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