FIG. 2.

Diagrams of classical and present views of the N cycle in the surface waters of oligotrophic oceans. The composition of the dissolved N pool is shown with approximate relative concentrations of inorganic and organic constituents indicated by the size of the box. Dashed lines indicate transformations and processes included in the newer view of nitrogen cycling. (A) Some phytoplankton use simple organic compounds as a source of nitrogen. (B) There are multiple species of phytoplankton (cyanobacteria) in the open ocean that fix N₂. (C) Bacteria can compete for nitrate and ammonium. (D) Bacteria can excrete urea and can also be a source of high-molecular-weight DON. (E). Some oceanic bacterioplankton appear to fix N₂.