Phylum Chlorobi (green sulphur bacteria) and phylum Chloroflexi (green non-sulphur bacteria)

Phylum Chlorobi (green sulphur bacteria) and phylum Chloroflexi (green non-sulphur bacteria)

We have already come across three distinct groups of photosynthetic bacteria: 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 , 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. 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 filamen-tous 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).