Mobilisation of lipophilic pollutants from blubber in northern elephant seal pups (Mirounga angustirostris) during the post-weaning fast

Abstract

Northern elephant seals (NES) (Mirounga angustirostris) from the Año Nuevo State Reserve (CA, USA) were longitudinally sampled during the post-weaning fast in order to study the mobilisation and redistribution of various classes of persistent organic pollutants (POPs), such as polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), dichlorodiphenyldichloroethylene (p,p'-DDE) and hexachlorobenzene (HCB) between blubber and blood. Inner and outer blubber layers were analysed separately. Organohalogenated compounds were detected in all blubber samples in the decreasing order of their concentrations: p,p'-DDE > PCBs ⪢ HCB > PBDEs. The concentrations of all studied compounds were homogeneously distributed in the blubber layer at early fast, since the concentrations of POPs were statistically not different in the inner and outer layers. With the progression of the fast, the concentrations of PBDEs, PCBs and p,p'-DDE increased more sharply in inner blubber than in outer blubber. As a result, their levels became significantly higher in inner blubber as compared to outer blubber at late fast. The rise of pollutant concentrations in blubber might result from a less efficient mobilisation than triglycerides and/or a reuptake by adipocytes of some of the pollutants released into the circulation. The mobilisation of pollutants from blubber was higher at late fast. An increase of pollutant concentrations was observed in serum between early and late fast. Lower halogenated congeners (i.e. tetra-CBs) were present in higher proportions in serum, whereas the higher halogenated congeners (i.e. hepta-CBs) were mainly found in the inner and outer blubber layers. The transfer ratios of both PBDEs and PCBs from inner blubber to serum decreased with the number of chlorine and bromine atoms. In addition, the distribution of both types of compounds between serum and blubber was strongly influenced by their lipophilic character (logKow values), with more lipophilic compounds being less efficiently released from blubber to serum.

Mobilisation of blubber fatty acids of northern elephant seal pups (Mirounga angustirostris) during the post-weaning fast

“…Since TGs are composed of by FAs, and TGs are the main lipid fraction of adipose tissue, the present results may be compared to the total lipid percentages of blubber. Similar trends in the inner blubber layer of weaned NES pups were observed for the total lipid percentages, which were determined by gravimetry (Louis et al, 2014). This means that TG content remains stable in blubber throughout the fast, which may be surprising because TGs are heavily mobilised.…”

“…This means that TG content remains stable in blubber throughout the fast, which may be surprising because TGs are heavily mobilised. Indeed, the lipid mass of weaned NES pups is reduced by 24-49% (Noren et al, 2003) and a net increase of free FA concentration between early and late fast is noted in their serum (our study and Louis et al (2014)). Moreover, a morphometric study of the weaned NES pup blubber was conducted earlier and showed that the proportions of small adipocytes increased in both blubber layers, whereas the proportions of large adipocytes decreased over the fast, reflecting a reduction of lipid content in the adipocytes (Louis et al, 2014).…”

“…Since the inner blubber layer exhibited the highest range of FM, we can expect that this layer is the most metabolically active (compared to the outer one), as already suggested elsewhere (e.g. Louis et al, 2014) and thus, participates largely to the increase of FAs in serum. Most of the major FAs encountered in serum (C18:1n-9 N C16:0 N C16:1n-7 ≈ C22:6n-3) were indeed similar to those of inner blubber.…”

“…However, in those studies, the FA profiles were assessed in the total lipid fraction of each tissue. In weaned NES pups, the total lipid fraction of blood is dominated by phospholipids (52%), followed by cholesterol (31%) and TGs (10%) (Louis et al, 2014), for which the liver is the main source. The free FA fraction, which comes from the lipolytic process in blubber, represents only 5% of the total lipids of blood (Louis et al, 2014).…”

“…The technique of sampling was described elsewhere (Louis et al, 2014). Twenty free-ranging pups were specifically marked with dye and were followed by observing the colony each day.…”