title: 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 (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 (SC/O) for Form I Rubisco from 11 diatom species. Diatom Rubisco shows greater variation in K C (23-68 µM), SC/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.
authors.affiliation: Department of Geosciences, Princeton University, Princeton, NJ 08544, USA youngjn@uw.edu.
authors.forename: Jodi N
authors.initials: JN
authors.lastname: Young
authors.affiliation: Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK.
authors.forename: Ana M C
authors.initials: AM
authors.lastname: Heureux
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Sharwood
authors.affiliation: Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK.
authors.forename: Rosalind E M
authors.initials: RE
authors.lastname: Rickaby
authors.affiliation: Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
authors.forename: François M M
authors.initials: FM
authors.lastname: Morel
authors.affiliation: Plant Science Division, Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia.
authors.forename: Spencer M
authors.initials: SM
authors.lastname: Whitney
mesh_terms: D002244:Carbon / Q000378:metabolism*; D017377:Diatoms / Q000201:enzymology / Q000378:metabolism*; D007700:Kinetics; D010788:Photosynthesis; D010839:Phytoplankton / Q000201:enzymology / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*
chemical_list: D002244:Carbon; D012273:Ribulose-Bisphosphate Carboxylase
keywords: Algae; Rubisco.; carbon fixation; diatoms; kinetics; photosynthesis