Life in acid: pH homeostasis in acidophiles

Microorganisms that have a pH optimum for growth of less than pH 3 are termed ‘acidophiles’. To grow at low pH, acidophiles must maintain a pH gradient of several pH units across the cellular membrane while producing ATP by the influx of protons through the F0F1 ATPase. Recent advances in the biochemical analysis of acidophiles coupled to sequencing of several genomes have shed new insights into acidophile pH homeostatic mechanisms. Acidophiles seem to share distinctive structural and functional characteristics including a reversed membrane potential, highly impermeable cell membranes and a predominance of secondary transporters. Also, once protons enter the cytoplasm, methods are required to alleviate effects of a lowered internal pH. This review highlights recent insights regarding how acidophiles are able to survive and grow in these extreme conditions. Microorganisms that have a pH optimum for growth of less than pH 3 are termed ‘acidophiles’. To grow at low pH, acidophiles must maintain a pH gradient of several pH units across the cellular membrane while producing ATP by the influx of protons through the F0F1 ATPase. Recent advances in the biochemical analysis of acidophiles coupled to sequencing of several genomes have shed new insights into acidophile pH homeostatic mechanisms. Acidophiles seem to share distinctive structural and functional characteristics including a reversed membrane potential, highly impermeable cell membranes and a predominance of secondary transporters. Also, once protons enter the cytoplasm, methods are required to alleviate effects of a lowered internal pH. This review highlights recent insights regarding how acidophiles are able to survive and grow in these extreme conditions. A measurement of the energized state of the cell membrane as a result of a charge separation between the cytoplasm and external milieu created by the membrane potential (ΔΨ) and pH gradient (ΔpH) across the membrane. In acidophiles, the PMF is primarily made up of the ΔpH. For example, an acidophile with a near neutral cytoplasm (pH 6) separated from an acidic environment (pH 2) will experience a net force (i.e. the PMF) across the cell membrane that can drive energy dependent processes. Acidophiles have an inside positive ΔΨ potentially formed by the influx of potassium ions, which inhibits proton entry into the cell. The inside positive ΔΨ detracts from the large PMF formed by the ΔpH. A difference in electrical potential formed between two solutions separated by an ion-exchange membrane without any current flowing through the membrane. A stable Donnan potential or ‘Donnan equilibrium’ is formed when the diffusion of ions in one direction equals the electromigrational flux of ions in the opposite direction, resulting in net zero mass and charge transport. Active secondary transporters are membrane proteins that use the transmembrane electrochemical gradient of protons or sodium ions to drive transport. As a result of the predominance of secondary transporters evident in acidophile genome sequences, they are believed to be an important functional adaptation to life at low pH. Synthetic, uncharged compounds (such as protonated acids or a conjugate base) that have dissociable protons and permeate bilayers to easily pass across cell membranes. Uncouplers shuttle the net uniport of protons and ‘uncouple’ proton transport from cellular processes. In acidophiles, uncouplers include organic acids that are protonated (and therefore neutral) at the low external pH and pass into the near-neutral cytoplasm where the proton dissociates, leading to cytoplasmic protonation.