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.