Inorganic Carbon Acquisition and Photosynthetic Metabolism in Marine Photoautotrophs: A Summary

The diffusive availability of CO₂ for photosynthesis is orders of magnitude lower in water than in air. This, and the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) for CO₂, implies that most marine photoautotrophs (cyanobacteria, microalgae, macroalgae and marine angiosperms or seagrasses) would be severely restricted were they to rely only on dissolved CO₂ for their photosynthetic performance. On the other hand, the ~120 times higher concentration of bicarbonate (HCO₃^-) makes this inorganic carbon (Ci) form more available for utilisation by marine photosynthesisers. The most common way in marine macrophytes to utilise HCO₃^- is to convert it to CO₂ within acidic micro-zones of diffusion boundary layers (DBLs), including the cell walls, as catalysed by an outwardly acting carbonic anhydrase (CA). This would then generate an intra-chloroplastic (or for cyanobacteria intra-carboxysomal) CO₂-concentrating mechanism (CCM). Some algae (e.g., the common macroalgae Ulva spp.) and most cyanobacteria and microalgae feature direct HCO₃^- uptake as the most efficient CCM, while others (e.g., some red algae growing under low-light conditions) may rely on CO₂ diffusion only. We will in this contribution summarise our current understanding of photosynthetic carbon assimilation of submerged marine photoautotrophs, and in particular how their 'biophysical' CCMs differ from the 'biochemical' CCMs of terrestrial C₄ and Crassulacean Acid Metabolism (CAM) plants (for which there is very limited evidence in cyanobacteria, algae and seagrasses).