A deep dive into fat: Investigating blubber lipidomics fingerprint of killer whales and humpback whales in northern Norway

<p class="xl24">In cetaceans, blubber is the primary and largest lipid body reservoir. Our current understanding about lipid stores and uses in cetaceans is still limited and most studies only focused on a single narrow snapshot of the lipidome. We documented an extended lipidomics fingerprint in two cetacean species present in northern Norway during wintertime. We were able to detect 817 molecular lipid species in blubber of killer whales (<i>Orcinus orca</i>) and humpback whales (<i>Megaptera novaeangliae</i>). The profiles were largely dominated by triradylglycerols in both species and to a lesser extent, by other constituents including glycerophosphocholines, phosphosphingolipids, glycerophosphoethanolamines and diradylglycerols. Through a unique combination of traditional statistical approaches, together with a novel bioinformatics tool (LION/web), we showed contrasting fingerprints composition between species. The higher content of triradylglycerols in humpback whales is necessary to fuel their upcoming half a year fasting and energy-demanding migration between feeding and breeding grounds. In adipocytes, we assume that the intense feeding rate of humpback whales prior to migration translates into an important accumulation of triacylglycerols content in lipid droplets. Upstream, the endoplasmic reticulum is operating at full capacity to supply acute lipid storage, consistent with the reported enrichment of glycerophosphocholines in humpback whales, major components of the endoplasmic reticulum. There was also an enrichment of membrane components which translates into higher sphingolipids content in the lipidome of killer whales, potentially as a structural adaptation for their higher hydrodynamic performance. Finally, the presence of both lipid-enriched and lipid-depleted individuals within the killer whale population in Norway suggests dietary-specialization, consistent with significant differences of δ¹⁵N and δ¹³C isotopic ratios in skin between the two groups, with higher values and a wider niche for the lipid-enriched individuals. Results suggest the lipid-depleted killer whales were herring-specialists, while the lipid-enriched individuals might feed on both herrings and seals.</p>