Isotopologue profiling enables insights into dietary routing and metabolism of trophic biomarker fatty acids

The application of fatty acids (FAs) as biomarkers in food web ecology is based on the concept of dietary routing, i.e., FA assimilation as entire molecule and transfer into consumer tissue with no or minor modification. However, the lack in knowledge in lipid metabolism of the soil fauna hampers the FA trophic marker approach. This study used labeled palmitic acid ((13)C16:0, 99 atom%) as tracer in lipid pathways of two common soil Collembola, Heteromurus nitidus and Protaphorura fimata. The incorporation and dietary routing of this precursor were determined by C-13:C-12 FA analysis (gas chromatography-combustion-isotope-ratio-monitoring), while metabolic modifications were assessed with isotopologue profiling (mass spectrometry with single ion monitoring). The C-13 enrichment in FAs assigned the transfer of the labeled precursor predominantly into the neutral lipid fraction of Collembola. Thereby, isotopologue profiling demonstrated that dietary routed FAs are marginally metabolically modified. In contrast, the phospholipid fraction showed a considerable degree of FA modification, most markedly a fast degradation of palmitoleic acid. Further, de novo synthesis of linoleic acid was approved for both Collembola species. While 16:0 and 18:0, and their descendants 16:1 omega 7 and 18:1 omega 9, followed the concept of dietary routing and thus are good markers, arachidonic acid showed a different metabolism. This comprised elongation/desaturation of (13)C16:0 and de novo synthesis, including C-13 labeled acetyl-CoA from degradation of the precursor, indicating its utility to assign carbon flow but not as trophic marker. We propose isotopologue profiling as a valuable tool for evaluation of the lipid metabolism in small soil animals to disentangle trophic interactions.