Bacterial Photosynthesis Lecture Notes PDF

**Bacterial Photosynthesis** **OBJECTIVES:** The main objectives of this lecture are: - to study about the oxygenic photosynthetic bacteria - to study about the anoxygenic photosynthetic bacteria - to discuss the light reaction of photosynthesis in eukaryotes and cyanobacteria. - t...

**Bacterial Photosynthesis** **OBJECTIVES:** The main objectives of this lecture are: - to study about the oxygenic photosynthetic bacteria - to study about the anoxygenic photosynthetic bacteria - to discuss the light reaction of photosynthesis in eukaryotes and cyanobacteria. - to discuss the light reaction of photosynthesis in green and purple bacteria. **INTRODUCTION** The trapping of light energy and its conversion to chemical energy which is then used to reduced CO~2~ and incorporate it into organic form is termed as photosynthesis. This synthesis primarily of the sugars by using carbon atoms from CO~2~ gas is also termed as carbon fixation. Phototrophs are the organisms that carry out photosynthesis. Photosynthesis is one of the most significant metabolic processes on earth because almost all our energy is derived from solar energy. The process is also responsible for fulfilling the supply of oxygen on earth and over half of the photosynthesis on earth is carried out by microorganisms. Cyanobacteria (blue green algae), green sulphur bacteria, green non sulphur bacteria, purple sulphur bacteria, purple non-sulphur are some of the bacteria that undergo photosynthesis. **CLASSIFICATION OF PHOTOSYNTHETIC BACTERIA** Photosynthetic bacteria are classified into two -- 1. Oxygenic photosynthetic bacteria 2. Anoxygenic photosynthetic bacteria **OXYGENIC PHOTOSYNTHETIC BACTERIA** **Oxygenic photosynthetic bacteria** carry out photosynthesis in a way plants do so. They contain light-harvesting pigments, absorb carbon dioxide, and release oxygen. The only form of oxygenic photosynthetic bacteria is the cyanobacteria. Cyanobacteria are blue-green in color and converts the Earth's early oxygen-deficient atmosphere to an oxygen-rich environment. Cyanobacteria are mostly found in water but can survive on land, in rocks, and even in animal shells, and in coral. They are also known to be endosymbiont i.e.they can live within the cells or body of another organism in a mutually beneficial way. Image result for CYANOBACTERIA Fig: Cyanobacteria **ANOXYGENIC PHOTOSYNTHETIC BACTERIA** **Anoxygenic photosynthetic bacteria** consume carbon dioxide but do not release oxygen. These include Green and Purple bacteria, Filamentous Anoxygenic Phototrophs (FAPs) and **Phototrophic Heliobacteria **. **Purple bacteria** produce sulfur particles inside their cells. They are found in either stagnant water or hot sulfuric springs.This organisms instead of using water to photosynthesize, like plants and cyanobacteria, purple sulfur bacteria use hydrogen sulfide as their reducing agent, and because of this they give off sulfur rather than oxygen. **Purple non-sulfur bacteria** do not release sulfur because instead of using hydrogen sulfide as its reducing agent, they use hydrogen. **Phototrophic Heliobacteria **are found in soils, and they use a particular type of bacteriochlorophyll which differentiates them from other types of photosynthetic bacteria. They are photoheterotroph, i.e they cannot use carbon dioxide as their primary source of carbon. **Green and red filamentous anoxygenic phototrophs (FAPs)** uses filaments to move around. The color depends on the type of bacteriochlorophyll the particular organism uses. This form of bacteria can either be photoautotrophic, i.e. they make their own energy through the sun's energy or chemoorganotropic, which requires a source of carbon or photoheterotrophic i.e. they don't use carbon dioxide for their carbon source. Image result for purple bacteria Fig: (a) Heliobacteria (b) Purple bacteria **PHOTOSYNTHESIS** Photosynthesis is divided into two parts -- 1. Light reaction 2. Dark reaction In the light reactions light energy is trapped and converted to chemical energy whereas in the dark reactions the energy from the light reaction is use to reduce or fix CO~2~ and synthesized cell constituents. **LIGHT REACTION IN EUKARYOTES AND CYANOBACTERIA** For the absorption of light all photosynthetic organisms have pigments. Pigments includes the chlorophyll, carotenoiods, phycoerythrin and phycocynin. From among this pigments chlorophyll is the most important. Chlorophyll are the large planar rings composed of four substituted pyrrole rings with a magnesium atom coordinated to the four central nitrogen atoms. Although several chlorophyll are found in eucaryotes the most important are the chlorophyll a and chlorophyll b. Carotenoids are long molecules usually yellowish in colour an possess an extensive conjugated double bond system. Certain red algae and cyanobacteria possess another photosynthetic pigment known as phycobiliproteins (phycoerythrin and phycocynin) which consist of a protein with a tetrapyrrole attached. Phycoerythrin is a red pigment with maximum absorption at around 550nm while phycocynin is blue with maximum absorption at around 620-640 nm. This phycobiliproteins along with the chlorophylls are called the