Photosynthesis is synthesis of carbohydrate (food) from carbon dioxide in presence of light. Such 'converted' carbon dioxide or carbohydrate formed is then used for metabolism. In short, it is light dependent energy yielding process. Bacteria that utilize light energy in nutrition are phototrophic and hence photosynthetic. Photosynthesis in plants, algae and cyanobacteria is similar to bacterial photosynthesis in requirement for large amount of energy in the form of ATP (Adenosine Tri-phosphate) but different with respect to form of chemical reductants and resultant end products of photosynthesis. Water is chemical reductant of plants, algae and cyanobacteria. Inorganic compounds such as H2 or H2S and organic compounds lactate, succinate or malate are reductants of bacterial photosynthesis. Phototrophic bacteria using inorganic and organic chemical reductants are known as photolithotrophs and photoorganotrophs respectively. Bacterial photosynthesis is anoxygenic means end product or oxidation product is not oxygen like that in plants, algae and cyanobacteria.
Photosynthetic bacteria: Principle groups of phototrophic bacteria are: Purple sulfur bacteria (Chromatium sp.), Purple non-sulfur bacteria (Rhodospirillum rubrum) and Green bacteria (Chlorobium sp.). Photosynthesis in prokaryotic blue green algae or cyanobacteria resembles plants and higher algae regarding photosystem, pigments, reductants and end products.
Bacterial photosynthetic pigments: Bacteriochlorophyll is the principle light harvesting pigment of photobacteria. It is present as a, b, c, d or e types. It is different from plant chlorophyll in structure and light absorbing properties. Bacteriochlorophyll absorb light in infrared region (wavelength of 725-1035nm). They are not contained in chloroplasts instead scattered in cytoplasm and cell membrane system. Bacteria also contain carotenoids and other accessory pigments which absorb light of shorter wavelength and transfer energy to bacteriochlorophyll.
Process of bacterial photosynthesis: Bacterial photosynthesis is based on cyclic photophosphorylation mechanism and only one pigment system (PS-I) is involved. During the process, bacteriochlorophyll absorbs light and this light energy raises the chlorophyll molecule to an excited state. Excited bacteriochlorophyll gives off an electron and becomes positively charged. It serves as a strong oxidising agent and electron acceptor. Some of the light energy is carried successively to electron transport system via electron. The first energy receiver is ferredoxin followed by ubiquinone, cytochrome b and to cytochrome f and finally back to excited bacteriochlorophyll. An electron thus completes the cycle of energy transfer beginning with and returning to bacteriochlorophyll, hence it is called cyclic photophosphorylation. Energy in the form of ATP is generated from ADP (Adenosine Di-phosphate) and inorganic phosphate, in the step between cytochrome b and cytochrome f. In photobacteria, photosynthetic reducing power for generation of ATP is obtained from inorganic and organic compounds as stated earlier and not by photolysis of water or reduction of NADP+ (Nicotinamide Adenine Di-phosphate) reduction.
Importance of bacterial photosynthesis: The most important usefulness of photobacteria is in analysis of evolution of photosynthetic systems. Since all photosynthetic bacteria still possess ancient arrangement and structure of their photosynthetic apparatus. It also gives an evolutionary evidence for origin of chloroplasts. The use of chemical reductants other than water by photosynthetic bacteria is a strong geological evidence to support the theory of ancient reducing atmosphere on the Earth. Genetic approaches involving mutational analysis and directed mutagenesis are very useful to study photosynthetic reaction centers, electron transfer mechanisms and gene arrangements; because this knowledge about plant photosynthesis is still in infancy. Photobacteria could have multiple biotechnological applications such as production (also the overproduction if necessary) of enzymes and pharmaceuticals for the simplest reason that no carbon source needs to be added in their growth medium. Photosynthetic bacteria find potential application in bioremediation of polluted aquatic environments since they can grow and utilize toxic substances like H2S or H2S2O3. The ongoing research is to use these bacteria to produce clean fuels using light energy in the process of photosynthesis.
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