Abstract
It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is known and where there are gaps in knowledge. Irreversible decarboxylases produce CO2, and CO2 is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO2; others use HCO3(-). The relative role of permeation through the lipid bilayer versus movement through CO2-selective membrane proteins in the downhill, non-energized, movement of CO2 is not clear. Passive permeation explains most CO2 entry, including terrestrial and aquatic organisms with C3 physiology and biochemistry, terrestrial C4 plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO3(-) use in CO2 concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there is some evidence of active CO2 influx at the plasmalemma of algae. HCO3(-) active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO3(-) can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO2 and HCO3(-) pools consequent upon CCMs result in leakage involving CO2, and occasionally HCO3(-). Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering in the CCM; leakage from terrestrial C4 plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.
Highlights
The textbook equations for oxygenic photosynthesis and for dark respiration have CO2 as, respectively, the inorganic C substrate and the inorganic C product
The second over-simplification is that there are known CO2 concentrating mechanisms (CCMs), involving active transport of some inorganic C species and/or C4 or crassulacean acid metabolism (CAM) biochemistry, accounting for about half of global primary productivity. This assertion is based on the global net primary productivity values of 56 Pg C per year on land and 49 Pg C per year in the ocean (Field et al, 1998), the assumption that the ratio of global C4 gross primary productivity to total gross primary productivity, i.e. 0.23 (Still and Berry, 2003), applies to net primary productivity, and the assumption that not less than 0.8 of the marine global net primary productivity is carried out by organisms with CCMs (Raven et al 2012, 2014; Raven and Beardall, 2014)
As for C4 plants, so with cyanobacterial and algal CCMs: the prediction is an increasing fraction of the inorganic C pumped into the intracellular pool being lost as CO2 efflux with decreasing incident photosynthetically active radiation, and that algae relying on diffusive CO2 entry from the medium to Rubisco would be more common in low-irradiance habitats
Summary
The textbook equations for oxygenic photosynthesis and for dark respiration have CO2 as, respectively, the inorganic C substrate and the inorganic C product. The assumption is that terrestrial C4 and CAM vascular plants rely on CO2 entry from the cell wall to the cytosol where carbonic anhydrase equilibrates CO2 with HCO3–, the inorganic C substrate for PEPc (Colman and Espie, 1985; Nelson et al, 2005).
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