Phytoplankton nitrate 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 ... 

The ongoing research into such specific N fractions is critical to address the fundamental issue of isotopic alteration of exported organic N in the water column and sediments. This work also raises a host of new questions. Consider the case of diatom frustule-bound N (Fig. 34.7). The organic N of diatoms exported from the surface layer could differ from the integrated sinking N, for instance, if the isotope effect of nitrate assimilation by diatoms differs from that of the entire phytoplankton population. Moreover, the of N trapped and preserved within the diatom... 

During nitrate assimilation, phytoplankton preferentially consume " N-nitrate relative to i N-nitrate (Montoya and McCarthy, 1995 Waser et al., 1998), leaving the surface nitrate pool enriched in (Sigman et al., 1999a). This results in a correlation between the ratio... 

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 major N product of organic matter decomposition in seawater is NH4+, but NH4+ is present at trace or undetectable levels in the huge volume of the deep ocean. Rather, deep water contains NOs" at 20-40 pM concentrations, which would seem to imply that nitrification occurs mainly in the deep ocean. Nitrate concentrations in the surface ocean are usually maintained at low levels because phytoplankton assimilate N03 more rapidly than it can be suppHed by mixing or diffusion from the deep NOs" reservoir. Ammonium, which is produced in the photic zone by heterotrophic processes, is also usually immediately assimilated by phytoplankton and heterotrophic bacteria before it can be nitrified. The important physical and biological differences in the source functions of NH4+ and NOs" are... 

Glibert, P. M., and McCarthy, J. J. (1984). Uptake and assimilation of ammonium and nitrate by phytoplankton Indices of nutritional status for natural assemblages. J. Plankton Res. 6, 677—697. 

In contrast to zinc, the crucial role of Fe in the bioenergetics of carbon (C) and N metabolism is well recognized (e.g., Morel et al., 1991 Sunda, 1989). Substantial amounts of Fe are required in both photosynthetic and respiratory electron transport chains (e.g., Raven, 1988), the synthesis of chlorophyll (Chereskin and Castelfranco, 1982), and the assimilation ofNOj. Theoretical calculations based on Fe utilization efficiencies and cellular metabolic Fe demands, predict that phytoplankton growing on NOJ require 60% more Fe than those growing on NH (Raven, 1988, 1990), and greater cellular Fe requirements for NO growth have indeed been demonstrated for laboratory cultures of diatoms (Maldonado and Price, 1996). The extra Fe is needed to reduce NO to NH4 before it can be incorporated into amino acids. This process requires the assimilatory enzymes nitrate reductase (requires one... 

Dohler, G., and Hagmeier, E. (1997). UV efFects on pigments and assimilation oF N-ammonium and N-nitrate by natural marine phytoplankton oF the North Sea. Botanica Acta 110, 481—488. 

Francis C. A. and Tebo B. M. (2002) Enzymatic manganese(II) oxidation by metabolically dormant spores of diverse Bacillus species. Appl. Environ. Microbiol. 68, 874-880. Galvan A. and Fernandez E. (2(X)1) Eukaryotic nitrate and nitrite transporters. Cell. Mol. Life Sci. 58, 225 -233. Glibert P. M. and McCarthy J. J. (1984) Uptake and assimilation of ammonium and nitrate by phytoplankton—indexes of nutritional-status for natural assemblages. J. Plankton Res. 6, 677-697. 

G. Dohler (1995). Effects of UV-A and UV-B radiation on pigments and assimilation of 15N-ammonium and 15N-nitrate by natural phytoplankton of the Wadden Sea. Beitr. Biol. Pflan., 69,1-16. 

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