Oxygenic photosynthesis with reference to photosynthesis in cyanobacteria

 

Oxygenic means that oxygen is released during photosynthesis, distinguishing it from anoxygenic photosynthesis, which does not produce oxygen.

Anoxygenic and oxygenic photosynthesis differ in their electron donors, byproducts, and bacteriochlorophyll pigments. Oxygenic photosynthesis, performed by cyanobacteria and plants, uses water as an electron donor and releases oxygen as a byproduct. Anoxygenic types like green sulfur bacteria (GSB) using electron donors like hydrogen sulfide to produce elemental sulfur instead of oxygen. 

$$$$Oxygenic photosynthesis is summarized as, CO₂+6H₂O + light energy----->C₆H₁₂O₆+6O₂

Steps of oxygenic photosynthesis

1. Light-dependent reactions
• Occur in the thylakoid membranes of cell.
• Chlorophyll absorbs sunlight.
• Water molecules are split (photolysis), producing: Oxygen (O₂) + Electrons+ Protons (H⁺)
• ATP and NADPH are produced.

2. Light-independent reactions (Calvin cycle)
   • Occur in the cytoplasm.
   • ATP and NADPH are used to convert carbon dioxide (CO₂) into glucose (C₆H₁₂O₆).

Key features

• Electron donor: Water (H₂O)
• By-product: Oxygen (O₂)
• Pigment involved: Chlorophyll
• Organisms: Plants, algae, and cyanobacteria

Importance

• Produces the oxygen required for aerobic life.
• Forms the basis of most food chains by producing glucose.
• Removes carbon dioxide from the atmosphere, helping regulate Earth's climate.

The Light Reaction in Oxygenic Photosynthesis 

Phototrophic eucaryotes and the cyanobacteria carry out oxygenic photosynthesis, so named because oxygen is generated when light energy is converted to chemical energy. Central to this process, and to all other phototrophic processes, are light-absorbing pigments. In oxygenic phototrophs, the most important pigments are the chlorophylls.

 Several chlorophylls are found in eucaryotes, the two most important are chlorophyll a- absorption peak at 665 nm and chlorophyll b -absorption peak at 645 nm. 

Chlorophyll a is the primary photosynthetic pigment in cyanobacteria.  Chlorophyll a is embedded in the thylakoid membranes inside cyanobacterial cells. Chlorophylls absorb primarily in the red and blue ranges and green light is transmitted. Consequently many oxygenic phototrophs are green in color.

Chlorophyll a, the primary photosynthetic pigment in cyanobacteria, has absorption peaks at approximately:

~430 nm (blue region)

~662 nm (red region)

Cyanobacteria also contain accessory pigments that help absorb additional wavelengths of light, including:

Phycocyanin (blue pigment)

Phycoerythrin (red pigment, in many species)

Carotenoids (yellow to orange pigments)

Accessory pigments also trap light energy. The most widespread of these are the carotenoids, usually yellowish in color. Carotene is present in cyanobacteria belonging to the genus Prochloron and most photosynthetic protists; fucoxanthin is found in protists such as diatoms and dinoflagellates.

 Red algae and cyanobacteria have photosynthetic pigments called phycobiliproteins-phycoerythrin is a red pigment and phycocyanin is blue (maximum absorption at 620 to 640 nm). 

Carotenoids and phycobiliproteins are often called accessory pigments because of their role in photosynthesis. Accessory pigments are important because they absorb light in the range not absorbed by chlorophylls (the blue-green through yellow range; about 470–630 nm). This light is very efficiently transferred to chlorophyll. In this way accessory pigments make photosynthesis more efficient over a broader range of wavelengths. In addition, this allows organisms to use light not used by other phototrophs in their habitat. For instance, the microbes below a canopy of plants can use light that passes through the canopy. Accessory pigments also protect microorganisms from intense sunlight, which could oxidize and damage the photosynthetic apparatus. 

Chlorophylls and accessory pigments are assembled in highly organized arrays called antennas, which creates a large surface area to trap as many photons as possible. An antenna has about 300 chlorophyll molecules. Light energy is captured in an antenna and transferred from chlorophyll to chlorophyll until it reaches a special reaction-center chlorophyll pair directly involved in photosynthetic electron transport.

