Organic matter transformations in the upper mesopelagic zone of the North Pacific: Chemical composition and linkages to microbial community structure

Transformation processes in the euphotic and mesopelagic zones are of crucial importance to the biological pump and global elemental cycles. In this study, elemental stoichiometries and chemical compositions of particulate and dissolved organic matter (DOM) were investigated in the euphotic and upper mesopelagic zones of the North Pacific Subtropical Gyre. The distributions of bacterial biomarkers (D‐amino acids, muramic acid) and major biochemicals (amino acids, neutral sugars, amino sugars) indicated a direct link between microbial community structure and the biochemical composition of organic matter. Bacteria were major sources of organic C, N, and P in the upper mesopelagic zone. Heterotrophic bacterial transformations were important in the formation of biorefractory organic matter that is retained in the ocean on timescales of decades to millennia. Net removal rates for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and major biochemicals were calculated for the upper mesopelagic zone (110–300 m). Dissolved hydrolyzable amino acids, neutral sugars, and amino sugars comprised 5–18% of DOC and 4–5% of DON removed in the upper mesopelagic zone, indicating these biochemicals were important components of semilabile DOM. Net removal rates of neutral sugars were 3–10 times higher than net removal rates of amino acids and amino sugars. This suggested that neutral sugars were the most reactive component among the three classes of biochemicals. Depth‐integrated net DOC removal rates indicated that DOC comprised 19–31% of total carbon export flux in the North Pacific gyre and supplied 27–93% of bacterial carbon demand in the upper mesopelagic zone.