Phytoplankton nitrogen assimilation

If ammonium concentrations in seawater are low, phytoplankton will assimilate nitrate and nitrite using chemical-specific permeases. Once inside the cell, these DIN species are transformed into ammonium via redox reactions in which nitrogen is reduced to the -III oxidation state ... 

Mitamura, O. (1986). Urea metaboHsm and its significance in the nitrogen cycle in the euphoric layer of Lake Biwa. II. Half-samrarion constant for nitrogen assimilation by fractionated phytoplankton in different trophic areas. Arch. Hydrobiol. 107, 167—182. 

Ronner, U., Sorensson, F., Holm-Hansen, O. (1983). Nitrogen assimilation by phytoplankton in the Scotia Sea. Polar Biol. 2, 137-147. 

Nickel and molybdenum, like iron, play important roles in nitrogen assimilation. Nickel occurs in the enzyme urease, and thus is required by phytoplankton grown on urea as a nitrogen source. It also occurs in Ni-superoxide dismutase found in many marine cyanobacteria, which, like the Mn and Fe forms of the enzyme, removes harmful superoxide radicals from cells. Little is currently known about the potential for nickel limitation in the ocean. 

Granger, J., Sigman, D.M., Needoba, J.A., and Harrison, P.J. (2004) Coupled nitrogen and oxygen isotopic fractionation of nitrate during assimilation by cultures of marine phytoplankton. Limnol. Oceanogr. 49, 1763-1773. 

The primary engine that drives these ocean interior variations is the photosynthetic fixation of carbon into organic matter by marine phytoplankton in the fight-illuminated upper ocean (euphoric zone) (see Fig. 1.1). Along with carbon, nutrient elements such as nitrogen, phosphorus, iron and many others are taken up and assimilated. Most of the resulting organic matter is either respired or reminerafized... 

DON concentrations of 4-8 pM have been observed in the relatively weak upweUing area off the northwest coast of Spain (Alvarez-Salgado et al., 1999). HUl and Wheeler (2002) reported values of 7 pM DON in upweUed waters off the Oregon coast. These are similar DON concentrations to the open ocean as summarized Table 17.3 of HiU and Wheeler (2002). Some of this DON is upweUed e.g., water at 50 m depth off Oregon had a mean value of 5.98 (HUl and Wheeler, 2002). Another source is from production by phytoplankton as they partition newly assimilated NO3 into particulate nitrogen and DON that is released (Bronk et al., 2006 Wetz and Wheeler, 2003). 

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