Influence of Temperature and CO2 On Plasma-membrane Permeability to CO2 and HCO3- in the Marine Haptophytes Emiliania huxleyi and Calcidiscus leptoporus (Prymnesiophyceae)

Abstract

Membrane permeabilities to CO₂ and HCO₃^- constrain the function of CO₂ concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO₂ but effectively impermeable to HCO₃^- . Here, CO₂ and HCO₃^- membrane permeabilities were measured using an ¹⁸ O-exchange technique on two species of haptophyte algae, Emiliania huxleyi and Calcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO₂ (0.006-0.02 cm · s^-1 ) but minimally permeable to HCO₃^- . Increased temperature and CO₂ generally increased CO₂ membrane permeabilities in both species, possibly due to changes in lipid composition or CO₂ channel proteins. Changes in CO₂ membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma-membrane permeabilities to CO₂ were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret ¹⁸ O-exchange data assumes that carbonic anhydrase, which catalyzes ¹⁸ O-exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two-compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate ¹⁸ O-exchange data and then inferring plasma-membrane CO₂ permeabilities from the simulated data. This analysis showed that the inferred plasma-membrane CO₂ permeabilities are minimal estimates but should be quite accurate under most conditions.

Keywords: CO2; carbon concentrating mechanism; haptophyte; membrane; permeability.