SummaryHigh-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 À1°C, the Rubisco turnover rate, k cat c , was 0.4 C s À1 per site and the half saturation constant for CO 2 was 15 lM (vs c. 3 C s À1 per site and 50 lM 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 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 Rubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.

We use autonomous gas measurements to examine the metabolic balance (photosynthesis minus respiration) of coastal Antarctic waters during the spring/summer growth season. Our observations capture the development of a massive phytoplankton bloom and reveal striking variability in pCO 2 and biological oxygen saturation (ΔO 2 /Ar) resulting from large shifts in community metabolism on time scales ranging from hours to weeks. Diel oscillations in surface gases are used to derive a high-resolution time series of net community production (NCP) that is consistent with 14 C-based primary productivity estimates and with the observed seasonal evolution of phytoplankton biomass. A combination of physical mixing, grazing, and light availability appears to drive variability in coastal Antarctic NCP, leading to strong shifts between net autotrophy and heterotrophy on various time scales. Our approach provides insight into the metabolic responses of polar ocean ecosystems to environmental forcing and could be employed to autonomously detect climate-dependent changes in marine primary productivity.

Summary This study explores some of the physiological mechanisms responsible for high productivity near the shelf in the Western Antarctic Peninsula despite a short growing season and cold temperature. We measured gross and net primary production at Palmer Station during the summer of 2012/2013 via three different techniques: incubation with H218O; incubation with 14CO2; and in situ measurements of O2/Ar and triple oxygen isotope. Additional laboratory experiments were performed with the psychrophilic diatom Fragilariopsis cylindrus. During the spring bloom, which accounted for more than half of the seasonal gross production at Palmer Station, the ratio of net‐to‐gross production reached a maximum greater than c. 60%, among the highest ever reported. The use of multiple techniques showed that these high ratios resulted from low heterotrophic respiration and very low daylight autotrophic respiration. Laboratory experiments revealed a similar ratio of net‐to‐gross O2 production in F. cylindrus and provided the first experimental evidence for an important level of cyclic electron flow (CEF) in this organism. The low ratio of community respiration to gross primary production observed during the bloom at Palmer Station may be characteristic of high latitude coastal ecosystems and partially supported by a very active CEF in psychrophilic phytoplankton.

SummaryThe goal of this study is to investigate the CO 2 concentrating mechanism (CCM) of the dominant phytoplankton species during the growing season at Palmer station in the Western Antarctic Peninsula.Key CCM parameters (cellular half-saturation constants for CO 2 fixation, carbonic anhydrase activity, CO 2 /HCO 3 À uptake, d 13 C org ) in natural phytoplankton assemblages were determined. Those results, together with additional measurements on CO 2 membrane permeability from Fragilariopsis cylindrus laboratory cultures, were used to develop a numerical model of the CCM of cold water diatoms. The field data demonstrate that the dominant species throughout the season possess an effective CCM, which achieves near saturation of CO 2 for fixation. The model provides a means to examine the role of eCA activity and HCO 3 À /CO 2 uptake in the functioning of the CCM. According to the model, the increase in d 13 C org during the bloom results chiefly from decreasing ambient CO 2 concentration (which reduces the gross diffusive flux across the membrane) rather than a shift in inorganic carbon uptake from CO 2 to HCO 3 À . The CCM of diatoms in the Western Antarctic Peninsula functions with a relatively small expenditure of energy, resulting chiefly from the low half-saturation constant for Rubisco at cold temperatures.

High-latitude oceans, in particular the coastal Western Antarctic Peninsula (WAP) region of the Southern Ocean, are experiencing a rapidly changing environment due to rising surface ocean temperatures and CO 2 concentrations. However, the direct effect of increasing CO 2 on polar ocean primary production is unclear, with a number of experiments showing conflicting results. It has been hypothesized that increased CO 2 may cause a reduction of the energy-intensive carbon concentrating mechanism (CCM) in phytoplankton, and these energy savings may lead to increased productivity. To test this hypothesis, we incubated natural phytoplankton communities in the WAP under high (800 ppm), current (400 ppm) and low (100 ppm) CO 2 for 2 to 3 wk during the austral spring-summer of 2012/2013. In 2 incubations with diatom-dominated phytoplankton assemblages, high CO 2 led to a clear down-regulation of CCM activity, as evidenced by an increase in half-saturation constants for CO 2 , a decrease in external carbonic anhydrase activity and a higher biological fractionation of stable carbon isotopes. In a third incubation, there was no observable regulation of the CCM, possibly because HCO 3 − served as the major inorganic carbon source in all treatments for this phytoplankton assemblage. We did not observe a significant effect of CO 2 on growth rates or community composition in the diatom-dominated communities. The lack of a measureable effect on growth despite CCM down-regulation is likely explained by a very small energetic requirement to concentrate CO 2 and saturate Rubisco at low temperatures.