Many Earth habitats have an extreme cold, heat, salinity, and pressure conditions. Such places are populated by extremophiles. Thermophiles prefer environments with high temperatures. Psychrophiles live at very low temperatures. Halophiles can tolerate high salt, alkaliphiles tolerate high and acidophiles low pH. Barophiles adapt to live under high pressure. Extremophiles have permanent modifications in their membranes and in the amino acid composition of their proteins that allow them to maintain a stable internal environment under extreme conditions. Many environments have several extreme conditions, e.g. cold and high pressure on the ocean floor; such places are populated by polyextremophiles.