ION PERMEABILITY OF THE CYTOPLASMIC MEMBRANE LIMITS THE MAXIMUM GROWTH TEMPERATURE OF BACTERIA AND ARCHAEA

Protons and sodium ions are the most commonly used coupling ions in en ergy transduction in bacteria and archaea. At their growth temperature , the permeability of the cytoplasmic membrane of thermophilic bacteri a to protons is high compared with that of sodium ions. In some thermo philes, sodium is the sole energy-coupling ion. To test whether sodium is the preferred coupling ion at high temperatures, the proton- and s odium permeability was determined in liposomes prepared from lipids is olated from various bacterial and archaeal species that differ in thei r optimal growth temperature. The proton permeability increased with t he temperature and was comparable for most species at their respective growth temperatures. Liposomes of thermophilic bacteria are an except ion in the sense that the proton permeability is already high at the g rowth temperature. In all liposomes, the sodium permeability was lower than the proton permeability and increased with the temperature. The results suggest that the proton permeability of the cytoplasmic membra ne is an important parameter in determining the maximum growth tempera ture.