Journal
NEW PHYTOLOGIST
Volume 205, Issue 1, Pages 172-181Publisher
WILEY
DOI: 10.1111/nph.13021
Keywords
carboxylation; cold water; diatoms; phytoplankton; primary production; psychrophilic; Rubisco; Western Antarctic Peninsula (WAP)
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Funding
- US National Science Foundation [OPP 1043593, EF 1040965, OCE 0825192]
- Natural Science and Engineering Research Council of Canada
- Directorate For Geosciences [1043593] Funding Source: National Science Foundation
- Emerging Frontiers [1040965] Funding Source: National Science Foundation
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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 degrees C, the Rubisco turnover rate, k(cat)(c), was 0.4Cs(-1) per site and the half saturation constant for CO2 was 15M (vs c. 3Cs(-1) per site and 50M at 20 degrees C). To achieve high carboxylation rates, psychrophilic diatoms increased Rubisco abundance to c. 8% of biomass (vs c. 0.6% at 20 degrees 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 CO2 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 Rubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.
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