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At present (2015) seven distinct groups of photosynthetic bacteria are recognized. All but one contain bacteriochlorophyll (Bchl), one or more of Bchl a through Bchl g, and are anoxygenic. The only oxygenic group has chlorophyll (Chl) instead of Bchl, and is oxygenic (Cyanobacteria). Green sulfur bacteria, e.g. Chlorobium and Pelodictyon, are strict anaerobes found only in anoxic habitats such as the hypolimnia of mesotrophic to eutrophic lakes, and subsurface layers of salt marsh mats. Purple sulfur bacteria such as Chromatium are restricted to low concentrations of dissolved oxygen (microaerobes) and anoxic habitats. They often lie above the green sulfurs in microlayers of lakes and salt marsh mats. Purple nonsulfur bacteria such as Rhodospirillum, Rhodobacter and several others are diverse in their nutrition and habitats They are mainly anaerobic photoheterotrophs that survive under at low light intensity with several organic substrates. Some ferment. Many are also photoautotrophs or chemoheterotrophs that grow in microaerobic or even aerobic conditions. Some species switch modes of metabolism das a result of habitat conditions. One of another species occupies various diverse habitats include freshwater and marine aquatic habitats, also soils, sewage lagoons and plants. Acidobacteria, first recognized in 1997, are soil dwellers that was described in 2007. Some are acidophlic such as Choroacidobacterium. Heliobacteria were discovered by a field class at Indiana University in 1981 given the name Heliobacterium chlorum. They live in soils and contain Bchl g, and lack intracytoplasmic memberanes so that all their pigments are in the cytoplasmic membrane. They are strict anaerobes and nitrogen fixers. Green and red filamentous bacteria, formerly known as green non-sulfurs, are anaerobes that can be photoautotrophs, chemoorganotrophs or photoheterotrophs. Cyanobacteria are the first known oxygenic photosynthesizers and contain Chl a along with two water-soluble ‘phycobilin’ pigments’ = phycocyanin, a blue pigment, and phycoerithrin, a red pigment. the phycobilins absorb in the light spectrum where chlorophyll a is least efficient, thus increase the light harvesting ability of cyanobacteria at low light intensity. Many if not all cyanobacteria are capable of switching between anoxygenic and oxygenic photosynthesis. |
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Distribution of bacteriochlorophylls among the groups of photosynthetic bacteria: |
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References |
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