Identification of the sea ice diatom biomarker IP25 and related lipids in marine mammals: a potential method for investigating regional variations in dietary sources within higher trophic level marine systems

Marine mammals are often the top predators in a given food web; however, since these animals spend much of their time underwater and often away from the coast, this potentially limits our ability to obtain a complete ecological understanding of these important animals, including the determination of their dietary preferences and adaptations. Recently, the analysis of so-called highly branched isoprenoid (HBI) lipids from marine diatoms has provided new insights into the feeding habits of lower trophic level marine animals. In the present study, we extended this approach to higher trophic levels by examining for the presence of these marine diatom HBI biomarkers in seven marine mammal species from both Arctic and temperate regions. Analysis of sub-samples of the livers of Arctic marine mammals revealed the presence of the sea ice diatom biomarker IP25, thus providing evidence for trophic transfer of this lipid from sea ice algae. Our analysis also demonstrated the occurrence of additional HBI lipids in both Arctic and Atlantic marine mammals that are likely indicative of trophic transfer from phytoplanktic diatoms. Although the reasons for the highly variable abundances of the HBIs in individual specimens are uncertain at this stage, the differences in the relative distributions of the individual HBI isomers in mammals from the Arctic and the Atlantic, suggests that these have the potential to provide novel information regarding different dietary sources from the two regions. Further, our analyses demonstrate that HBIs are distributed homogeneously within striped dolphin (Stenella coeruleoalba) liver tissue, thus validating the sub-sampling technique employed for other specimens here. Validation of this sampling technique also provides the necessary confidence to facilitate advancement of the study of HBIs in large marine mammals