pH regulation in an acidophilic green alga - a quantitative analysis

Short-term cytosolic pH regulation has three components: H(+) binding by buffering groups; H(+) transport out of the cytosol; and H(+) transport into the vacuole. To understand the large differences plants show in their tolerance to acidic environments, these three components were quantified in the acidophilic unicellular green alga Eremosphaera viridis. Intracellular pH was recorded using ion-selective microelectrodes, whereas constant doses of weak acid were applied over different time intervals. A mathematical model was developed that describes the recorded cytosolic pH changes. Recordings of cytosolic K(+) and Na(+) activities, and application of anion channel inhibitors, revealed which ion fluxes electrically compensate H(+) transport. The cytosolic buffer capacity was beta = 30 mM. Acidification resulted in a substantial and constant H(+) efflux that was probably driven by the plasmalemma H(+)-ATPase, and a proportional pH regulation caused by H(+) pumped into the vacuole. Under severe cytosolic acidification (> or = 1 pH) more than 50% of the ATP synthesized was used for H(+) pumping. While H(+) influx into the vacuole was compensated by cation release, H(+) efflux out of the cell was compensated by anion efflux. The data presented here give a complete and quantitative picture of the ion fluxes during acid loading in an acidophilic green plant cell.