A basal carbon concentrating mechanism in plants?

Many photosynthetic organisms have developed inorganic carbon (Ci) concentrating mechanisms (CCMs) that increase the CO2 concentration within the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). Several CCMs, such as four carbon (C4) and crassulacean acid metabolism (CAM), bicarbonate accumulation systems and capsular structures around RubisCO have been described in great detail. These systems are believed to have evolved several times as mechanisms that acclimate organisms to unfavourable growth conditions. Based on recent experimental evidence we propose the occurrence of another more general CCM system present in all plants. This basal CCM (bCCM) is supposed to be composed of mitochondrial carbonic anhydrases (a β-type carbonic anhydrase and the γ-type carbonic anhydrase domain of the mitochondrial NADH dehydrogenase complex) and probably further unknown components. The bCCM is proposed to reduce leakage of CO2 from plant cells and allow efficient recycling of mitochondrial CO2 for carbon fixation in chloroplasts.

► We propose a basal carbon concentrating mechanism in plants and eukaryotic algae.
► bCCM allows recycling of mitochondrial CO2 for carbon fixation in chloroplasts.
► Mitochondrial γ carbonic anhydrases are proposed to export HCO3− to the cytosol.
► Bicarbonate reaches chloroplast by translocators or by a single celled C4 like cycle.
► Differences between theoretically and experimentally carbon fixation are explained.