A highly water-soluble substance that accumulates within the cell(s) of an organism and enables it to maintain its water balance with the external environment. Halophilic archaea, for example, which are adapted to living in very salty water, contain intracellular solutes (e.g. glycine betaine, trehalose) that lower the water potential of their cell contents so that the organism stays in physiological balance with its surroundings. Some plants employ a similar strategy to adjust the water potential of their cells as a means of protecting against damage due to water shortage. Compatible solutes are generally sugar alcohols, sugars, alcohols, amino acids, or their derivatives. Some are synthesized by the organism, whereas others are accumulated from the environment.