Compound-specific amino acid δ15N patterns in marine algae: Tracer potential for cyanobacterial vs. eukaryotic organic nitrogen sources in the ocean

Stable nitrogen isotopic analysis of individual amino acids (delta N-15-AA) has unique potential to elucidate the complexities of food webs, track heterotrophic transformations, and understand diagenesis of organic nitrogen (ON). While delta N-15-AA patterns of autotrophs have been shown to be generally similar, prior work has also suggested that differences may exist between cyanobacteria and eukaryotic algae. However, delta N-15-AA patterns in differing oceanic algal groups have never been closely examined. The overarching goals of this study were first to establish a more quantitative understanding of algal delta N-15-AA patterns, and second to examine whether delta N-15-AA patterns have potential as a new tracer for distinguishing prokaryotic vs. eukaryotic N sources. We measured delta N-15-AA from prokaryotic and eukaryotic phytoplankton cultures and used a complementary set of statistical approaches (simple normalization, regression-derived fractionation factors, and multivariate analyses) to test for variations. A generally similar delta N-15-AA pattern was confirmed for all algae, however significant AA-specific variation was also consistently identified between the two groups. The relative delta N-15 fractionation of Glx (glutamine + glutamic acid combined) vs. total proteinaceous N appeared substantially different, which we hypothesize could be related to differing enzymatic forms. In addition, the several other AA (most notably glycine and leucine) appeared to have strong bio-marker potential. Finally, we observed that overall patterns of delta N-15 values in algae correspond well with the Trophic vs. Source-AA division now commonly used to describe variable AA delta N-15 changes with trophic transfer, suggesting a common mechanistic basis. Overall, these results show that autotrophic delta N-15-AA patterns can differ between major algal evolutionary groupings for many AA. The statistically significant multivariate results represent a first approach for testing ideas about relative eukaryotic vs. prokaryotic ON sources in the sea. (C) 2012 Elsevier Ltd. All rights reserved.