What is anoxygenic photosynthesis?

Photosynthesis is the process by which some organisms use the energy of solar radiation to synthesize organic molecules that they can later use as fuel for their cellular metabolism.

In this way, thanks to photosynthesis, these organisms are able to produce their own food with the help of light, which is why they are classified as living organisms. photoautotrophs.

There are two main types of photosynthesis: oxygenated photosynthesis and anoxygenic photosynthesis.

All floors, along with most algae and cyanobacteria, perform oxygenated photosynthesis. This type of photosynthesis uses light energy to combine water (H2O) and carbon dioxide (CO2) to form glucose.

in this reaction water acts as an electron donor (reducing agent) and molecular oxygen (O) is produced as a waste product or, hence it is called oxygenated photosynthesis.

The overall reaction of oxygenated photosynthesis responds to the following equation:

6COtwo + 12HtwoO + Light → C6H12ANY6 + 6Htwo + 6HtwoANY

Anoxygenic photosynthesis is another type of photosynthesis in which no oxygen is released and they only do certain groups of bacteria. Instead of water, another inorganic molecule intervenes as an electron donor, for example, hydrogen sulfide (H2S):

COtwo + 2HtwoS + Light → [CH2O] + 2S + HtwoANY

anoxygenic photosynthetic bacteria

Among eukaryotic organisms, only plants and algae are capable of photosynthesis, and in both cases perform oxygenated photosynthesis.

In the case of prokaryotic organisms, we find bacteria that perform oxygenated photosynthesis (the cyanobacteria) and bacteria that perform anoxygenic photosynthesis.

Among the anoxygenic photosynthetic bacteria there is a great variety, both in their phylogeny and in the composition of the photosynthetic apparatus, while the organisms that carry out oxygenic photosynthesis (plants, algae and cyanobacteria) share a common mechanism.

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In fact, plant and algae chloroplasts are thought to come from ancient cyanobacteria that entered into symbiosis with primitive plants and algae, which is why they would share the same common mechanism of photosynthesis.

None of the anoxygenic photosynthetic bacteria are able to use water as an electron donor, that is, they are not able to oxidize water. Instead, they use compounds of sulfur, hydrogen, or organic substrates that donate electrons to reduce CO2 molecules and turn it into organic carbon.

The most studied anoxygenic photosynthetic bacteria are purple sulfur bacteria that may be related to our mitochondria, but there are other groups:

purple sulfur bacteria: family Chromatiaceae family Ectotiorhodospiraceae. They are anaerobic or microaerophilic. They use hydrogen sulfide (HtwoS, in solution called hydrogen sulfide) and produce gaseous sulfur (Stwo). They cannot tolerate the presence of oxygen, so they usually live in stagnant water or sulfurous hydrothermal vents. Green sulfur bacteria: Chlorobiaceae family. They use hydrogen sulfide (H2S) or sulfur (S) as an electron donor. acidic bacteria: as an electron donor can use various compounds. For example, iron bacteria use ferrous oxide, which, by giving up electrons, oxidizes and forms insoluble ferric oxide that gives the typical brown color of the waters where these bacteria live. heliobacterium: the photosynthetic pigment bacteriochlorophyll g is unique to this type of bacteria. Exclusively anaerobic. They are considered photoheterotrophs, as they obtain energy from light or chemicals, but do not use CO2 as a carbon source (their carbon source is exclusively organic compounds). anoxygenic filamentous bacteria: formerly known as green and purple non-sulfur bacteria, but later some were found to use sulfur compounds. They can be photoautotrophs, chemoorganotrophs, or photoheterotrophs.
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Purple sulfur bacteria in a Winogradsky column Green sulfur bacteria in a Winogradsky column Stream where iron bacteria live

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