CHEMICAL-COMPOSITION OF DISSOLVED ORGANIC NITROGEN IN THE OCEAN

Fixed nitrogen is one of the main limiting nutrients for primary produ ction in the ocean(1-3), where it is biologically available in the for m of dissolved inorganic and organic matter. Inorganic nitrogen concen trations are consequently very low in surface waters of temperate ocea n gyres, yet fixed nitrogen persists in the form of dissolved organic matter, The small, rapidly cycling organic compounds fundamental to mi crobial and planktonic growth (such as free amino acids, amines and ur ea(4,5)) account for only a minor fraction of total dissolved organic nitrogen (DON). In contrast, the vast majority of DON, especially in t he deep ocean, resides the form of nitrogenous substances that are res istant to biological degradation, These substances, which represent an enormous reservoir of fixed nitrogen, are not readily identified by c onventional biochemical techniques, but have been assumed to consist l argely of structurally complex macromolecules resulting from the degra dation and spontaneous abiotic condensation of biochemical. precursors (6). Here we present N-15 NMR measurements that contradict this view. Our results show that most higher-molecular-weight DON in the ocean ex ists in amide form, rather than as a collection of nitrogen heterocycl es that might be indicative of spontaneous condensation products. Beca use these amides are unlikely to form abiotically, the bulk of the oce an's DON reservoir appears to derive directly from degradation-resista nt biomolecules.