Hidden Bioavailable Dissolved Organic Matter in the Deep Northwestern Pacific Ocean

Abstract The deep ocean is widely viewed as a stable reservoir of dissolved organic carbon (DOC) that is highly resistant to microbial degradation. However, this paradigm may be oversimplified. In a cross‐latitude survey conducted in the northwestern Pacific Ocean (155°E, 28.5°N–41.5°N), bulk DOC concentrations were found to be relatively constant below 1,000 m with little variation across latitude. Despite this stability, compound‐specific analyses revealed unexpectedly high abundance of hydrolyzable amino acids, up to 1.1% of DOC, at depths of 1,000–4,000 m between 30°N and 36°N. This amino acid enrichment, doubling the typical levels observed in other deep‐sea basins and comparable to upper ocean values, reflected episodic inputs of semi‐labile DOC likely derived from actively settling fresh particles. These findings reveal hidden reservoirs of bioavailable DOC in the deep ocean, a feature not captured by bulk analysis, challenging the conventional view of deep‐sea DOC as predominantly refractory and stable.