| Summary: | 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 (kcat<sup>c</sup>), oxygenation (kcat<sup>o</sup>), 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. Uniquely diatom Rubisco shows extensive variations in KC (23 to 68 µM), SC/O (57 to 116 mol.mol<sup>-1</sup>) and KO (23 to 68 µM) relative to plant and algal Rubisco. The broad range of KC values mostly exceed those of C4-plant Rubisco, suggesting the carbon concentrating mechanism (CCM) efficiency in diatoms is more diverse, and more effective than previously predicted. The measured kcat^c for each diatom Rubisco showed less variation (2.1 to 3.7 s<sup>-1</sup>) thus averting the canonical trade-off typically observed between KC and kcat<sup>c</sup> 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 limiting cellular resources between Rubisco synthesis and CCM efficiency. 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.
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