Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 40

Figure2.11 Reactionsofphotosynthesis
sulphide, which is the principal gas responsible for the foul smell, reminiscent of
rotten eggs, typically found in wet and untilled soil, for example in the bottom
of ponds, the product is sulphate or elemental sulphur. Examination of bacterial
photosystems explains how this occurs.
Green and purple bacteria possess only one photosystem which is a fairly basic
equivalent to photosystem 1 of eukaryotes and cyanobacteria, but employing a
different set of electron carriers. In purple nonsulphur bacteria, this is only capable
of cyclic electron flow which produces a proton gradient and thus allows ATP
photosynthesis, but this process does not lead to NADPH production. A similar
system exists in green nonsulphur bacteria. The lack of a photosystem equivalent
to photosystem 2 in eukaryotes, requires these bacteria to provide a different
route for the regeneration of NADH which serves much the same function as
NADPH in carbohydrate synthesis. Their solution to this problem is to use as