Osmolites or compatible solutes are organic molecules which are compatible with cell metabolism even at molar concentrations and play protective roles in adaptation to extreme environments. Microorganisms also accumulate organic solutes for osmotic adjustment. Thermophilic and hyperthermophilic organisms accumulate alcohol phosphates (e.g., inositol-phosphates) as compatible solutes for thermal stabilization, something which does not occur in bacteria or archaea that grow at low or moderate temperature. Osmolites or soluble compatibles can be classified into three groups: poly-alcohol molecules (e.g., manytol), quaternary ammines (e.g., glycine betaine), and amino acids (e.g., proline). Compatible solutes have been studied in vitro and have shown significant effects on biomolecules as stabilizers of native macromolecules (proteins or nucleic acid structures).