Journal Article

Slow carboxylation of Rubisco constrains the rate of carbon fixation during Antarctic phytoplankton blooms

Tortell, Philippe D, Young, Jodi N, Kranz, Sven A, Goldman, Johanna A. L, Morel, Francois M. M

2015

Overview

High‐latitude oceans are areas of high primary production despite temperatures that are often well below the thermal optima of enzymes, including the key Calvin Cycle enzyme, Ribulose 1,5 bisphosphate carboxylase oxygenase (Rubisco). We measured carbon fixation rates, protein content and Rubisco abundance and catalytic rates during an intense diatom bloom in the Western Antarctic Peninsula (WAP) and in laboratory cultures of a psychrophilic diatom (Fragilariopsis cylindrus). At −1°C, the Rubisco turnover rate, kcₐₜᶜ, was 0.4 C s⁻¹per site and the half saturation constant for CO₂was 15 μM (vs c. 3 C s⁻¹per site and 50 μM at 20°C). To achieve high carboxylation rates, psychrophilic diatoms increased Rubisco abundance to c. 8% of biomass (vs c. 0.6% at 20°C), along with their total protein content, resulting in a low carbon : nitrogen ratio of c. 5. In psychrophilic diatoms, Rubisco must be almost fully active and near CO₂saturation to achieve carbon fixation rates observed in the WAP. Correspondingly, total protein concentrations were close to the highest ever measured in phytoplankton and likely near the maximum possible. We hypothesize that this high protein concentration, like that of Rubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.

• Publisher: Academic Press,New Phytologist Trust,Wiley Subscription Services, Inc
• Subject: Abundance / algal blooms / Antarctic region / Antarctic Regions / Bacillariophyceae / Bacillariophycidae / Biomass / Blooms / Calvin cycle / Carbon / Carbon - metabolism / Carbon Cycle / Carbon dioxide / Carbon Dioxide - metabolism / Carbon fixation / Carboxylation / Chlorophylls / cold water / Diatoms / Diatoms - enzymology / Enzymes / Eutrophication / Fragilariopsis cylindrus / Kinetics / latitude / Marine microorganisms / Nitrogen / Nitrogen - metabolism / Oceans / Oxygenase / Phytoplankton / Phytoplankton - enzymology / Phytoplankton - growth & development / Plankton / Primary production / Primary productivity / protein content / Proteins / Proteins - metabolism / psychrophilic / ribulose / Ribulose-bisphosphate carboxylase / Ribulose-Bisphosphate Carboxylase - metabolism / Rubisco / Saturation / Seasons / Temperature / Turnover rate / Water temperature / Western Antarctic Peninsula (WAP)