The polar lipid fraction E (PLFE) is a major tetraether lipid component in the thermoacidophilic archaeon Sulfolobus acidocaldarius. Using differential scanning and pressure perturbation calorimetry as well as ultrasound velocity and density measurements, we have determined the compressibilities and volume fluctuations of PLFE liposomes derived from different cell growth temperatures (T-g = 68, 76, and 81 degrees C). The compressibility and volume fluctuation values of PLFE liposomes, which are substantially less than those detected from diester lipid membranes (e.g., DPPC), exhibit small but significant differences with Tg. Among the three T(g)s employed, 76 C leads to the least compressible and most tightly packed PLFE membranes. This temperature is within the range for optimal cell growth (75-80 degrees C). It is known that a decrease in Tg decreases the number of cyclopentane rings in archael tetraether lipids. Thus, our data enable us to present the new view that membrane packing in PLFE liposomes varies with the number of cyclopentane rings in a nonlinear manner, reaching maximal tightness when the tetraether lipids are derived from cells grown at optimal Tgs. In addition, we have studied the effects of pressure on total layer thickness, d, and neutron scattering length density, rho(n), of a silicon-D2O interface that is covered with a PLFE membrane using neutron reflectometry (NR). At 55 degrees C, d and rho(n) are found to be rather insensitive to pressure up to 1800 bar, suggesting minor changes of the thickness of the membrane's hydrophobic core and headgroup orientation upon compression only.