Assimilatory nitrate reduction

Assimilatory nitrate reduction Conversion of nitrate to reduced forms of nitrogen, generally ammonium, for the synthesis of amino acids and proteins. 

Assimilatory nitrate reduction is the reduction of NOT, followed by uptake of the nitrogen by the organism as biomass. 

A heterogeneous group of microorganisms, including many bacterial, fungal, and algal species, are capable of assimilatory nitrate reduction, a process that reduces nitrate and nitrite to ammonia, which can be subsequently incorporated into amino acids. 

Figure 16.2 The redox intermediate, nitrite (N02 ), occupies a central position in the marine N-cycle. Shown are the various processes that are either sources or sinks for N02. Abbreviations include (clockwise from top) ANR = assimilatory nitrate reduction, Nit = nitrification, photolysis = UV-driven photocatalysis, DNR/DN = dissimilatory nitrate reduction/denitrification, ANR and Nit = as above, Anammox = Anaerobic ammonium oxidation.

Several comprehensive studies of N assimilation in the North Pacific trades biome have been conducted over the past several decades. Gundersen and his colleagues (1974, 1976) were the first to estabhsh N2 fixation as a source of new N to the open ocean ecosystem, and concluded that it was a more important source of fixed N than wet deposition from the atmosphere (see Case Studies section). They also made measurements of the rates of nitrification, denitrification and assimilatory nitrate-reduction. These latter experiments involved the addition of fairly high concentrations of exogenous N substrates (NH4 , N02, NOa ) and extended incubations (days to months), so the rates reported must be viewed as potential rates at best. 

Assimilatory nitrate reduction is the uptake of NO by an organism and incorporation as biomass through nitrate reduction. It is an important process, because it allows the mobile nitrate ion to be transported to a receptor which can then reduce it to ammonia for subsequent uptake. It is an important input of nitrogen for many plants and organisms. 

Figure 19. Biological transformation /N species. I-nitrogen fixation 2-ammonia assimilation 3-nitrification 4-assimilatory nitrate reduction 5-ammoniafication 6-denitrification.

Two different pathways can be monitored for nitrates in terrestrial and aquatic ecosystems. The first is related to assimilatory nitrate reduction and the second to denitrification. 

The nitrate produced by nitrification diffuses downwards into the upper layers of anoxic sediment, where it is used by denitrifiers (sometimes called nitrate reducers, but this term can lead to confusion between dissimilatory and assimilatory nitrate reduction) for the oxidation of organic matter, with nitrite as the by-product. Nitrate can also be converted into organic-N, by the process of assimilatory nitrate reduction, which is performed by many bacteria, fungi and algae. Under anaerobic conditions there are many genera of facultative anaerobes (e.g. Es-... 

Figure 2. Biological transformation of N species, /—nitrogen fixation, 2—ammonia assimilation. 3—nitrification, 4—assimilatory nitrate reduction, 5—ammoniaficatinn, 6—denitrification.

Assimilatory nitrate reduction is simultaneous reduction of nitrate and uptake of N into the biomass of any organism. This process might be dominant when nitrogen is in low supply, which is typical in aerobic soil and water column conditions. We can consider this as a primary N input to many microorganisms and plants. 

FIGURE 8.48 Effect of sediment depth on partitioning of nitrate reductive processes of denitrification, dissimilatory nitrate reduction to ammonia (DNRA), and assimilatory nitrate reduction (ANR). Each value represents the mean of six relications (D Angelo and Reddy, 1993). 

Nitrate assimilation, assimilatory nitrate reduction see Nitrate reduction. 

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