 In oxygenic phototrophs, there are two kinds of antennas associated with two different photosystems. Photosystem I absorbs longer wavelength light (700 nm) and funnels the energy to a special chlorophyll a pair called P700. The term P700 signifies that this molecule most effectively absorbs light at a wavelength of 700 nm. Photosystem II traps light at shorter wavelengths (680 nm) and transfers its energy to the special chlorophyll pair P680.

 When the photosystem I antenna transfers light energy to the reaction-center P700 chlorophyll pair, P700 absorbs the energy and is excited; it donates its excited, high-energy electron to a specific acceptor, probably a special chlorophyll a molecule or an iron-sulfur protein. The electron is eventually transferred to ferredoxin and can then travel in either of two directions- cyclic or non-cyclic.

 In the cyclic pathway, the electron moves in a cyclic route through a series of electron carriers and back to the oxidized P700. The pathway is termed cyclic because the electron from P700 returns to P700 after traveling through the photosynthetic electron transport chain. PMF is formed during cyclic electron transport in the region of cytochrome b6 and used to synthesize ATP. 

This process is called cyclic photophosphorylation because electrons travel in a cyclic pathway and ATP is formed. Only photosystem I participates. Two ATP's are generated

Cyclic photophosphorylation

 

    Electrons also can travel in a noncyclic pathway involving both photosystems. P700 is excited and donates electrons to ferredoxin. In the noncyclic route,  reduced ferredoxin reduces NADP to NADPH. Because the electrons contributed to NADP cannot be used to reduce oxidized P700, photosystem II participation is required. It donates electrons to oxidized P700 and generates ATP in the process. 

The photosystem II antenna absorbs light energy and excites P680, which then reduces pheophytin a. Pheophytin a is chlorophyll a in which two hydrogen atoms have replaced the central magnesium. Electrons subsequently travel to the plastoquinone pool and down the electron transport chain to P700. 

Although P700 has been reduced, P680 must also be reduced if it is to accept more light energy. Thus, H₂O can be used to donate electrons to P680 resulting in the release of oxygen. 

ATP is synthesized by noncyclic photophosphorylation. One ATP and one NADPH are formed when two electrons travel through the noncyclic pathway. Both photosystem I & photosystem II participate.

 

 

In cyanobacteria, photosynthetic light reactions are located in thylakoid membranes within the cell.

The dark reactions require three ATPs and two NADPHs to reduce one CO₂ and use it to synthesize carbohydrate (CH₂O). 

CO₂ + 3ATP + 2NADPH +2H+ H2O ⎯⎯→ (CH₂O) + 3ADP + 3Pi +2NADP_ 

The noncyclic system generates one NADPH and one ATP per pair of electrons; therefore four electrons passing through the system will produce two NADPHs and two ATPs. A total of 8 quanta of light energy (4 quanta for each photosystem) is needed to propel the four electrons from water to NADP_. Cyclic photophosphorylation operates independently to generate the extra ATP. This requires absorption of another 2 to 4 quanta.

 Thus, around 10 to 12 quanta of light energy are needed to reduce and incorporate one molecule of CO₂ during photosynthesis.

Why are Cyanobacteria the most significant group of photosynthetic microorganisms?

Cyanobacteria are the largest group of photosynthetic prokaryotes, which harvest solar energy and perform photosynthesis through chlorophyll-a by fixing CO₂ and generating O₂. Cyanobacteria are important in global carbon fixation, and reduce atmospheric CO₂ levels. In addition to chlorophyll-a (green pigment), cyanobacteria produce accessory photosynthetic pigments carotenoids, which are protect against photooxidative damages & blue and red pigments known as phycobilin (phycocyanin (PC) and phycoerythrin (PE)), which enable them to grow under low-light conditions. Some cyanobacteria can also fix atmospheric nitrogen.

Cyanobacteria use chlorophyll a to perform oxygenic photosynthesis, releasing oxygen as a byproduct. They are among the earliest organisms to have produced oxygen on Earth, contributing significantly to the oxygenation of the atmosphere.