Editing Carbon dioxide (section)

=== Photosynthesis and carbon fixation ===
[[File:Calvin-cycle4.svg|thumb|left|upright=1|Overview of the [[Calvin cycle]] and carbon fixation]]
[[Carbon fixation]] is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and cyanobacteria into [[fuel|energy-rich]] organic molecules such as [[glucose]], thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and [[water]] to produce sugars from which other [[organic compound]]s can be constructed, and [[oxygen]] is produced as a by-product.

[[RuBisCO|Ribulose-1,5-bisphosphate carboxylase oxygenase]], commonly abbreviated to RuBisCO, is the [[enzyme]] involved in the first major step of carbon fixation, the production of two molecules of [[3-phosphoglycerate]] from {{CO2}} and [[ribulose bisphosphate]], as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.<ref>{{cite journal |vauthors=Dhingra A, Portis AR, Daniell H |date=April 2004 |title=Enhanced translation of a chloroplast-expressed RbcS gene restores small subunit levels and photosynthesis in nuclear RbcS antisense plants |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=101 |issue=16 |pages=6315–6320 |bibcode=2004PNAS..101.6315D |doi=10.1073/pnas.0400981101 |pmc=395966 |pmid=15067115 |quote=(Rubisco) is the most prevalent enzyme on this planet, accounting for 30–50% of total soluble protein in the chloroplast |doi-access=free}}</ref>

[[Phototroph]]s use the products of their photosynthesis as internal food sources and as raw material for the [[biosynthesis]] of more complex organic molecules, such as [[polysaccharide]]s, [[nucleic acid]]s, and proteins. These are used for their own growth, and also as the basis of the [[food chain]]s and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the [[coccolithophore]]s synthesise hard [[calcium carbonate]] scales.<ref>{{Cite book |title=Evolution of primary producers in the sea |vauthors=Falkowski P, Knoll AH |date=1 January 2007 |publisher=Elsevier, Academic Press |isbn=978-0-12-370518-1 |oclc=845654016}}</ref> A globally significant species of coccolithophore is ''[[Emiliania huxleyi]]'' whose [[calcite]] scales have formed the basis of many [[sedimentary rock]]s such as [[limestone]], where what was previously atmospheric carbon can remain fixed for geological timescales.[[File:Auto-and heterotrophs.png|thumb|Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by [[photosynthesis]] (green), which can be [[cellular respiration|respired]] (red) to water and {{CO2}}.]]

Plants can grow as much as 50% faster in concentrations of 1,000&nbsp;ppm {{CO2}} when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.<ref>{{cite web |date=December 2002 |title=Carbon Dioxide In Greenhouses |url=http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm |url-status=live |archive-url=https://web.archive.org/web/20190429202513/http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm |archive-date=29 April 2019 |access-date=12 June 2007 |vauthors=Blom TJ, Straver WA, Ingratta FJ, Khosla S, Brown W}}</ref> Elevated {{CO2}} levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated {{CO2}} in FACE experiments.<ref>{{cite journal |vauthors=Ainsworth EA |year=2008 |title=Rice production in a changing climate: a meta-analysis of responses to elevated carbon dioxide and elevated ozone concentration |url=http://www.plant-biotech.dk/Meetings/PBD_Symposium_Plant%20Stress_litterature/LisaAinsworth_pdf2.pdf |journal=Global Change Biology |volume=14 |issue=7 |pages=1642–1650 |bibcode=2008GCBio..14.1642A |doi=10.1111/j.1365-2486.2008.01594.x |archive-url=https://web.archive.org/web/20110719130608/http://www.plant-biotech.dk/Meetings/PBD_Symposium_Plant%20Stress_litterature/LisaAinsworth_pdf2.pdf |archive-date=19 July 2011 |s2cid=19200429}}</ref><ref>{{cite journal |vauthors=Long SP, Ainsworth EA, Leakey AD, Nösberger J, Ort DR |date=June 2006 |title=Food for thought: lower-than-expected crop yield stimulation with rising {{CO2}} concentrations |url=http://www.as.wvu.edu/biology/bio463/Long%20et%20al%202006%20Lower%20yield%20than%20expected%20under%20increased%20CO2.pdf |url-status=live |journal=Science |volume=312 |issue=5782 |pages=1918–1921 |bibcode=2006Sci...312.1918L |citeseerx=10.1.1.542.5784 |doi=10.1126/science.1114722 |pmid=16809532 |archive-url=https://web.archive.org/web/20161020165354/http://www.as.wvu.edu/biology/bio463/Long%20et%20al%202006%20Lower%20yield%20than%20expected%20under%20increased%20CO2.pdf |archive-date=20 October 2016 |access-date=27 October 2017 |s2cid=2232629}}</ref>

