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Types of Bacterial Photosynthesis

Types of Bacterial Photosynthesis - Bacteria which use the radiant energy of light in nutrition arc called phototrophs, while those utilizing chemical bond energy are called chemotrophs.

Phototrophs normally utilize CO2 as the source of carbon. The energy required for the reduction of CO2 during its assimilation may be obtained from inorganic or organic substrates.

Metabolism in which inorganic substances like H2S or thiosulphate are the sources of reducing power is called photolithotrophy. When the oxidation of organic compounds like malate and succinate yields reducing power, the process is called photoorganotrophy.
The general equation for photosynthesis is : 2H2A + CO2-->C(H2O) + H2O + 2A

In green plants growing aerobically 10 light H2A is H2O. In photosynthetic bacteria that are obligatory anaerobes H2A is hydrogen sulphide (H.S) or organic molecules. In the Chromatiaceae and Chlorobiaceae the reducing substrate for replacing electrons are hydrogen sulphide or, hlosulphate.

In the Rhodospirillaceae the substrates are organic molecules like malate or succinate. Many members of the Rhodospirillaceae can grow in the dark.
They apparently obtain their energy by electron, transport dependent phosphorylation linked to oxidation of organic molecules. An outline of the different types of reactions in bacterial photosynthesis is given below:

(i) Purple sulphur bacteria, e.g. Chromatium, utilize H2S as the reducing substrate instead of H2O in photosynthesis. Sulphur(S) is produced instead of oxygen.
CO2 + 2 H2S -->C(H2O) + 2 S + H2O
(ii) Purple sulphur bacteria can also use thiosulphate as a reductant.
2 CO2 + Na2S2O3 + 5 H2O -->2 C(H2O) + 2 H2O -I- 2 NaHSO4

(iii) In non-sulphur bacteria like Rhodospirillum rubrum, the electron donor & are organic compounds like ethanol, isopropanol or succinate.
CO2 + 3C2H5OH-->C(H2O) + CH3CHO + H2O

It will be seen that in green plants there is evolution of oxygen during photosynthesis, while in bacteria no oxygen is evolved.
The cyanobacteria differ from purple and green bacteria in the nature of their photosynthetic pigment system and in their ability for oxygenic photosynthesis

The purple and green bacteria carry out anoxygenic photosynthesis, i.e. there is no evolution of oxygen. There is only one photosystem involved in photosynthesis. The electron donors are sulphur, reduced sulphur compounds, molecular hydrogen or simple organic compounds.

These are substances with lower redox potentials than water. It should be noted that even in cyanobacteria there may be anoxygenic photosynthesis with only one photosystem when H2S is the electron donor.
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