Alanine δ15N trophic fractionation in heterotrophic protists

We evaluated differences in the 15N isotopic enrichment factors of trophic amino acids (AA) for protistan (microzooplankton) and metazoan (mesozooplankton) consumers, testing the hypothesis that δ15N of alanine (ala) increases in both consumer types, while glutamic acid (glu) enriches mainly in mesozooplankton. AA δ15N values were measured for dinoflagellate and ciliate grazers and their respective algal prey (Oxyrrhis marina/Dunaliella tertiolecta and Favella sp./Heterocapsa triquetra) in four two-stage chemostat experiments, including treatments with different nitrogen : phosphorous nutrient ratios and light/dark recycling conditions. Propagation of AA 15N enrichment to a metazoan consumer was also assessed in two- and three-stage chemostat experiments simulating simple “classical” (Calanus pacificus and the diatom Thalassiosira weissflogii) and “multivorous” (C. pacificus, O. marina, and D. tertiolecta) food chains. We found small or negligible 15N-enrichment of glu for both protistan grazers, while ala enrichment was consistently greater and similar to that in metazoan consumers. Ala and glu δ15N values were both highly elevated in C. pacificus relative to prey, and enrichment was higher with autotrophic diets. These laboratory results suggest that ala may be used as an alternate, accurate isotopic proxy for quantifying protistan contributions to trophic structure in aquatic systems.