A scenario for the origin of life: Volume regulation by bacteriorhodopsin required extremely voltage sensitive...

The osmotic activity produced by internal, non-permeable, anionic nucleic acids and metabolites causes a persistent and life-threatening cell swelling, or cellular edema, produced by the Gibbs-Donnan effect. This evolutionary-critical osmotic challenge must have been resolved by LUCA or its ancestors, but we lack a cell-physiology look into the biophysical constraints to the solutions. Like mycoplasma, early cells conceivably preserved their volume with Cl^- , Na⁺ , and K⁺ -channels, Na⁺ /H⁺ -exchangers, and a light-dependent bacteriorhodopsin-like H⁺ -pump. Here, I simulated protocells having these ionic-permeabilities and inhabiting an oceanic pond before the Great-Oxygenation-Event. Protocells showed better volume control and stable resting potentials at lower external pH and higher temperatures, favoring a certain type of extremophile life. Prevention of Na⁺ -influx at night, with low bacteriorhodopsin activity, required deep shutdown of highly voltage-sensitive Na⁺ -channels and extremely selective K⁺ -channels, two conserved features essential for modern neuronal encoding. The Gibbs-Donnan effect universality implies that extraterrestrial cells, if they exist, may reveal similar volume-controlling mechanisms.