Hydrogen isotopes (d2H) of polyunsaturated fatty acids track bioconversion by zooplankton

Organisms at the base of aquatic food webs synthesize essential nutrients, such as omega-3 polyunsaturated fatty acids (n-3 PUFA), which are transferred to consumers at higher trophic levels. Many consumers, requiring n-3 long-chain (LC) PUFA, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have limited ability to bioconvert them from the essential dietary precursor α-linolenic acid (ALA) and thus rely on dietary provision of LC-PUFA. We investigated LC-PUFA metabolism in freshwater zooplankton using stable hydrogen isotopes (d2H) of fatty acids as tracers. We conducted feeding experiments with the freshwater keystone grazer Daphnia to quantify changes in the d2H value of body FA in response to the FA composition of their food and the d2H value of the ambient water. The isotopic composition of LC-PUFA changed in Daphnia, depending on the integration of 2H from ambient water during de novo synthesis or bioconversion from dietary precursors, allowing us to distinguish ...