The deep ocean is known to harbor vast reservoirs of dissolved organic carbon (DOC) that has long been believed to resist microbial degradation, featuring its refractory nature. However, recent studies have revealed spatial and temporal variability in the concentration, chemical composition, and radiocarbon content of the deep-sea DOC, suggesting a broader spectrum of bioavailability than previously recognized. The conventional approach, which relies on bulk DOC measurements, lacks the sensitivity to detect subtle variations if deep-sea DOC degrades. In this study, we employed an alternative approach, analyzing the amino acid content of DOC along a broad latitudinal gradient (28.5˚N-41.5˚N) and across depths of 1000-5300 m in the northwest Pacific Ocean. We observed that, while bulk DOC concentrations exhibited minimal variation with depth and latitude in the deep ocean, the concentrations and DOC-normalized yields of amino acids exhibited significant heterogeneity, with pronounced peaks in the 1000-4000 m layers between 30˚-35˚N. These unusually high amino acid contents suggest the presence of localized pockets of bioavailable DOC, challenging the conventional view of deep-sea DOC as commonly resistant to degradation. Our findings point to hidden reservoirs of bioavailable organic matter in the deep ocean, potentially influencing microbial metabolisms and biogeochemical cycles in ways not yet fully understood.

dissolved organic matter,bioavailability,deep ocean,Northwest Pacific Ocean