accessory pigments. Chlorophylls cannot absorb light energy effectively in the blue-green through yellow range but accessory pigments do absorb light in this region and transfer the trapped energy to chlorophyll. Hence, they make photosynthesis more efficient over a broader range. Chlorophyll and accessory pigments are assembled in a highly organised arrays known as the antennas which function to create a large surface area to trap many photons of light. An antennas has about 300 chlorophyll molecules. Two kinds of antennas are associated with two different photosystems in eukaryotes and cyanobacteria. In photosystem I it absorbs longer wavelength light i.e. ≥680 nm and funnels the energy to a special chlorophyll a molecule called P700. P700 absorbs light effectively at 700nm wavelength and hence the term is given. Photosystem II traps light at shorter wavelength i.e. 680 nm and transfer the energy to special chlorophyll P680. When the antenna in the photosystem I transfer light energy to the reaction center P700 chlorophyll, it absorbs the energy and is excited and ultimately its reduction potential becomes very negative. Further, it donates the electron to chlorophyll a molecule or an iron-sulphur protein and the electron is eventually transferred to ferridoxin and can travel in any of the two directions i.e the cyclic photophosphorylation and non-cyclic photophosphorylation. In the cyclic pathway the electron moves in cyclic route through the series of electron carriers and then comes back to the oxidised P700. This pathway is termed as cyclic because the electrons from P700 returns to P700 after travelling to the photosynthetic electron transport chain. Here ATP is formed during cyclic electron transport in the region of cytochrome b~6~. Hence this process is called the cyclic photophosphorylation because electrons travel in cyclic pathway and ATP is formed. While in noncyclic pathway it involves both the photosystems. Like in cyclic pathway here also P700 is excited and donates electron to ferridoxin. In this pathway reduced ferridoxin reduces NADP^+^ to NADPH. Since, the electrons contributed to the NADP^+^ cannot be used to reduce oxidised P700, photosystem II involvement is necessary. It donates the electrons to oxidised P700 and generates ATP in the process. The antenna in the photosystem II absorbs light energy and excites P680 which then reduce pheophytin a. Further, electrons travels to Q i.e. plastoquinone and down the electron transport chain to P700. Oxidised P680 than obtains an electron from the oxidation of water to O~2~. Thus, electrons flow from water all the way to the NADP^+^ with the aid of energy from two photosystems and ATP is synthesized by noncyclic photophosphorylation.  The dark reaction require three ATPs and two NADPHs to reduce one CO~2~ and used it to synthesized carbohydrate. In noncyclic pathway it generates one NADPH and one ATP per pair of electrons. Therefore four electrons passing through the system will produce two NADPHs and two ATPs. For carbon dioxide fixation the ratio of ATP to NADPH require is 3:2 which means one more ATP needs to be supplied. Hence cyclic photophosphorylation operates independently to generate the extra ATP. **LIGHT REACTION IN GREEN AND PURPLE BACTERIA** Green and purple photosynthetic bacteria greatly differs from the cyanobacteria and the eukaryotic photosynthesizers. The green and purple bacteria are anoxygenic which means they do not use water as an electron source or produce oxygen photosynthetically. While the cyanobacteria and the eukaryotes are always oxygenic. This green and purple bacteria possess a pigment known as the bacteriochlorophyll with absorption maxima at longer wavelength. Purple and green bacteria lack the photosystem II and hence cannot used water as the electron donor in the noncyclic electron transport. Also, without photosystem II they cannot produce O~2~ from H~2~O. In this bacteria, when a reaction centre chlorophyll P870 is excited, it donates an electron to the bacteriopheophytin. Further, electrons then flow to quinones and through an electron transport chain back to P870 while driving ATP. This bacteria synthesized NADH in three ways. When they are growing in the presence of hydrogen gas the hydrogen can used directly in the production of NADH. Also many purple photosynthetic bacteria use ATP or the protonmotive force to reverse the flow of electrons in an electron transport chain and move them from inorganic or organic donors to NAD^+^. The green sulphur bacteria carry out a simple form of noncyclic photosynthetic electron flow to reduce NAD^+^. Related image **CONCLUSION** Photosynthesis is a light dependent energy yielding process. It is the synthesis of carbohydrate from carbon dioxide in the presence of light and hence the carbohydrate that is form is used for metabolism. Photosynthesis in plants, algae and cyanobacteria is similar to bacterial photosynthesis in the requirement for large amount of energy in the form of ATP but different with respect to form of chemical reductants and end products of photosynthesis. **GLOSSARY** **Carbon dioxide fixation**: Conversion of inorganic carbon to organic compounds by living organisms. **Phototrophs**: Organisms that make their own energy through the sun's energy **Photosynthesis**: The trapping of light energy and its conversion to chemical energy which is then used to reduced CO~2~ and incorporate it into organic form **Endosymbiont**: They can live within the cells or body of another organism in a mutually beneficial way. **Photoheterotrophs:** Organisms that do not use carbon dioxide for their carbon source. **Bacteriochlorophyll**: Photosynthetic pigments that occur in various phototrophic bacteria. **ATP**: Adenosine triphosphate **NADP**: Nicotinamide adenine dinucleotide phosphate **FAPs**: **Filamentous anoxygenic phototrophs** **FAQs:** 1. **Differentiate between anoxygenic and oxygenic photosynthetic bacteria.