Photosynthetic Proteobacteria

Photosynthetic Proteobacteria

The purple sulphur and purple non-sulphur bacteria are the only members of the Pro-teobacteria to have retained the photosynthetic ability of their presumed ancestor. The type of photosynthesis they carry out, however, is quite distinct from that carried out by plants, algae and cyanobacteria, differing in two important respects:

·              it is anoxygenic – no oxygen is produced by the process

·              it utilizes bacteriochlorophyll a and/or b, which have different absorbance prop-erties to chlorophylls a and b.

Like organisms that carry out green photosynthesis, however, they incorporate CO2 into carbohydrate by means of the Calvin cycle . 

All are at least facultatively anaerobic, and are typically found in sediments of stagnant lakes and salt marsh pools, where they may form extensive coloured blooms. Because the absorption spectrum of bacteriochlorophylls lies mostly in the infrared part of the spectrum, they are able to utilise light energy that penetrates beyond the surface layers of water.

The coloration, ranging from orange/brown to purple, is due to the presence of carotenoid pigments such as lycopene and spirillixanthin, which mask the blue/green colour of the bacteriochlorophylls. The photosynthetic pigments are located on highly folded extensions of the plasma membrane. Photosynthetic proteobacteria include rods, cocci and spiral forms.

Under anaerobic conditions, the purple sulphur bacteria typically utilise hydrogen sulphide or elemental sulphur as an electron donor for the reduction of CO2.

H2S +CO2−−−−−−−→  (CH2O)n+ S0

S0+CO2+  H2O  âˆ’−−−−−−→  (CH2O)n+  H2SO4

Many store sulphur in the form of intracellular granules. The purple sulphur bacteria all belong to the Î³ -Protobacteria. They are typically found in surface muds, and sulphur springs, habitats that provide the right combination of light and anaerobic conditions.

Representative genera: Thiospirillum, Chromatium

The purple non-sulphur bacteria were distinguished from the above group because of their apparent inability to use H2S as an electron donor. It is now known, however, that the majority can do this, but are able to tolerate much lower concentrations in comparison with the purple sulphur bacteria. The purple non-sulphur bacteria are fac-ultative anaerobes able to grow as photoheterotrophs, that is, with light as an energy source and a range of organic molecules such as carbohydrates and organic acids as sources of both carbon and electrons. In addition, many are able to grow aerobically as chemoheterotrophs in the absence of light. Under present classification systems, purple nonsulphur bacteria are divided between the Î±- and Î²-Proteobacteria.

Representative genera: Rhodospirillum, Rhodopseudomonas