Polar and neutral lipid composition in the pelagic tunicate Pyrosoma atlanticum.

Structure and functioning of colonial pyrosomes are largely undescribed and their lipid characteristics have received limited attention. The aim of this paper is to fill this gap on one of the dominant species Pyrosoma atlanticum. Lipid content is tightly coupled to size and weight. Lipid composition shows a large dominance of structural polar lipids. Neutral lipids were dominated by sterols with low levels of acylglycerols and free fatty acids. Phospholipids show a dominance of PC with intermediate percentages of PE and DPG. Other constituents (PS, PI, LPC, sphingolipids) were present at lower levels. Fatty acid composition of DAG and TAG showed a dominance of saturated acids (16:0, 14:0), DHA and intermediate levels of MUFA. Phospholipids were dominated by DHA with values exceeding 30% of total FA in all categories except for PI, where lower percentages occurred. Saturated acids were second in abundance with MUFA showing intermediate concentrations. Sterols were dominated by 24-methylcholesta-5,22E-dien-3beta-ol with more than 22% of the total sterol. Cholesterol (cholest-5-en-3beta-ol) represented only 12 % of the total while 24-methylcholesta-5,24(28)E-dien-3beta-ol accounted for 11% of the total sterols. The low levels of triacylglycerols and free fatty acids, coupled with high concentrations of glycolipids and phytoplankton-derived degraded chloropigments, is evidence of a direct link with the digestive activity and substantiate the idea of a high physiological turnover as an alternative to large lipid accumulation. The fatty acid and sterol profiles are consistent with a diverse phytoplankton diet, and a strong contribution of phospholipid classes to energy needs, including locomotion.