Chapter 7 - Nitrogen Uptake and Assimilation

Abstract

This chapter focuses on general patterns of nitrogen (N) uptake and assimilation, primarily in the open ocean. N uptake was reviewed mainly in terms of the kinetics of ammonium (NH4+) and nitrate (NO3–) uptake and the linkages to internally versus externally controlled cellular growth rate by phytoplankton. These two compounds were thought to be the major nitrogenous nutrients supporting oceanic primary production and so measuring their uptake, the relative affinity of phytoplankton for NH4+ and NO3–, and the applicability of the different models of marine phytoplankton growth were of primary concern. In addition to these it is now recognized that organic compounds also contributes substantially to primary productivity. Methods for measuring dissolved organic nitrogen (DON) and specific components of the DON pool have improved dramatically. The view of the N cycle has expanded; global estimates of new production from marine N2 fixation have been revised upward, and uptake of DON regenerated through a variety of processes by phytoplankton in the upper water column is now widely recognized. Further complications in interpreting uptake data in the context of new production arise when one considers that water column regeneration of NO3 (i.e., nitrification), can be an important source of regenerated NO3 and NO2 at least regionally, and that bacteria and phytoplankton seem to compete for the same inorganic and organic N sources.