Lipid extraction has tissue-dependent effects on isotopic values (δ34S, δ13C, and δ15N) from different marine predators

Rationale: The use of sulfur isotopes to study trophic ecology in marine ecosystems has increased in the past decade. Unlike other commonly used isotopes (e.g., carbon), sulfur can better discriminate benthic and pelagic productivity. However, how lipid extraction affects sulfur isotopic values has not been assessed, despite its frequent use to remove lipid effects on delta C-13 values. Methods: We used white muscle and liver samples from two species of sharks and skin samples from two species of pinnipeds (sea lion and fur seal) to assess the effects of lipid extraction on stable isotope values for delta S-34, delta C-13, and delta N-15. Isotopic values were determined using a continuous flow-isotope ratio mass spectrometer coupled to an elemental analyzer. Results: Lipid extraction significantly decreased delta S-34 values in shark tissues, more so for liver than muscle (-4.6 +/- 0.9 parts per thousand vs 0.8 +/- 0.3 parts per thousand, average change), with nearly no change in their standard deviations. Lipid extraction did not affect delta S-34 values from pinniped skin samples (0.2 +/- 0.8 parts per thousand, average change). After lipid extraction, consistent increases in delta C-13 values (0.2 parts per thousand-7.3 parts per thousand) were detected as expected, especially in tissue with high lipid content (C:N >4). After lipid extraction, significant increases in delta N-15 values (0.5 parts per thousand-1.4 parts per thousand) were found in shark muscle and liver tissues. For pinniped skin samples, delta N-15 values were not significantly lower after lipid extraction (-0.4 parts per thousand to -0.1 parts per thousand). Conclusions: Lipid extraction did not have a strong impact on delta S-34 values of shark muscle and pinniped skin (<= 1 parts per thousand). However, our results suggest it is essential to consider the effects of lipid extraction when interpreting results from delta S-34 values of shark liver tissue, as they significantly depleted values relative to bulk tissue (similar to 5 parts per thousand). This may reflect selective removal of sulfolipids and glutathione present in higher concentrations in the liver than in muscle and skin and requires further investigation.