Increased atmospheric {{CO2}} concentrations result in fewer stomata developing on plants<ref>{{cite journal |vauthors=Woodward F, Kelly C |year=1995 |title=The influence of {{CO2}} concentration on stomatal density |journal=New Phytologist |volume=131 |issue=3 |pages=311–327 |doi=10.1111/j.1469-8137.1995.tb03067.x |doi-access=free|bibcode=1995NewPh.131..311W }}</ref> which leads to reduced water usage and increased [[water-use efficiency]].<ref>{{cite journal |vauthors=Drake BG, Gonzalez-Meler MA, Long SP |date=June 1997 |title=More Efficient Plants: A Consequence of Rising Atmospheric {{CO2}}? |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=48 |issue=1 |pages=609–639 |doi=10.1146/annurev.arplant.48.1.609 |pmid=15012276 |s2cid=33415877}}</ref> Studies using [[Free-Air Concentration Enrichment|FACE]] have shown that {{CO2}} enrichment leads to decreased concentrations of micronutrients in crop plants.<ref>{{cite journal |vauthors=Loladze I |year=2002 |title=Rising atmospheric {{CO2}} and human nutrition: toward globally imbalanced plant stoichiometry? |journal=Trends in Ecology & Evolution |volume=17 |issue=10 |pages=457–461 |doi=10.1016/S0169-5347(02)02587-9 |s2cid=16074723}}</ref> This may have knock-on effects on other parts of [[ecosystem]]s as herbivores will need to eat more food to gain the same amount of protein.<ref>{{cite journal |vauthors=Coviella CE, Trumble JT |year=1999 |title=Effects of Elevated Atmospheric Carbon Dioxide on Insect-Plant Interactions |journal=Conservation Biology |volume=13 |issue=4 |pages=700–712 |doi=10.1046/j.1523-1739.1999.98267.x |jstor=2641685 |bibcode=1999ConBi..13..700C |s2cid=52262618}}</ref>

The concentration of secondary [[metabolites]] such as [[phenylpropanoid]]s and [[flavonoid]]s can also be altered in plants exposed to high concentrations of {{CO2}}.<ref>{{Cite journal |vauthors=Davey MP, Harmens H, Ashenden TW, Edwards R, Baxter R |year=2007 |title=Species-specific effects of elevated {{CO2}} on resource allocation in ''Plantago maritima'' and ''Armeria maritima'' |journal=Biochemical Systematics and Ecology |volume=35 |issue=3 |pages=121–129 |doi=10.1016/j.bse.2006.09.004}}</ref><ref>{{cite journal |vauthors=Davey MP, Bryant DN, Cummins I, Ashenden TW, Gates P, Baxter R, Edwards R |date=August 2004 |title=Effects of elevated {{CO2}} on the vasculature and phenolic secondary metabolism of Plantago maritima |journal=Phytochemistry |volume=65 |issue=15 |pages=2197–2204 |doi=10.1016/j.phytochem.2004.06.016 |pmid=15587703|bibcode=2004PChem..65.2197D}}</ref>

Plants also emit {{CO2}} during respiration, and so the majority of plants and algae, which use [[C3 photosynthesis]], are only net absorbers during the day. Though a growing forest will absorb many tons of {{CO2}} each year, a mature forest will produce as much {{CO2}} from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.<ref>{{cite web |title=Global Environment Division Greenhouse Gas Assessment Handbook – A Practical Guidance Document for the Assessment of Project-level Greenhouse Gas Emissions |url=http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2002/09/07/000094946_02081604154234/Rendered/INDEX/multi0page.txt |url-status=dead |archive-url=https://web.archive.org/web/20160603011630/http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2002/09/07/000094946_02081604154234/Rendered/INDEX/multi0page.txt |archive-date=3 June 2016 |access-date=10 November 2007 |publisher=[[World Bank]]}}</ref> Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon<ref>{{cite journal |display-authors=6 |vauthors=Luyssaert S, Schulze ED, Börner A, Knohl A, Hessenmöller D, Law BE, Ciais P, Grace J |date=September 2008 |title=Old-growth forests as global carbon sinks |url=https://hal-cea.archives-ouvertes.fr/cea-00910763/file/Luyssaert2008.pdf |journal=Nature |volume=455 |issue=7210 |pages=213–215 |bibcode=2008Natur.455..213L |doi=10.1038/nature07276 |pmid=18784722 |s2cid=4424430}}</ref> and remain valuable [[carbon sink]]s, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved {{CO2}} in the upper ocean and thereby promotes the absorption of {{CO2}} from the atmosphere.<ref>{{cite journal |display-authors=6 |vauthors=Falkowski P, Scholes RJ, Boyle E, Canadell J, Canfield D, Elser J, Gruber N, Hibbard K, Högberg P, Linder S, Mackenzie FT, Moore B, Pedersen T, Rosenthal Y, Seitzinger S, Smetacek V, Steffen W |date=October 2000 |title=The global carbon cycle: a test of our knowledge of earth as a system |journal=Science |volume=290 |issue=5490 |pages=291–296 |bibcode=2000Sci...290..291F |doi=10.1126/science.290.5490.291 |pmid=11030643 |s2cid=1779934}}</ref>