Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here, we reveal a convergent Rubisco kinetic adaptation in phylogenetically distant seagrass species that shared catalytic efficiencies and CO2 and O2 affinities up to 3-times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO2 concentrating mechanisms (CCMs), which likely evolved in response to the constant CO2 limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C4 plants. Thus, our results demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses across distantly related aquatic lineages.
