The marine green sulfur bacterium Chlorobium vibrioforme strain 1930 produced H2 and elemental sulfur from sulfide or thiosulfate under N limitation in the light. H2 production depended on nitrogenase and occurred only in the absence of ammonia. Methionine sulfoximine, an inhibitor of glutamine synthetase, prevented the switch-off by ammonia. In defined syntrophic cocultures of the acetate-oxidizing, sulfur-reducing bacterium Desulfuromonas acetoxidans with green sulfur bacteria, H2 was produced from acetate via a light-driven sulfur cycle. The sulfur-reducing bacterium could not be replaced by sulfate-reducing bacteria in these experiments. In a coculture of the marine Chlorobium vibrioforme strain 1930 and the sulfur-reducing bacterium Desulfuromonas acetoxidans strain 5071, optimum long-term H2 production from acetate was obtained with molecular nitrogen as N source, at low light intensity (110 μmol · m-2 · s-1), in sulfide-reduced mineral medium (2 mM Na2S) at pH 6.8. Traces of sulfide (10 μM) were sufficient to keep the sulfur cycle running. The coculture formed no poly-β-hydroxyalkanoates (PHA), but 20%–40% polysaccharide per cell dry mass. Per mol acetate added, the coculture formed 3.1 mol of H2 (78% of the theoretical maximum). Only 8% of the reducing equivalents was incorporated into biomass. The maximum rate of H2 production was 1300 ml H2 per day and g cell dry mass.