Carbon assimilation in upper subtidal macroalgae is determined by an inverse correlation between Rubisco carboxylation efficiency and CO 2 concentrating mechanism effectiveness.

Seaweeds have a wide ecophysiological and phylogenetic diversity with species expressing different Rubisco forms that frequently coexist with biophysical CO 2 concentrating mechanisms (CCMs), an adaptation that overcomes the low CO 2 availability and gas diffusion in seawater. Here, we assess the possible coevolution between the Rubisco catalysis and the type and effectiveness of CCMs present in six upper subtidal macroalgal species belonging to the three phylogenetic groups of seaweeds. A wide diversity in the Rubisco kinetic traits was found across the analysed species, although the specificity factor was the only parameter explained by the expressed Rubisco form. Differences in the catalytic trade-offs were found between Rubisco forms, indicating that ID Rubiscos could be better adapted to the intracellular O 2 /CO 2 ratio found in marine organisms during steady-state photosynthesis. The biophysical components of the CCMs also differed among macroalgal species, resulting in different effectiveness to concentrate CO 2 around Rubisco active sites. Interestingly, an inverse relationship was found between the effectiveness of CCMs and the in vitro Rubisco carboxylation efficiency, which possibly led to a similar carboxylation potential across the analysed macroalgal species. Our results demonstrate a coevolution between Rubisco kinetics and CCMs across phylogenetically distant marine macroalgal species sharing the same environment.