** **Answer:** **Oxygenic photosynthetic bacteria** contain light-harvesting pigments, absorb carbon dioxide, and release oxygen whereas **anoxygenic photosynthetic bacteria **consume carbon dioxide but do not release oxygen. 2. **What are endosymbiont? Give one example of endosymbiont.** **Answer:** Organisms that can live within the cells or body of another organism in a mutually beneficial way are called the endosymbiont. E.g Cyanobacteria. 3. **Give a detailed account on the anoxygenic photosynthetic bacteria.** **Answer:** **Anoxygenic photosynthetic bacteria** consume carbon dioxide but do not release oxygen. Example - Green and Purple bacteria, Filamentous Anoxygenic Phototrophs (FAPs) and **Phototrophic Heliobacteria**. **Purple bacteria** produce sulfur particles inside their cells. This organisms instead of using water to photosynthesize, like plants and cyanobacteria use hydrogen sulfide as their reducing agent, and because of this they give off sulfur rather than oxygen. **Purple non-sulfur bacteria** do not release sulfur because instead of using hydrogen sulfide as its reducing agent, they use hydrogen. **Phototrophic Heliobacteria **are found in soils, and they use a particular type of bacteriochlorophyll which differentiates them from other types of photosynthetic bacteria. They are photoheterotroph, i.e they cannot use carbon dioxide as their primary source of carbon. **Green and red filamentous anoxygenic phototrophs (FAPs)** uses filaments to move around. The colour depends on the type of bacteriochlorophyll the particular organism uses. This form of bacteria can either be photoautotrophic, i.e. they make their own energy through the sun's energy or chemoorganotropic, which requires a source of carbon or photoheterotrophic i.e. they don't use carbon dioxide for their carbon source. 4. **Name some of the photosynthetic pigments present in the eukaryotes and cyanobacteria.** **Answer:** Chlorophyll, carotenoiods, phycoerythrin and phycocynin are some of the photosynthetic pigments present in the eukaryotes and cyanobacteria. 5. **Write a note on cyclic photophosphorylation.** **Answer:** In the cyclic pathway the electron moves in cyclic route through the series of electron carriers and then comes back to the oxidised P700. This pathway is termed as cyclic because the electrons from P700 returns to P700 after travelling to the photosynthetic electron transport chain. Here ATP is formed during cyclic electron transport in the region of cytochrome b~6~. Hence this process is called the cyclic photophosphorylation because electrons travel in cyclic pathway and ATP is formed. 6. **Differentiate between phycocynin and phycoerythrin pigments.** **Answer:** Phycoerythrin is a red pigment with maximum absorption at around 550nm whereas phycocynin is blue with maximum absorption at around 620-640 nm. 7. **What is photosynthesis?** **Answer:** The trapping of light energy and its conversion to chemical energy which is then used to reduced CO~2~ and incorporate it into organic form is termed as photosynthesis. This synthesis primarily of the sugars by using carbon atoms from CO~2~ gas is also termed as carbon fixation. Phototrophs are the organisms that carry out photosynthesis. Photosynthesis is one of the most significant metabolic processes on earth because almost all our energy is derived from solar energy. The process is also responsible for fulfilling the supply of oxygen on earth and over half of the photosynthesis on earth is carried out by microorganisms. Cyanobacteria (blue green algae), green sulphur bacteria, green non sulphur bacteria, purple sulphur bacteria, purple non-sulphur are some of the bacteria that undergo photosynthesis. 8. **What are photoautotrophs and chemoorganotrophs?** **Answer:** Photoautotrophic organisms make their own energy through the sun's energy and chemoorganotropic organisms requires a source of carbon. 9. **Differentiate between light reaction and dark reaction in photosynthesis.** **Answer:** In the light reactions light energy is trapped and converted to chemical energy whereas in dark reactions the energy from the light reaction is use to reduce or fix CO~2~ and synthesized cell constituents. **References** Prescott, L.M., Harley, J.P. and Klein, D.A. 1999. *Microbiology*. 4^th^ Edition, McGraw-Hill. Pelczar, M.J., Chan, E.C.S. and Krieg, N.R. 1998. *Microbiology*. TATA McGRAW-HILL. Tortora, G.J., Funke, B.R. and Case, C.L. 2013. *Microbiology: An Introduction*. Pearson publication. **Links:**