isolated heterocysts of Anabaena 7120

Isolated heterocysts of Anabaena 7120 evolve H₂ in an ATP-dependent nitrogenase-catalyzed process that is inhibited by N₂ and C₂H₂. Heterocysts have an active uptake hydrogenase that only requires an electron acceptor of positive redox potential, e.g., methylene blue, dichlorophenolindophenol or potassium ferricyanide. O₂ supplied at low partial pressures is a very effective physiological oxidant for H₂ uptake. High concentrations of O₂ are inhibitory to H₂ uptake. The oxyhydrogen reaction in heterocysts appears to be mediated by a cytochrome-cytochrome oxidase system, and it supports ATP synthesis via oxidative phosphorylation. Attempts to demonstrate acetylene reduction in isolated heterocysts employing H₂ as an electron donor were unsuccessful. It is suggested that the uptake hydrogenase functions to conserve reductant that otherwise would be dissipated via nitrogenase-catalyzed H₂ evolution.