Transformations of particulate organic matter from the surface to the abyssal plain in the North Pacific as inferred from compound-specific stable isotope and microbial community analyses

Particulate organic matter (POM) produced in surface waters undergoes extensive reworking and breakdown by microbial and metazoan communities as it sinks to the abyssal seafloor and serves as the base of benthic and pelagic food webs. Here, we examined how various size classes of POM in the oligotrophic North Pacific Subtropical Gyre (Station ALOHA) and in the eutrophic California Current System (Station M) undergo microbial alteration throughout the water column. Compound-specific stable isotope analysis showed that sampling method strongly impacts the type of POM quantified as export to the deep sea. Moored abyssal sediment traps captured material that matched the isotopic composition of surface POM, indicating they collected large, fast-sinking particles, in contrast to the more heavily reworked particles collected with in situ filtration at the same depths. Extending δ¹⁵N analyses of amino acids to bathypelagic and abyssopelagic depths for the first time, we confirmed that most particle remineralization and reworking occurs within the upper ∼400 m of the water column regardless of initial surface productivity. At Station ALOHA, we further used 16S rRNA barcoding to characterize the microbial communities associated with the POM. We found that chemolithoautotrophic ammonia-oxidizing archaea are abundant in the upper water column at Station ALOHA and that their abundance corresponded to regions of high heterotrophic reworking as indicated by amino acid isotope analysis.