A Voltage-Gated H+ Channel Underlying pH Homeostasis in Calcifying Coccolithophores
Figure 7
Model of the major ion fluxes associated with calcification and pH homeostasis in coccolithophores.
The scheme illustrates the requirement for efficient H+ efflux pathways in coccolithophores as a result of intracellular calcification. Mature coccoliths are arranged on the extracellular surface, surrounding the cell to form a coccosphere (A). However, coccolith formation occurs within the intracellular Golgi-derived coccolith vacuole. Calcium carbonate (CaCO3) precipitation requires the production of carbonate (CO32−) from bicarbonate (HCO3−) and results in the net production of H+ (B). H+ must be rapidly removed from the coccolith vacuole in order to maintain a suitable pH for CaCO3 precipitation. Once in the cytosol (C), some H+ may potentially be utilised by photosynthesis in the production of CO2 from HCO3−, however H+ efflux provides an efficient mechanism to prevent cytosolic acidosis during fluctuations in photosynthetic rate. At normal seawater pH 8.2, pHi of 7.2, and Vm ∼−46 mV maintained by a Cl– inward rectifier (D)[18], a drop in pHi alone or in combination with membrane depolarisation would result in a net outward proton motive force across the plasma membrane. This enables passive H+ efflux via a plasma membrane localised H+ channel (E), providing a rapid mechanism for maintaining constant intracellular pH. Other membrane transporters (yet to be characterised) are likely involved in longer term maintenance of cytoplasmic pH (F). Nevertheless, the pH- and voltage-sensitive gating mechanism of the H+ channel coupled to its high transport capacity suggests it plays a major role in the modulation of intracellular pH in coccolithophores. Patch clamp studies indicate that Cl– and H+ are the dominant transmembrane conductances in coccolithophores.