Large variation in the Rubisco kinetics of diatoms reveals diversity among their carbon-concentrating mechanisms

While marine phytoplankton rival plants in their contribution to global primary productivity, our understanding of their photosynthesis remains rudimentary. In particular, the kinetic diversity of the CO₂-fixing enzyme, Rubisco, in phytoplankton remains unknown. Here we quantify the maximum rates of carboxylation (k _cat ^c), oxygenation (k _cat ^o), Michaelis constants (K _m) for CO₂ (K _C) and O₂ (K _O), and specificity for CO₂ over O₂ (S_C/O) for Form I Rubisco from 11 diatom species. Diatom Rubisco shows greater variation in K _C (23-68 μM), S_C/O (57-116mol mol^-1), and K _O (413-2032 μM) relative to plant and algal Rubisco. The broad range of K _C values mostly exceed those of C₄ plant Rubisco, suggesting that the strength of the carbon-concentrating mechanism (CCM) in diatoms is more diverse, and more effective than previously predicted. The measured k _cat ^c for each diatom Rubisco showed less variation (2.1-3.7s^-1), thus averting the canonical trade-off typically observed between K _C and k _cat ^c for plant Form I Rubisco. Uniquely, a negative relationship between K _C and cellular Rubisco content was found, suggesting variation among diatom species in how they allocate their limited cellular resources between Rubisco synthesis and their CCM. The activation status of Rubisco in each diatom was low, indicating a requirement for Rubisco activase. This work highlights the need to better understand the correlative natural diversity between the Rubisco kinetics and CCM of diatoms and the underpinning mechanistic differences in catalytic chemistry among the Form I Rubisco superfamily.