The deep-sea archaeon Methanococcus jannaschii was grown at 86؇C and under 8, 250, and 500 atm (1 atm ‫؍‬ 101.29 kPa) of hyperbaric pressure in a high-pressure, high-temperature bioreactor. The core lipid composition of cultures grown at 250 or 500 atm, as analyzed by supercritical fluid chromatography, exhibited an increased proportion of macrocyclic archaeol and corresponding reductions in archaeol and caldarchaeol compared with the 8-atm cultures. Thermal analysis of a model core-lipid system (23% archaeol, 37% macrocyclic archaeol, and 40% caldarchaeol) using differential scanning calorimetry revealed no well-defined phase transition in the temperature range of 20 to 120؇C. Complementary studies of spin-labeled samples under 10 and 500 atm in a special high-pressure, high-temperature electron paramagnetic resonance spectroscopy cell supported the differential scanning calorimetry phase transition data and established that pressure has a lipid-ordering effect over the full range of M. jannaschii's growth temperatures. Specifically, pressure shifted the temperature dependence of lipid fluidity by ca. 10؇C/500 atm.For archaea to flourish in the extreme environments that many inhabit, normal membrane structure and function must be preserved. Preservation of membrane function may derive partly from the unique physical structures of archaeal lipids, which differ greatly from those of their eubacterial counterparts (7,(16)(17)(18). In contrast to the straight or minimally branched, mostly unsaturated fatty acids of variable lengths found in eubacteria, archaeal apolar chains are based on saturated isopranoid alcohols typically containing 20 or 40 carbon atoms. These isopranyl chains are connected to a glycerol (or more rarely, a complex polyol) head group by ether bonds, not the fatty acid ester bonds common to eubacteria. Moreover, the stereochemistry of these ether lipids (D or sn-2,3) is opposite to that seen in eubacteria (L or sn-1,2). Some archaea also possess bilayer-spanning tetraether lipids, in which a pair of biphytanyl-based chains are linked to two glycerol moieties via ether bonds. In total, the structural differences are pronounced enough to raise the question of whether archaeal lipids respond differently to temperature and pressure than do eubacterial lipids, thus contributing to microbial viability under extreme conditions.Given that eubacterial thermophiles and barophiles do exist (6), the unique structural features of archaeal lipids are not necessarily required for retention of membrane structure and function at elevated temperatures and pressures. However, considering the abundance of archaea and relative scarcity of eubacteria at the outer regions of the pressure-temperature envelope at which life is known to exist, many archaea appear to possess an intrinsic capacity for thermophilic and barophilic behavior.To adapt to moderate fluctuations in temperature, virtually all eubacteria can maintain optimal membrane fluidity by modulating their lipid composition in a process termed homeoviscous ad...

Methanococcus jannaschii, an extremely thermophilic methanogen isolated from a deep-sea hydrothermal vent, was grown at 80'C in continuous culture on a mineral salts medium gassed with H2 and COZ at three different flow rates. The maximum specific growth rate was 0.56 h-', and the maximum specific methane productivity was 0.32 (mol g-' h-'). Uncoupling of growth and methane production was evidenced by an increase in the non-growth-associated rate of methane formation, p, with increasing gaseous input.The specific hydrogenase activity exhibited growthassociated behavior at low growth rates, but showed no dependence on growth at higher growth rates. The growth dependence of hydrogenase activity is consistent with the pressure dependence of hydrogenase activity measured in previous experiments. In contrast, the specific protease activity was independent of the growth rate over the entire range of dilution rates studied.