An acid can be simplified as anything that releases H+ ions when dissolved in another chemical, most often water. These H+ ions are the things that will react with metal and wood to give the characteristic corrosive nature of acids. Concentrations of hydrogen ions can build up for many different reasons, for instance without oxygen your blood streams pH begins to fall and it is this increase in acidity that the body uses to tell you that you’re not breathing in enough air. Of course all sufficiently advanced organisms have evolved a way to remove hydrogen ions from themselves otherwise their internal chemistry could become extremely damaged. One of the most common methods is using a active transport mechanism that is powered by sunlight.
Now it has also been found that there are some hydrogen ion pumps that work in the opposite direction. A bacterium that exists in the deep ocean doesn’t have the luxury of much light and any energy it does get can’t be used to discard any chemical with some use left. If fact this bacteria uses a protein that is normally found in creature’s eyes called rhodopsin. Now this protein is incredibly sensitive to light and if directly exposed will bleach almost instantly. Now this protein is sensitive enough to still pick up energy at the depth this bacteria live and actually gathers H+ ions from the surroundings and gives them to the bacteria. These kind of inward H+ ion pumps are very rare while their inverse would be considered common. Finding and experimenting on an example of a photo active version is very important discovery indeed.