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

Abstract

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 (kcatc), oxygenation (kcato), Michaelis constants (Km) for CO2 (KC) and O2 (KO), and specificity for CO2 over O2 (SC/O) for Form I Rubisco from 11 diatom species. Diatom Rubisco shows greater variation in KC (23–68 µM), SC/O (57–116mol mol−1), and KO (413–2032 µM) relative to plant and algal Rubisco. The broad range of KC 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 kcatc for each diatom Rubisco showed less variation (2.1–3.7s−1), thus averting the canonical trade-off typically observed between KC and kcatc for plant Form I Rubisco. Uniquely, a negative relationship between KC 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.