Survival strategies and membrane properties of extremophiles

The cytoplasmic membranes of Bacteria and Archaea determine to a large extent the composition of the cytoplasm. Since the ion and, in particular, the proton and/or the sodium ion electrochemical gradients across the membranes are crucial for the bioenergetic conditions of these microorganisms, strategies are needed to restrict the permeation of these ions across their cytoplasmic membranes. The proton and sodium permeabilities of biological membranes increase with the temperature. Psychrophilic and mesophilic Bacteria, and mesophilic, (hyper)thermophilic, and halophilic Archaea are capable of adjusting the lipid composition of their membranes in such a way that the proton permeability at the respective growth temperature remains constant (homeoproton permeability). Thermophilic Bacteria, however, have more difficulties to restrict the proton permeation across their membranes at high temperatures and often they rely on the less permeable sodium ions for maintaining a high sodium-motive force, which then drives energy requiring membrane-bound processes.