Editing Pyrenoid (section)

==Origin==

The CCM is only induced during periods of low CO<sub>2</sub> levels, and it was the existence of these trigger levels of CO<sub>2</sub> below which CCMs are induced that led researchers to speculate on the likely timing of origin of mechanisms like the pyrenoid.

There are several [[scientific hypothesis|hypotheses]] as to the origin of pyrenoids. With the rise of large terrestrial based flora following the colonisation of land by ancestors of [[charophyceae|Charophyte algae]], CO<sub>2</sub> levels dropped dramatically, with a concomitant increase in O<sub>2</sub> atmospheric concentration. It has been suggested that this sharp fall in CO<sub>2</sub> levels acted as an evolutionary driver of CCM development, and thus gave rise to pyrenoids<ref>Badger, M. R., & Price, G. D. (2003). CO<sub>2</sub> concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution. ''Journal of Experimental Botany'', 54(383), 609-622. {{PMID|12554074}}</ref> in doing so ensuring that rate of supply of CO<sub>2</sub> did not become a limiting factor for photosynthesis in the face of declining atmospheric CO<sub>2</sub> levels.

However, alternative hypotheses have been proposed. Predictions of past CO<sub>2</sub> levels suggest that they may have previously dropped as precipitously low as that seen during the expansion of land plants: approximately 300 MYA, during the [[proterozoic|Proterozoic Era]].<ref>Riding, R. (2006). Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition.Geobiology, 4(4), 299-316.</ref> This being the case, there might have been a similar evolutionary pressure that resulted in the development of the pyrenoid, though in this case, a pyrenoid or pyrenoid-like structure could have developed, and have been lost as CO<sub>2</sub> levels then rose, only to be gained or developed again during the period of land colonisation by plants. Evidence of multiple gains and losses of pyrenoids over relatively short geological time spans was found in hornworts.<ref name =bop />