Cellular pH measurements in Emiliania huxleyi reveal pronounced membrane proton permeability

Abstract

• To understand the influence of changing surface ocean pH and carbonate chemistry on the coccolithophore Emiliania huxleyi, it is necessary to characterize mechanisms involved in pH homeostasis and ion transport. • Here, we measured effects of changes in seawater carbonate chemistry on the fluorescence emission ratio of BCECF (2′,7′‐bis‐(2‐carboxyethyl)‐5‐(and‐6)‐carboxyfluorescein) as a measure of intracellular pH (pH_i). Out of equilibrium solutions were used to differentiate between membrane permeation pathways for H⁺, CO₂ and HCO₃⁻. • Changes in fluorescence ratio were calibrated in single cells, resulting in a ratio change of 0.78 per pH_i unit. pH_i acutely followed the pH of seawater (pH_e) in a linear fashion between pH_e values of 6.5 and 9 with a slope of 0.44 per pH_e unit. pH_i was nearly insensitive to changes in seawater CO₂ at constant pH_e and HCO₃⁻. An increase in extracellular HCO₃⁻ resulted in a slight intracellular acidification. In the presence of DIDS (4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid), a broad‐spectrum inhibitor of anion exchangers, E. huxleyi acidified irreversibly. DIDS slightly reduced the effect of pH_e on pH_i. • The data for the first time show the occurrence of a proton permeation pathway in E. huxleyi plasma membrane. pH_i homeostasis involves a DIDS‐sensitive mechanism.