The pressure-volume-temperature (PVT) properties of a lipid mixture from a marine copepod, Calanus plumchrus: Implications for buoyancy and sound scattering

Abstract

The density of a lipid mixture composed of nearly 85% wax esters and isolated from the copepod Calanus plumchrus was determined at 5.13, 14.49, and 23.65°C and at pressures up to 783 bars. At atmospheric pressure, the density was determined at temperatures between 1.64 and 15.54°C. The experimental isotherms and isobars were fitted to several equations of state and these were used to calculate coefficients of thermal expansion and compressibility. The wax ester mixture visibly melted as a function of temperature at atmospheric pressure. The high pressure isotherms show a volume discontinuity as a function of pressure that suggests a first-order liquid-solid phase transition. The coefficient of thermal expansion at atmospheric pressure is quite large (up to ten times that of water) and is apparently due in part to the phase change and its concomitant volume change. The coefficient of compressibility at atmospheric pressure is larger for the more solid form of the wax ester mixture than for the more liquid form. The contribution of wax esters to the buoyant properties of organisms is discussed. The buoyant force on the wax ester in seawater changes more as a function of temperature than as a function of pressure. The data also have implications to understanding sound scattering by wax ester-containing organisms. At depths defined by the pressure-temperature curve for the phase change, the wax ester mixture is on the verge of either melting or freezing. This results in a large coefficient of compressibility and may result in distinctive sound scattering by organisms containing wax esters and swimming at such depths. Furthermore, sound scattering should be different in organisms with solid wax ester mixtures than in those with liquid mixtures.