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

• Published in: New Phytologist
• Main Authors: Young, Jodi N., Goldman, Johanna A. L., Kranz, Sven A., Tortell, Philippe D., Morel, Francois M. M.
• Other Authors: US National Science Foundation, Natural Science and Engineering Research Council of Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1111/nph.13021; http://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fnph.13021; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fnph.13021; https://onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13021; https://onlinelibrary.wiley.com/doi/full-xml/10.1111/nph.13021; https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13021

Summary High‐latitude oceans are areas of high primary production despite temperatures that are often well below the thermal optima of enzymes, including the key C alvin C ycle enzyme, R ibulose 1,5 bisphosphate carboxylase oxygenase ( R ubisco). We measured carbon fixation rates, protein content and R ubisco abundance and catalytic rates during an intense diatom bloom in the W estern A ntarctic P eninsula ( WAP ) and in laboratory cultures of a psychrophilic diatom ( F ragilariopsis cylindrus ). At −1°C, the R ubisco turnover rate, k cat c , was 0.4 C s −1 per site and the half saturation constant for CO 2 was 15 μM (vs c . 3 C s −1 per site and 50 μM at 20°C). To achieve high carboxylation rates, psychrophilic diatoms increased R ubisco 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, R ubisco must be almost fully active and near CO 2 saturation 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 R ubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.