Extracellular Carbonic Anhydrase Supports Constitutive HCO3− Uptake in Fragilariopsis cylindrus Regardless of Temperature Changes

Diatoms, including Fragilariopsis cylindrus ( Fcyl ), are the major primary producers in productive polar oceans. Little is known about carbon concentrating mechanisms (CCMs) in polar diatoms and their sensitivity to ocean warming and acidification. Here we characterized the CCM response to temperature in Fcyl using Membrane Inlet Mass Spectrometry. Fcyl increases RuBisCO expression at lower temperatures to compensate slower catalytic rates but maintains a reliance on HCO3− uptake across different temperatures (−2 °C to 9 °C) despite higher CO2 solubility at colder temperatures. However, when external carbonic anhydrase (eCA) is inhibited, inorganic carbon usage switches from HCO3− uptake to a dependency on CO2 diffusion. Incorporating these measurements with modeling, we propose that relying on eCA supported HCO3− uptake is an adaptive strategy to the highly dynamic polar ocean environment which experience large fluctuations in [CO2] but where HCO3− is constantly available. ### Competing Interest Statement The authors have declared no competing interest.