Depicted are typical Chlamydomonas cells with a single cup-shaped chloroplast (green) and a nucleus (grey circle, N). When carbon dioxide (CO2) levels in the air surrounding the alga are higher than oxygen (O2; top), Rubisco enzymes (blue and yellow) are dispersed in the chloroplast and efficiently fix carbon dioxide. When carbon dioxide levels are lower than oxygen (left), Rubisco fixes oxygen instead. This leads to harmful metabolites that need to be recycled, resulting in the generation of the by-product hydrogen peroxide (H2O2; orange). To counteract this, Chlamydomonas employ a carbon dioxide concentrating mechanism (CCM) which sequesters Rubisco enzymes in to pyrenoids that preclude the entry of oxygen (grey circular structure), and produces proteins that transport carbon dioxide into the chloroplast (Ci; purple). When oxygen levels are high (also known as hyperoxia; right), this leads to oxygen fixation and production of hydrogen peroxide as well. Neofotis et al. found that Chlamydomonas also contain pyrenoids under these conditions, even when there are high amounts of carbon dioxide and the full CCM is suppressed. This led them to propose that hydrogen peroxide triggers the formation of pyrenoids via an unknown mechanism when oxygen levels are high and when there are insufficient amounts of carbon dioxide.
Image credit: Britta Foerster (CC BY 4.0)