Low temperature reduces the energetic requirement for the CO 2 concentrating mechanism in diatoms

Summary The goal of this study is to investigate the CO 2 concentrating mechanism ( CCM ) of the dominant phytoplankton species during the growing season at P almer station in the W estern A ntarctic P eninsula. Key CCM parameters (cellular half‐saturation constants for CO 2 fixation, carbonic anhydrase activity, CO 2 / HCO 3 − uptake, δ 13 C org ) in natural phytoplankton assemblages were determined. Those results, together with additional measurements on CO 2 membrane permeability from F ragilariopsis 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 e CA activity and HCO 3 − / CO 2 uptake in the functioning of the CCM . According to the model, the increase in δ 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 − . The CCM of diatoms in the W estern A ntarctic P eninsula functions with a relatively small expenditure of energy, resulting chiefly from the low half‐saturation constant for R ubisco at cold temperatures.