IDENTIFYING NOVEL ISOTOPIC TRACERS OF MARINE PRIMARY PRODUCERS TO STUDY FOOD WEB CARBON CYCLES

Abstract

Eukaryotic microalgae play critical roles in global biogeochemical cycling, the efficacy of the biological pump, and the structure and function of most marine food webs. Compound-specific isotope analysis of amino acids (CSIA-AA) couples molecular-level biomarkers with stable isotope analyses to track the contribution of primary producers to food webs. Previous work has developed δ13CAA fingerprints for major groups of primary producers but categorized all marine algae together despite their vast taxonomic and ecological diversity. This work characterizes the δ13CAA fingerprints of four major groups of eukaryotic microalgae — diatoms, dinoflagellates, raphidophytes, and prasinophytes — in controlled cultures, and quantifies the effect of temperature on the fidelity of these fingerprints. We found excellent separation of essential amino acid δ13CEAA values among the four microalgal groups with mean posterior probability reclassification of 99.2±2.9%. Surprisingly, temperature did not have a large impact on variance in δ13CAA values or the diagnostic microalgal δ13CEAA fingerprints. The δ13CEAA fingerprints generated in this study successfully identified primary production sources to published consumer δ13CAA data in two contrasting systems: penguins feeding in a diatom-based food web and mixotrophic corals receiving organic matter from endosymbiotic dinoflagellates and water column diatoms via heterotrophy. The increased taxonomic specificity of CSIA-AA fingerprints developed in this study will greatly improve future efforts to reconstruct the contribution of diverse eukaryotic microalgae to the sources and cycling of organic matter in studies of food web dynamics and biogeochemical cycling.