Biological carbon fixation
Carbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It refers mostly to the processes found in autotrophs (organisms that produce their own food), usually driven by photosynthesis, whereby carbon dioxide is changed into sugars. Carbon fixation can also be carried out by the process of calcification in marine calcifying organisms such as Emiliania huxleyi and also by heterotrophic organisms in some circumstances.
Biological
Plants
The Calvin cycle is the most common biological method of carbon fixation.
In plants, there are three types of carbon fixation during photosynthesis:
- C3 plants that use the Calvin cycle for the initial steps that incorporate CO2 into organic matter, forming a 3-carbon compound as the first stable. This form of photosynthesis occurs in the majority of terrestrial species of plants. Plants that use this pathway have a carbon isotope signature of -24 to -33‰.[1]
- C4 plants that preface the Calvin cycle with reactions that incorporate CO2 into a 4-carbon compound. C4 plants have a distinctive internal leaf anatomy. Tropical grasses, such as sugar cane and maize are C4 plants, but there are many broadleaf plants that are C4. Overall, 7600 species of terrestrial plants use C4 carbon fixation, representing around 3% of all species.[2] These plants have a carbon isotope signature of -16 to -10 ‰.[1]
- CAM-plants that use Crassulacean acid metabolism as an adaptation for arid conditions. CO2 enters through the stomata during the night and is converted into organic acids, which release CO2 for use in the Calvin cycle during the day, when the stomata are closed. The jade plant (Crassula ovata) and cacti are typical of CAM plants. Sixteen thousand species of plants use CAM.[3] These plants have a carbon isotope signature of -20 to -10 ‰.[1]
Microorganisms
In addition to the Calvin cycle, the following alternative pathways are currently known to be used in certain autotrophic microorganisms:
- Reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle). The reaction is the Citric acid cycle run in reverse and is used by photolitho-autotrophic bacteria of the Chlorobiales and some chemolitho-autotrophic sulfate-reducing bacteria.
- Reductive acetyl CoA Pathway is found in methanogenic archaea and in acetogenic and some sulfate-reducing bacteria as a way of fixing carbon.
- 3-Hydroxypropionate Pathway is found in photolitho-autotrophically grown bacteria of the genus Chloroflexus and, in modified form, in some chemolitho-autotrophically grown archaea as a way of fixing carbon.
Heterotrophs
Although almost all heterotrophs cannot synthesize complete organic molecules from carbon dioxide, some carbon dioxide is incorporated in their metabolism.[4] Notably pyruvate carboxylase consumes carbon dioxide (as carbonate ions) as part of gluconeogenesis.
References
- ^ a b c Attention: This template ({{cite jstor}}) is deprecated. To cite the publication identified by jstor:1310735, please use {{cite journal}} with
|jstor=1310735instead. - ^ Sage, Rowan (1999). "16". C4 Plant Biology. pp. 551–580. ISBN 0126144400.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1093/jexbot/53.369.569, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with
|doi=10.1093/jexbot/53.369.569instead. - ^ Nicole Kresge, Robert D. Simoni, Robert L. Hill (2005). "The Discovery of Heterotrophic Carbon Dioxide Fixation by Harland G. Wood". The Journal of Biological Chemistry.
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