Adaptations of anaerobic archaea to life under extreme energy limitation.

Some anaerobic archaea live on substrates that do not allow the synthesis of 1 mol of ATP per mol of substrate. Energy conservation in these cases is only possible by a chemiosmotic mechanism that involves the generation of an electrochemical ion gradient across the cytoplasmatic membrane that then drives ATP synthesis via an A1AO ATP synthase. The minimal amount of energy required is thus depending on the magnitude of the electrochemical ion gradient, the phosphorylation potential, and the ion/ATP ratio of the ATP synthase. Methanogens, Thermococcus, Pyrococcus, and Ignicoccus have evolved different ways to energize their membranes, such as methyltransferases, H+, or NAD+ reducing electron transport systems fueled by reduced ferredoxin or H2 -dependent sulfur reduction that all operate at the thermodynamic limit of life. The structure and function of the enzymes involved are discussed. Despite the differences in membrane energization, they have in common an A1AO ATP synthase that shows an extraordinary divergence in rotor composition and structural adaptations to life under these conditions. In sum, adaptation of anaerobic archaea to energy-limited substrates involves chemiosmotic energy coupling, often with Na+ as coupling ion and a structurally and functionally highly adapted ATP synthase.