What is Anoxygenic Photosynthesis

By | October 13, 2021
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Photosynthesis is a process, where organisms use inorganic molecules such as carbon di oxide and convert it into carbohydrates (food) using sunlight. In this manner organisms capable of performing photosynthesis can generate energy that helps their growth.

Different organisms perform Photosynthesis : Green sulfur bacteria, Acidobacteria, Cyanobacteria, Algae , Plants etc.

Depending on the products of the photosynthesis – They are divide into oxygenic and anoxygenic

Oxygenic Photosynthesis :

  • In oxygenic photosynthesis, molecular oxygen is released as a byproduct into the atmosphere – Algae, Cyanobacteria, and plants (green) release oxygen as Photosynthesis byproduct. Water is used as electron donor.
  • In the bacterial domain, oxygenic photosynthesis is limited to only one phylum, i.e.Cyanobacteria or blur green algae. Photosynthesis occur in thylakoid membranes present in its cytoplasm, as they lack chloroplasts found in plants and algae.
  • Cyanobacteria are the first organisms known to have produced oxygen on earth through photosynthesis.
  • Based on the theory of endosymbiosis, ancestors of Cyanobacteria may have evolved into present-day chloroplasts that exist in plants of today.

Anoxygenic Photosynthesis :

  • Anoxygenic photosynthesis is process where trapped sunlight is converted to ATP (chemical energy) and most important difference is that there is no release of oxygen. Water is, therefore, not used as an electron donor.
  • Anoxygenic photosynthetic bacteria vary broadly not only in terms of their phylogenetic positions, but also in the compositions of their photosynthetic apparatuses and pigments used for light reaction.
  • In anoxygenic process, sulfide, hydrogen or organic substrates are employed as electron donors.
  • Either Photosystem I or Photosystem II (never both), is used by anoxygenic bacteria for photosynthesis.
  • Phyla Chlorobi (green sulfur bacteria), Firmicutes (Heliobacterium spp.) and Acidobacteria (C. thermophilum) – Use Photosystem I.
  • In this process as electrons are transported in a cyclic manner and return to the same photosystem. Hence, anoxygenic photosynthesis is also known as cyclic photophosphorylation.
  • NADPH is not produced in anoxygenic photosynthesis.
  • Phyla Proteobacteria (purple bacteria), Chloroflexi (filamentous anoxygenic phototrophic bacteria), and Gemmatimonadetes ( G. phototrophica) – Employ photosystem II.
  • The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular structure and wavelength of absorbed light ( which differs from bacteria to bacteria – Green Sulphur bacteria : P840 , purple sulfur bacteria : P870 – P stand for Pigment and number denotes the maximal wavelength of light it can absorb ). They are called bacteriochlorophylls.
  •  Phyla ChlorobiChloroflexi and Acidobacteria – use infrared light at 740–750 nm ,  Firmicutes absorb : 786–792 nm.
  • The structural and pigmentary diversities found among anoxygenic photosynthetic bacteria have been attributed to their long evolutionary history.
  • Anoxygenic photosynthesis is considered to be more ancient and appeared before emergence of oxygenic photosynthesis on earth.
  • The photosynthetic system of the nonsulfur purple bacterium – Rhodospeudomonas viridis and Green sulfur bacteria such as Chlorobium thiosulfatophilum have photosynthetic machinery similar to oxygenic photosynthetic systems but appears to be more ancient.
  • Reduced sulfur species such as H2S are electron donors in the overall photosynthetic reaction.
  • However, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria.
Bacteria Electron donorOxygen use
Green sulfur H2, H2S, SulfurAnoxygenic
Green non sulfurVariety of amino acids and organic acids Anoxygenic
Purple sulfurH2, H2S, Sulfur Anoxygenic
Purple nonsulfurUsually organic molecules Anoxygenic
Cyanobacteria H2OOxygenic
  • When bacteria use Photosystem II reaction center, there will be no oxygen evolving center attached to it, but they are structurally and sequentially analogous to PS II of plants and Cyanobacteria.
  • The electron transport chain of purple non-sulfur bacteria starts with excitation of bacteriochlorophyll pair, P870 by means of light energy.
  • P870 will then donate an electron to bacteriopheophytin to become p870+ (oxidized), which then passes it on to a series of electron carriers down the electron transport chain.
  • This process of transfer of electrons through different carriers will generate an electrochemical gradient which will be used to synthesize ATP by chemiosmosis.
  • P870 has to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew.
  • Molecular hydrogen in the bacterial environment is the usual electron donor.
  • In todays world, these type of bacteria are found in only sulfur springs and deep-sea vents.
Cyclic photophosphorylation Purple non-sulfur bacteria

Difference between Oxygenic and Anoxygenic Photosynthesis

Feature Oxygenic Anoxygenic
DefinitionOxygenic photosynthesis is kind of photosynthesis exhibited by some organisms which convert light energy to fix Carbon di oxide into carbohydrates using ATP and NADPH formed during light phase and release molecular oxygen as byproductAnoxygenic photosynthesis is mostly similar to oxygenic with the exception of release of oxygen
Organisms exhibitPlants, Algae and CyanobacteriaMany different bacteria phyla such as Proteobacteria, Chloroflexi , Chlorobi etc.
BacteriaOnly phylum – CyanobacteriaMany bacteria phyla
Electron transport chainBoth photosystem I and II are involvedOnly one photosystem at a time
NADPHFormedNot formed
WaterWater is the electron donorWater doesn’t donate electrons. H2S is mostly act as reducing agent or electron donor. Sulfur is released as byproduct.
OxygenReleasedNot released
OriginEvolved after Anoxygenic process mostly by modifying photosynthetic machinery of anoxygenic bacteria ( by formation of chloroplast organelle and other modifications) First photosynthetic organisms on earth were anoxygenic.
PigmentsChlorophyll a is main pigmentBacteriochlorophylls ( similar to chlorophylls of eukaryotes)

Image Credit : Wikipedia commons

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