Na2S is known to support anoxygenic photosynthesis in some strains of cyanobacteria and to stimulate H2 production in N2 fixing filaments of Nostoc muscorum. We have shown electron transfer between Na2S and Photosystem I to be dependent on cytochrome b559 which was detected only in vegetative cells. An electron mediator was required to support Na2S driven nitrogenase activity in isolated heterocysts. Na2S was also found to deplete the ATP pool, probably by inhibiting electron transfer from Photosystem I.
