Diversity in Rubisco kinetics and CO2-concentrating mechanisms in Cyanobacteria.

Cyanobacteria are the most ancient oxygenic photosynthetic organisms on Earth and play a pivotal role in the global carbon cycle. Despite their ecological and evolutionary significance, the mechanisms of carbon acquisition and fixation in this phylum remain largely unexplored beyond a few model species. Here, we examined representative taxa spanning the full phylogenetic breadth of Cyanobacteria, assessing in vivo carbon acquisition pathways, the role and effectiveness of CO2-concentrating mechanisms (CCMs), as well as conducting in vitro biochemical characterizations of the kinetic traits and carbon isotope fractionation of Rubisco. We found significant strain-specific differences in Rubisco kinetics and CCM performance, but a common signature of high Rubisco catalytic turnover coupled with low CO2 affinity, consistent with the co-evolution of this enzyme together with effective CCMs. Furthermore, we identified a strong positive correlation between Rubisco intrinsic carbon isotope fractionation and its CO2/O2 specificity factor. Together, these results provide insight into Rubisco catalysis and shed light on its co-evolution with CCMs, underscoring their role in shaping Earth's carbon dynamics.