Diving deep into the origins of refractory dissolved amino acids in the ocean

Amino acids are key constituents of marine peptides and proteins and are generally considered biologically labile or semi‐labile, yet their widespread occurrence throughout the ocean suggests bioresistance over extended timescales. To investigate this paradox, we conducted two parallel shipboard incubation experiments to track microbial utilization of amino acids differing in molecular form and biological origin. Seawater was collected from 50 to 1500 m at the Bermuda Atlantic Time‐series Study site, unamended or amended with plankton‐derived dissolved amino acids in free and combined forms. In amended treatments, free amino acids were rapidly consumed (0.52–0.68 d −1 ) reflecting their labile nature, whereas combined forms degraded more slowly with rates decreasing from 0.10–0.37 d −1 within 7 d to 0.003–0.06 d −1 over 180 d, revealing a labile to semi‐labile reactivity spectrum. In unamended seawater, amino acids occurred predominantly in combined forms and only 7–13% were degraded at extremely low rates (~ 0.0004 d −1 ), indicating low bioavailability. Compositional analysis revealed elevated proportions of glycine (up to 36 ± 2%) and non‐protein d ‐amino acids (up to 18 ± 2%), particularly in deep waters, suggesting dominance of glycine‐rich compounds and non‐proteinaceous bacterial material in the refractory amino acid pool. Notably, the degradation trajectory of plankton‐derived amino acids shifted toward, but did not converge with, native seawater profiles, implying additional processes or sources beyond the experimental conditions that shape the native reservoir. These results demonstrate a broad reactivity spectrum of naturally‐occurring amino acids tied to molecular form and origin, and highlight the intricate mechanisms expected to sustain a persistent amino acid reservoir in the ocean.