This chapter describes the nitrogen cycle in the North pacific trades biome. Energy derived from sunlight, is used to reduce all partially or fully reduced forms of N to the level of NH4+ either directly by phototrophic microorganisms or indirectly via heterotrophs that depend upon the phototrophs for a continued supply of energy in the form of organic carbon. Upon death by grazing, viral lysis or autolysis, and during the remineralization, reduced dissolved N compounds including both organics (e.g., proteins and nucleic acids and their respective monomeric constituents, urea, vitamins) and NH4+ are released to the surrounding waters. A large proportion of the total flux of reduced N fuels the next round of combined photo- and heterotrophy, especially in near-surface waters. By utilizing NH4+ or dissolved nitrogen rather than a more oxidized form of N, an organism conserves energy and conducts a more efficient metabolism which would be of selective value in an energy-limited habitat like the open sea. Eventually, reduced N is oxidized back to N2/N2O or NO3- During this coupled ammonification/nitrification process the remaining bioavailable energy is extracted from the reduced N by specialized Bacteria and Archaea that can sustain chemolithoautotrophic growth in the absence of energy in the form of sunlight or reduced carbon compounds. On the basis of the cumulative database of N pools and fluxes in the North Pacific trades biome, a quantitative N budget is beginning to emerge. This contemporary view should be considered a ''work in progress'' and may change as new discoveries are made and new methodologies are developed and employed.