GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

18. PHOTOTROPHIC BACTERIA AND PHOTOSYNTHESIS

18.3. PHOTOSYNTHESIS PROCESSES

18.3.2. Anoxygenic photosynthesis

Anoxygenic Photosynthesis involves a single photoreaction that Supports Cyclic electron transport. To replenish the cycle with electrons, external electron Donors are required. These include hydrogen sulfide, sulfur, thiosulfate, Organic compounds (such as malate and succinate), and molecular hydrogen. Although analogous to the first photoreaction in green plants, this photoreaction results solely in the generation of a proton motive force and ATP synthesis, rather than the reduction of NAD (as it lacks the non-cyclic electron flow responsible for NAD reduction). In anaerobic phototrophs, NADH is formed via Reverse Electron Transport, which consumes ATP (analogously to aerobic chemolithoautotrophs). This type of photosynthesis is characteristic of purple phototrophic Bacteria.

During the photoreaction in green bacteria, the primary electron acceptor has a Redox Potential of approximately -0.5 V (compared to only -0.1 V in purple bacteria!). Such a high negative potential enables Electron transfer from the primary electron acceptor to reduce ferredoxin and subsequently NAD. Consequently, reducing equivalents in green photobacteria are generated directly by the photoreaction rather than through ATP-consuming reverse electron transport (as seen in purple photobacteria). From an evolutionary perspective, the photosynthesis of green bacteria may represent a missing link connecting the photosynthesis of purple bacteria with that of cyanobacteria and green plants.