A carboxylic acid, CH3COOH, that is used as a carbon source by certain green algae. Combined with coenzyme A (see acetyl coenzyme a), it plays a crucial role in the energy metabolism of all organisms. Acetic acid bacteria comprise a group of bacteria, e.g. Acetobacter spp., that perform incomplete oxidation of alcohols to yield organic acids as end products. When the initial substrate is ethanol, the product is acetic acid.