CO2 Fixation in Biosynthesis

Chemoautotrophic organisms, such as bacteria or Archea living in the dark and in hostile environments such as the deep sea, which obtain their energy through the oxidation of inorganics such as H2S, elemental sulfur, metal ions (Fe Mn , ammonia, or nitrite  

Photoautotrophic organisms, such as plants or algae, protists, and cyano bacteria, which use water as an electron source and light as an energy source. 

It is estimated that more than 98 % of the CO2 that enters the biological carbon cycle ( 200 Gtc/y) is fixed by carboxylases, which makes them crucial players in controlling atmospheric CO2 concentrations [8, 9]. Biological CO2 fixation can occur following four different pathways [10, 11]  

The reductive tricarboxylic acid (TCA Amon-Buchanan) cycle [14]. 

Acyl-CoA carboxylase pathways 3-hydroxypropionate/malonyl-CoA cycle [18, 19], 3-hydroxypropionate/4-hydroxybutyrate cycle [20], diearboxylate/ 4-hydroxybutyrate pathway [21], and the ethylmalonyl-CoA pathway [22]. 

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