Anoxygenic vs. oxygenic photosynthesis with reference to photosynthesis in cyanobacteria, green bacteria and purple bacteria

 

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

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. Chlorophylls absorb primarily in the red and blue ranges and green light is transmitted. Consequently many oxygenic phototrophs are green in color.

 Other photosynthetic 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 (680 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.

 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.

Cyclic photophosphorylation

 

Electrons also can travel in a noncyclic pathway involving both photosystems. P700 is excited and donates electrons to ferredoxin as before. In the noncyclic route, however, 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.

 

 

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.

The Light Reaction in Anoxygenic Photosynthesis

 Certain bacteria carry out a second type of photosynthesis called anoxygenic photosynthesis. This phototrophic process derives its name from the fact that water is not used as an electron source and therefore O₂ is not produced. 

The process also differs in terms of the photosynthetic pigments used, the participation of just one photosystem, and the mechanisms used to generate reducing power. Three groups of bacteria carry out anoxygenic photosynthesis: phototrophic green bacteria, phototrophic purple bacteria, and heliobacteria. 

Sulfur bacteria use hydrogen sulfide which they oxidize to elemental sulfur, while non-sulfur bacteria can use a wider range of compounds, including some organic molecules (lactate, succinate etc)

 

 

Many differences found in anoxygenic phototrophs are due to their having a single photosystem. Because of this, they are restricted to cyclic electron flow and are unable to produce O₂ from H₂O. Indeed, almost all anoxygenic phototrophs are strict anaerobes.

 Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls. The absorption maxima of bacteriochlorophylls (Bchl) are at longer wavelengths than those of chlorophylls. Bacteriochlorophylls a and b have maxima in ether at 775 and 790 nm, respectively.

This shift of absorption maxima into the infrared region better adapts these bacteria to their ecological niches.

 

Purple bacteria

Purple bacteria has only one photosystem (similar to Photosystem II) with Pheophytin-Quinone/Type II Reaction Center.  Bacteriochlorophyll molecules absorb light energy, which is transferred to a reaction center called P₈₇₀. This process takes place in anoxic (oxygen-free) conditions, which are common in aquatic environments where these bacteria are found.

 

When bacteriochlorophyll P870 is excited, it donates an electron to bacteriopheophytin. Electrons then flow to quinones (Pheophytin-Quinone/Type II Reaction Center) and through an electron transport chain back to P870.  PMF created is used to drive ATP synthesis.

Purple bacteria

Both green and purple bacteria lack two photosystems, but the purple bacteria have a photosynthetic apparatus similar to photosystem II, whereas the green sulfur bacteria have a system similar to photosystem I.

 

Green bacteria

Photosynthesis in green sulfur bacteria (GSB) is similar to purple bacteria. Green sulfur bacteria has Fe-S Reaction Center (Type I Reaction Center). Excitation causes an electron to pass through a quinone (MK, Menaquinone) to the cytochrome bc1 complex and back to P 840.  PMF created is used for ATP synthesis.

Green Bacteria

GSB possess specialized light-harvesting chlorosomes containing bacteriochlorophyll, which are efficient at absorbing light energy, even in low-light conditions. Chlorosomes consist of bacteriochlorophyll (BChl) pigments, carotenoids, quinones, and proteins in a lipid-monolayer envelope. They efficiently capture light energy, even at very low light levels, and funnel it to the reaction center for photosynthesis.  They are essential for GSB to grow in extremely low-light environments, such as the deep parts of lakes and oceans. 

Anoxygenic phototrophs also require reducing power (NAD[P]H or reduced ferredoxin) for CO₂ fixation and other biosynthetic processes.

They generate reducing power in at least three ways, depending on the bacterium. Some have hydrogenases that are used to produce NAD(P)H directly from the oxidation of hydrogen gas.

Others, such as the photosynthetic purple bacteria, must use reverse electron flow to generate NAD(P)H. In this mechanism, electrons are drawn off the photosynthetic electron transport chain and “pushed” to NAD(P)_ using PMF or the hydrolysis of ATP. Electrons from electron donors such as hydrogen sulfide, elemental sulfur, and organic compounds replace the electrons removed from the electron transport chain in this way.

Phototrophic green bacteria and heliobacteria also must draw off electrons from their electron transport chains. Because the reduction potential of the component of the chain where this occurs is more negative than NAD_ and oxidized ferredoxin, the electrons flow spontaneously to these electron acceptors. Thus, these bacteria exhibit a simple form of noncyclic photosynthetic electron flow.

 

 

Carbon fixation

The thylakoids of cyanobacteria use the energy of sunlight to drive photosynthesis, a process where the energy of light is used to synthesize organic compounds from carbon dioxide..

The dark reaction, or Calvin cycle, is the second stage of photosynthesis where carbon dioxide is fixed into organic compounds like glucose. This process is independent of direct light but requires the ATP and NADPH produced during the light-dependent reactions. Microorganisms use these to convert atmospheric CO₂ into sugars for energy and growth with the involvement of the enzyme RuBisCO. Cyanobacteria have microcompartments known as carboxysomes, which store this CO₂-fixing enzyme, RuBisCO.

In eukaryotes like algae, the dark reaction occurs in the stroma of the chloroplast. In prokaryotes like cyanobacteria, it happens in the cytoplasm. It is "light-independent" because it doesn't use light energy directly, but it relies on the products (ATP and NADPH) of the light-dependent reactions, meaning it can only happen when light is available for the first stage to occur. 

Calvin cycle is driven by a series of enzyme-catalyzed reactions. It ultimately produces glucose from carbon dioxide, with ADP and NADP+ being recycled back to the light reactions

 

In short,

Phototrophs use light to generate a proton motive force (PMF), which is then used to synthesize ATP by a process called photophosphorylation (photo phos). The process requires light-absorbing pigments. When the pigments are chlorophyll or bacteriochlorophyll, the absorption of light triggers electron flow through an electron transport chain, accompanied by the pumping of protons across a membrane.

 The electron flow can be either cyclic (dashed line) or noncyclic (solid line), depending on the organism and its needs. Many phototrophs are autotrophs and must use much of the ATP and reducing power they make to fix CO2. 

In oxygenic photosynthesis, eucaryotes and cyanobacteria trap light energy with chlorophyll and accessory pigments and move electrons through photosystems I and II to make ATP and NADPH (the light reactions). 

Cyclic photophosphorylation involves the activity of photosystem I alone and generates ATP only. In noncyclic photophosphorylation photosystems I and II operate together to move electrons from water to NADP⁺ producing ATP, NADPH, and O₂

 Anoxygenic phototrophs differ from oxygenic phototrophs in possessing  bacteriochlorophyll and having only one photosystem. They use cyclic photophosphorylation to make ATP. They are anoxygenic because they do not use water as an electron donor for electron flow though the photosynthetic electron transport chain.

Purple sulfur bacteria contribute to nutrient cycling and play a significant role in primary production. These organisms contribute to the carbon cycle through carbon fixation and the phosphorus cycle & the iron cycle. Although purple sulfur bacteria are found in the anoxic layer of their habitat, they supply inorganic nutrients to the above oxic layer. Purple sulfur bacteria act as a source of food to other organisms.

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.