Chapter 2 Photosynthetic bacteria

The high level of diversity among photosynthetic bacteria, including cyanobacteria, stands in contrast to the relative uniformity found among the chloroplasts of photosynthetic eukaryotes. Photosynthetic bacteria convert light energy into chemical free energy. Most of these bacteria belong to the five eubacterial groups, but some, the halobacteria, belong to the archaebacteria. Although cyanobacteria are certainly photosynthetic eubacteria, they have unique and important ability to evolve oxygen. All photosynthetic eubacteria contain photochemical reaction centers (RC), containing one or more chlorophyll molecules. Each reaction center consists of a primary electron donor P (bacteriochlorophyll), an initial electron acceptor I (bacteriochlorophyll or bacteriopheophytin), and one or more secondary acceptors (Fe-S centers, quinones). Sometimes a secondary electron donor D (Cyt c ) is tightly bound to the RC. The absorption cross-section of a single RC is so small that it cannot trap light fast enough to drive the organism's electron transport system up to capacity. Light-harvesting complexes (LHC) exist to enlarge the effective absorption cross-section for each RC. Filamentous photosynthetic bacteria are anoxygenic phototrophic bacteria that are grouped together on the basis of their distinctive filamentous morphology. Green sulfur bacteria are small unicellular organisms. While they may form dense planktonic blooms in the anaerobic zones of both freshwater and saline aquatic habitats, they rarely form the dense mat layers typical of ChlorofEexus.