Amino Acid δ13C and δ15N Fingerprinting of Sympagic and Pelagic Algae in the Northern Labrador Sea

Compound-specific stable isotope analysis of amino acids (AAs) is emerging as a powerful new tool to trace element origins and biogeochemical processing in food webs. Stable carbon isotopes of AAs (δ¹³C-AA) reflect primary producer phylogenetic origins of fixed carbon, while stable nitrogen isotopes of AAs (δ¹⁵N-AA) primarily reflect baseline sources of nitrogen as well as eukaryotic and bacterial heterotrophic processing in consumers and detrital organic matter. Existing δ¹³C-AA and δ¹⁵N-AA data of marine primary producers are relatively scarce and mostly derived from laboratory cultures. We measured δ¹³C-AA and δ¹⁵N-AA in natural communities of sympagic (ice-associated) and pelagic algae collected from the northern Labrador Sea. Sympagic algae were collected by size-fractionated filtering (0.2 μm and 3μm) from melted surface brash ice in early July 2019. Pelagic algae were filtered the same way from surface seawater. Significant difference in δ¹³C-AA patterns between sympagic and pelagic algae was observed. This difference was further confirmed by Principal component analysis (PCA) using normalized δ¹³C of essential amino acids (EAAs), which showed a clear separation between sympagic and pelagic algae. The separation was retained when performing PCA including data from existing endmembers (bacteria, eukaryotic microalgae, cyanobacteria, and phaeophyceae) produced by Larsen et al. (2009; 2013). Sympagic algae demonstrated a similar δ¹³C-EAA pattern with eukaryotic microalgae, consistent with a presumed diatom-dominated community, while pelagic algae mostly overlapped with eukaryotic microalgae and slightly with bacteria. Trophic Position (TP) estimated from δ¹⁵N of Glu and Phe and the SumV parameter for estimation of microbial resynthesis averaged 1.2 ± 0.1 and 1.9 ± 0.2 (n = 5) for sympagic algae, and 1.4 ± 0.1 and for 2.0 ± 0.3 (n = 8) for pelagic algae, respectively. The slightly higher TP and SumV values for pelagic algae were consistent with the bacterial component indicated in the δ¹³C-EAA data. Overall, our findings provide first insights into the differences in δ¹³C-AA and δ¹⁵N-AA patterns between sympagic and pelagic algae and highlight the potential to estimate their contribution to export production and efficiency of benthic-pelagic coupling in polar marine environments.