Characterization of Phytoplankton-Derived Amino Acids and Tracing the Source of Organic Carbon Using Stable Isotopes in the Amundsen Sea

We utilized amino acid (AA) and carbon stable isotope analyses to characterize phytoplankton-derived organic matter (OM) and trace the sources of organic carbon in the Amundsen Sea. Carbon isotope ratios of particulate organic carbon (δ¹³C-POC) range from -28.7‱ to -23.1‱, indicating that particulate organic matter originated primarily from phytoplankton. The dissolved organic carbon isotope (δ¹³C-DOC) signature (-27.1 to -21.0‱) observed in the sea-ice melting system suggests that meltwater contributes to the DOC supply of the Amundsen Sea together with OM produced by phytoplankton. A negative correlation between the degradation index and δ¹³C-POC indicates that the quality of OM significantly influences isotopic fractionation (r² = 0.59, <i>p</i> < 0.001). The AA distribution in the Amundsen Sea (5.43 ± 3.19 µM) was significantly larger than previously reported in the Southern Ocean and was associated with phytoplankton biomass (r² = 0.49, <i>p</i> < 0.01). Under conditions dominated by <i>P. antarctica</i> (DI = 2.29 ± 2.30), OM exhibited greater lability compared to conditions co-dominated by diatoms and <i>D. speculum</i> (DI = 0.04 ± 3.64). These results highlight the important role of <i>P. antarctica</i> in influencing the properties of OM, suggesting potential impacts on carbon cycling and microbial metabolic activity in the Amundsen Sea.