Anoxygenic Photosynthetic Electron Transport

This chapter describes experimental conditions for assaying anoxygenic photosynthetic reactions in intact cells as well as in membrane preparations of O. limnetica. The cyanobacteria are unique among the phototrophic prokaryotes in their ability to carry out complete plant-type, water-splitting, O₂-evolving photosynthesis utilizing both photosystems, photosystem I (PSI) and photosystem II (PSII). Many cyanobacterial strains, however, can also carry out anoxygenic photosynthesis, using PSI only. In this process, after a short induction period (2-3 hr), sulfide electrons flow freely through the electron transport chain to reduce one of the following three acceptors: (1) CO₂, (2) protons, to produce H₂ gas (when CO₂ is absent or when CO₂ fixation is otherwise inhibited), or (3) N₂, in a process of N₂ fixation induced by the absence of combined nitrogen. Anoxygenic photosynthesis is induced in O. limnetica in a process that requires sulfide, light, and protein synthesis. The capacity of an organism to fix CO₂ anoxygenically with sulfide electrons does not necessarily imply ability to actually grow anaerobically under these conditions. Such ability depends on the indispensibility of O₂ for the organism's metabolism and its tolerance of low redox potentials.