Fatty acid composition and N2 solubility in triacylglycerol-rich adipose tissue: the likely importance of intact molecular structure

Diving tetrapods (sea turtles, seabirds and marine mammals) are a biologically diverse group, yet all are under similar constraints: oxygen limitation and increased hydrostatic pressure at depth. Adipose tissue is important in the context of diving because nitrogen gas (N-2) is five times more soluble in fat than in blood, creating a potential N-2 sink in diving animals. Previous research demonstrates that unusual lipid composition [waxes and short-chained fatty acids (FA)] in adipose tissue of some whales leads to increased N-2 solubility. We evaluated the N-2 solubility of adipose tissue from 12 species of diving tetrapods lacking these unusual lipids to explore whether solubility in this tissue can be linked to lipid structure. Across all taxonomic groups, the same eight FA accounted for 70-80% of the entire lipid profile; almost all adipose tissues were dominated by monounsaturated FA (40.2-67.4 mol%). However, even with consistent FA profiles, there was considerable variability in N2 solubility, ranging from 0.051 +/- 0.003 to 0.073 +/- 0.004 ml N-2 ml(-1) oil. Interestingly, differences in N-2 solubility could not be attributed to taxonomic group (P=0.06) or FA composition (P>0.10). These results lead to two main conclusions: (1) in triacylglycerol-only adipose tissues, the FA pool itself may not have a strong influence on N-2 solubility; and (2) samples with similar FA profiles can have different N-2 solubility values, suggesting that 3D arrangement of individual FA within a triacylglycerol molecule may have important roles in determining N-2 solubility.