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. 2023 May 8;120(20):e2300466120. doi: 10.1073/pnas.2300466120

Fig. 1.

Fig. 1.

Comparing the cyanobacterial CO2 concentrating mechanism (CCM) to the traditional box model of photosynthetic C isotope discrimination. (A) Cyanobacterial CCMs rely on i) active Ci uptake into the cell, and ii) coencapsulation of carbonic anhydrase (CA) and rubisco within the carboxysome. Independent, powered transporters for HCO3− and CO2 are shown in brown and purple; both work to increase cytosolic concentrations of HCO3− (see ref. 12 for review). All CCM components work to produce a high carboxysomal CO2 concentration that enhances CO2 fixation by rubisco and suppresses oxygenation. Limited CO2 escapes from the carboxysome—some is scavenged by CO2 pumps while the rest leaves the cell. (B) Architecture of the traditional box model based on (14–17); see SI Appendix for full discussion of this model. Boxes denote carbon pools of interest, and fluxes between boxes are denoted by Φ. Each flux has its own isotopic fractionation denoted by ε; no fractionation is assumed for Φloss. Model assumes an infinitely large external carbon pool, that carbon not fixed by rubisco (Clost) returns to this pool, and that fluxes are at steady state. Note that this architecture does not include a box for the carboxysome. (C) Model solution for the traditional model is εP = a*εequil + f*εRubisco (Eq. 2), where εP is defined as the difference in δ13C of Cexternal and Cfixed, f is defined as the fraction of Ci lost (Φloss/Φin), and a is the fractional contribution of HCO3− to total Ci uptake. When a = 0, all Ci uptake is as CO2 (dotted line); when a = 1, all Ci uptake is as HCO3− (solid line). This model is presented in ref. 18, which is a generalization of (19) that accounts for the fact that Ci uptake (Φin in Panel B) ranges in composition between CO2 and HCO3− based on which Ci uptake system is used. Values of εRubisco = 25‰ and εequil = −9‰ were used for this illustration (20). Model outputs indicate that at high external CO2 concentrations (dark wedge under graph), there is greater net Ci leakage (larger f values) from the cell.