Low temperature reduces the energetic requirement for the CO₂ concentrating mechanism in diatoms

The goal of this study is to investigate the CO₂ 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 CO₂ fixation, carbonic anhydrase activity, CO₂/HCO₃^- uptake, δ¹³C_org) in natural phytoplankton assemblages were determined. Those results, together with additional measurements on CO₂ 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 CO₂ for fixation. The model provides a means to examine the role of eCA activity and HCO₃^-/CO₂ uptake in the functioning of the CCM. According to the model, the increase in δ¹³C_org during the bloom results chiefly from decreasing ambient CO₂ concentration (which reduces the gross diffusive flux across the membrane) rather than a shift in inorganic carbon uptake from CO₂ to HCO₃^-. 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.