We have already come across three distinct groups of photosynthetic bacteria in this chapter: the purple sulphur and purple nonsulphur bacteria and the Cyanobacteria: here we consider the remaining two groups, the green sulphur and green non-sulphur bacteria.
The green sulphur bacteria (phylum Chlorobi), like their purple counterparts (see above), are anaerobic photolithotrophs that utilise reduced sulphur compounds instead of water as an electron donor, and generate elemental sulphur. They differ, however, in a number of respects. The sulphur is deposited outside the cell, and CO2 is assimilated not by the Calvin cycle, but by a reversal of the steps of the TCA cycle (see Chapter 6). The photosynthetic pigments in the green sulphur bacteria are contained in sac-like structures called chlorosomes that are associated with the inside of the plasma membrane.
Most members of the green non-sulphur bacteria (phylum Chloroflexi) are filamentous thermophiles, living in non-acid hot springs, where they form thick bacterial mats. Like the purple non-sulphur bacteria, they are photoheterotrophs, but can also grow in the dark as chemoheterotrophs.
Representative genera: Chlorobium (green sulphur), Chloroflexus (green non-sulphur).
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