(H2O), the principal component of living organisms including humans, dissociates into H+ and OH-in aqueous environments, and the resulting H+ concentration determines both cellular pH and the proton motive force (PMF) across cellular membranes. These physicochemical parameters are fundamental regulators of a wide range of biological processes. Optogenetics enables the manipulation of biological and cellular functions using light, typically through the ectopic expression of microbial rhodopsins as photoreceptive proteins in target cells or organs.