What is anoxygenic photosynthesis?

It is called photosynthesis to the process by which some organisms use the energy of solar radiation to synthesize organic molecules that can later be used as fuel for their cellular metabolism.

In this way, thanks to photosynthesis, these organisms are able to manufacture their own food with the help of light, so they are classified as photoautotrophic organisms .

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

All plants , together with most algae and cyanobacteria , perform oxygenic photosynthesis . This type of photosynthesis uses light energy to combine water (H 2 O) and carbon dioxide (CO 2 ) and form glucose.

In this reaction , water acts as an electron donor (reducing agent) and molecular oxygen (O 2 ) is produced as a waste product , which is why it is called oxygenic photosynthesis.

The global reaction of oxygenic photosynthesis responds to the following equation:

6CO 2 + 12H 2 O + Light ? C 6 H 12 O 6 + 6O 2+ 6H 2 O

The anoxygenic photosynthesis is another type of photosynthesis in which oxygen is not clear and that perform only certain groups of bacteria . Instead of water, another inorganic molecule acts as an electron donor , for example hydrogen sulfide (H 2 S):

CO 2 + 2H 2 S + Light ? [CH 2 O] + 2S + H 2 O

Anoxygenic photosynthetic bacteria

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

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

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

In fact, the chloroplasts of plants and algaeare believed 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 photosynthetic anoxygenic bacteria is capable of using water as an electron donor, in other words, they are not capable of oxidizing water . Instead they use sulfur compounds, hydrogen or organic substrates that donate electrons to reduce CO 2 molecules and transform it into organic carbon.

The most studied anoxigenic photosynthetic bacteria are the purple sulfur bacteria , which could be related to our mitochondria, but there are other groups:

  • Purple sulfur bacteria : family Chromatiaceae , family Ectothiorhodospiraceae . They are anaerobic or microaerophilic. They use hydrogen sulfide (H 2 S, in solution called hydrogen sulfide) and produce sulfur gas (S 2 ). They do not tolerate the presence of oxygen, which is why they usually live in stagnant waters or sulphurous hydrothermal vents.
  • Green sulfur bacteria : family Chlorobiaceae . They use hydrogen sulfide (H 2 S) or sulfur (S) as an electron donor.
  • Acidobacteria : as an electron donor can use various compounds. For example, iron bacteria use ferrous oxide, which when oxidized electrons oxidizes and forms insoluble ferric oxide that gives the brown color typical of the waters where these bacteria live.
  • Heliobacteria : the photosynthetic pigment bacteriochlorophyll g is unique to this type of bacteria. Exclusively anaerobic They are considered photoheterotrophs, since they obtain the energy of light or chemical products, but do not use CO2 as a carbon source (their carbon source are exclusively organic compounds ).
  • Anoxygenic filamentous bacteria : formerly known as green and purple bacteria, not sulfur, but later it was discovered that some use sulfur compounds. They can be photoautotrophs, chemoorganotrophs or photoheterotrophs.


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