Biological process

Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a pH stimulus with pH < 7. pH is a measure of the acidity or basicity of an aqueous solution.

This term should be used to annotate instances where a cell or organism is responding to a chemical that is playing the role of an acid (e.g. proton donor) and therefore lowering the pH. If instead you wish to describe a response to a specific acid as a chemical, such as the anion portion of glutamate, please annotate to the appropriate child of GO:0001101 'response to acid chemical'.