pH regulation in an acidophilic green alga – a quantitative analysis
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- • 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 β = 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 (≥ 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.