Amino acid carbon isotope fingerprints are unique among eukaryotic microalgal taxonomic groups

Eukaryotic microalgae play critical roles in the structure and function of marine food webs. The contribution of microalgae to food webs can be tracked using compound-specific isotope analysis of amino acids (CSIA-AA). Previous CSIA-AA studies have defined eukaryotic microalgae as a single functional group in food web mixing models, despite their vast taxonomic and ecological diversity. Using controlled cultures, this work characterizes the amino acid delta C-13 (delta(13)CAA) fingerprints-a multivariate metric of amino acid carbon isotope values-of four major groups of eukaryotic microalgae: diatoms, dinoflagellates, raphidophytes, and prasinophytes. We found excellent separation of essential amino acid delta C-13 (delta(13)CEAA) fingerprints among four microalgal groups (mean posterior probability reclassification of 99.2 +/- 2.9%). We also quantified temperature effects, a primary driver of microalgal bulk carbon isotope variability, on the fidelity of delta C-13(AA) fingerprints. A 10 degrees C range in temperature conditions did not have significant impacts on variance in delta C-13(AA) values or the diagnostic microalgal delta C-13(EAA) fingerprints. These delta C-13(EAA) fingerprints were used to identify primary producers at the base of food webs supporting consumers in two contrasting systems: (1) penguins feeding in a diatom-based food web and (2) mixotrophic corals receiving amino acids directly from autotrophic endosymbiotic dinoflagellates and indirectly from water column diatoms, prasinophytes, and cyanobacteria, likely via heterotrophic feeding on zooplankton. The increased taxonomic specificity of CSIA-AA fingerprints developed here will greatly improve future efforts to reconstruct the contribution of diverse eukaryotic microalgae to the sources and cycling of organic matter in food web dynamics and biogeochemical cycling studies.