Bacterial photosynthesis

               Bacterial photosynthesis

Photosynthesis: Photosynthesis is the process that converts solar energy into chemical energy which is used to reduce Co₂ and incorporate it into organic forms or PHOTOSYNTHESIS is the process whereby plants, algae, some bacteria,
use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO₂ and water).

•         Directly or indirectly, photosynthesis nourishes almost the entire living world.

•         Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes.

•         Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part).

Light reaction: In light reaction light energy is trapped and converted to chemical energy.

Calvin cycle/Dark Reaction: In dark reaction the chemical energy that is formed in light reaction is used to reduce or fix Co₂ and synthesize cell constitution.

Different types of Bacteria:

Phototrophic Bacteria: phototrophic bacteria are group of bacteria whose energy for growth is derived from sunlight and their source of carbon comes from carbon dioxide and organic carbon. there are two groups of phototrophic bacteria  i.e.anoxygenic phototrophic bacteria and oxygenic phototrophic bacteria.                            Chemoorganotrophic: Requiring an organic source of carbon and   metabolic energy.

Photoheterotrophic bacteria: They are organisms that use light for their energy and but cannot use carbon dioxide as their sole carbon source. They use organic compound (carbohydrates, fatty acids and alcohols) from the environment to satisfy their carbon requirement .Examples of photoheterotrophic organisms include purple non-sulfur bacteria, and heliobacteria.

Chemosynthetic Bacteria: Chemosynthetic bacteria are organisms that use inorganic molecules as a source of energy and convert them into organic substances. Chemosynthetic bacteria, unlike plants, obtain their energy from the oxidation of inorganic molecules, rather than photosynthesis. Chemosynthetic bacteria use inorganic molecules, such as ammonia, molecular hydrogen, sulfur, hydrogen sulfide and ferrous iron, to produce the organic compounds needed for their subsistence.

Difference between Plant Photosynthesis and Bacterial

Photosynthesis:

Plant Photosynthesis

·       Plants contain definite chloroplasts

·        In plants photosynthesis, the pigments involved are chlorophylls, caretonoids and phycobilins

·       It takes place at wavelengths between 400 to 700 mμ

·       The CO2 reductants is NADPH + H+

·       The electron donor in this is only H2O

·       Oxygen is evolved

·       Two pigment systems are involved

·       The reaction centre of PS I is P700 and of PS II is P673 or P680

·       Non cyclic photophosphorylation is dominant

·        Emerson’s enhancement effect occurs.

Bacterial Photosynthesis

·       Bacteria lack definite chloroplasts

·       In bacterial photosynthesis, the pigments involved are bacterio chlorophylls, bacterioviridin and open chain aliphatic carotenoids

·       It takes place at wavelengths above 700 mμ

·       The reductants is NADH + H+

·       The electron donors in this are H2S, inorganic compounds and reduced organic compounds

·       Oxygen is not evolved

·        It involves only one pigment system

·       The reaction centre is only P890

·       Cyclic photophosphorylation is dominant

·       Emerson’s enhancement effect occurs is not reported.

                     Photosynthesis in Bacteria

Certain micro organisms like bacteria during photosynthesis are able to reduce Carbon dioxide (CO2) into essential organic constituents in presence of light using H2S, hydrogen and other inorganic and organic redunctants  instead of water. In such bacterial photosynthesis  O2 evolved and the bacteria inhabitant anaerobic environment. Such bacteria are called photosynthetic bacteria and photosynthesis as bacterial photosynthesis.

Bacteria that contain bacteriochlorophyll do not use water as an electron donor and therefore do not produce oxygen. This is known as anoxygenic photosynthesis. Cyanobacteria perform photosynthesis using water as an electron donor in a similar manner to plants. This results in the production of oxygen and is known as oxygenic photosynthesis.

Classification of Photosynthetic Bacteria

·       Oxygenic Photosynthesis in Bacteria:

Ø Oxygenic photosynthetic bacteria perform photosynthesis in a similar manner to plants. They contain light-harvesting pigments, absorb carbon dioxide, and release oxygen. Cyanobacteria are photosynthetic prokaryotic organisms that evolve O2 (Bryant, 1994). Fossil evidence indicates that Cyanobacteria existed over 3 billion years ago and it is thought that they were the first oxygen evolving organisms on earth (Wilmotte, 1994).

Ø Cyanobacteria or Cyanophyta are the only form of oxygenic photosynthetic bacteria known to date. There are, however, several species of Cyanobacteria. They are often blue-green in color and are thought to have contributed to the biodiversity on Earth by helping to convert the Earth’s early oxygen-deficient atmosphere to an oxygen-rich environment. This transformation meant that most anaerobic organisms that thrived in the absence of oxygen eventually became extinct and new organisms that were dependent on oxygen began to emerge.

·       Anoxygenic Photosynthesis in Bacteria:

Anoxygenic photosynthetic bacteria differ from oxygenic organisms in that each species has only one type of reaction center (Blankenship et al., 1995). In some photosynthetic bacteria the reaction center is similar to photosystem II and in others it is similar to photosystem I. Anoxygenic photosynthetic bacteria consume carbon dioxide but do not release oxygen. These include Green and Purple bacteria as well as Filamentous Anoxygenic Phototrophs (FAPs), Phototrophic Acidobacteria, and Phototrophic Heliobacteria.

Ø Purple bacteria can be divided into two main types – the Chromatiaceae, which produce sulfur particles inside their cells, and the Ectothiorhodospiraceae, which produce sulphur particles outside their cells. They cannot photosynthesize in places that have an abundance of oxygen, so they are typically found in either stagnant water or hot sulfuric springs. Instead of using water to photosynthesize, like plants and cyanobacteria, purple sulfur bacteria use hydrogen sulfide as their reducing agent, which is why they give off sulfur rather than oxygen.

Ø Purple bacteria are probably the most widely studied photosynthetic bacteria, being used for all sorts of scientific

endeavors including theories on possible microbiological life on other planets.

Ø Purple non-sulfur bacteria do not release sulfur because instead of using hydrogen sulfide as its reducing agent, they use hydrogen. While these bacteria can tolerate small amounts of sulfur, they tolerate much less than purple or green sulfur bacteria, and too much hydrogen sulfide is toxic to them.

Ø Green sulfur bacteria generally do not move (non-motile), and can come in multiple shapes such as spheres, rods, and spirals. These bacteria have been found deep in the ocean.  They have also been found underwater near Indonesia. These bacteria can survive in extreme conditions, like the other types of photosynthetic bacteria.

Ø Phototrophic Heliobacteria are also found in soils, especially water-saturated fields, like rice paddies. They use a particular type of bacteriochlorophyll, labeled g, which differentiates them from other types of photosynthetic bacteria.

Ø Green and red filamentous anoxygenic Phototrophs (FAPs) were previously called green non-sulfur bacteria, until it was discovered that they could also use sulfur components to work through their processes. This type of bacteria uses filaments to move around.

Pigments in Bacterial Photosynthesis:

The main light harvesting pigment in photosynthetic bacteria is not chlorophyll but bacteriochlorophyll (BChl).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 infra red region wavelength of (725-1035nm).They are not contain in chloroplast instead scattered in the cytoplasm and cell membrane system. Bacteria also contains carotenoids and other assessor pigments,i.e. Bacteriorhodopsin, Phycobilins etc, 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 oxidizing agent and electron acceptor. Some of the light energy is carried successively to electron transport system via electron.

             ADP + P_i                ATP

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:

Ø 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.

Ø 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.

Ø  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.

Ø 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.

Ø 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.