Cellular pH measurements in Emiliania huxleyi reveal pronounced membrane proton permeability

Authors


Author for correspondence:
Markus Bleich
Tel: +49 (0)431 880 2961
Email: m.bleich@physiologie.uni-kiel.de

Summary

  • 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 (pHi). 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 pHi unit. pHi acutely followed the pH of seawater (pHe) in a linear fashion between pHe values of 6.5 and 9 with a slope of 0.44 per pHe unit. pHi was nearly insensitive to changes in seawater CO2 at constant pHe 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 pHe on pHi.
  • The data for the first time show the occurrence of a proton permeation pathway in E. huxleyi plasma membrane. pHi homeostasis involves a DIDS-sensitive mechanism.

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