Occurrence of facultative anoxygenic photosynthesis among filamentous and unicellular cyanobacteria

Eleven of 21 cyanobacteria strains examined are capable of facultative anoxygenic photosynthesis, as shown by their ability to photoassimilate CO2 in the presence of Na2S, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 703-nm light. These include different cyanobacterial types (filamentous and unicellular) of different growth histories (aerobic, anaerobic, and marine and freshwater). Oscillatoria limnetica, Aphanothece halophytica (7418), and Lyngbya (7104) have different optimal concentrations of Na2S permitting CO2 photoassimilation, above which the rate decreases: 3.5, 0.7, and 0.1 mM, respectively. In A. halophytica, for each CO2 molecule photoassimilated two sulfide molecules are oxidized to elemental sulfur, which is excreted from the cells. The ecological and evolutionary significance of anoxygenic photosynthesis in the cyanobacteria is discussed.