Assimilate nitrate reduction

Green plants, algae, fungi, cyanobacteria and bacteria that assimilate nitrate also produce assimilatory nitrite reductases, which catalyze the six-electron reduction of nitrite to ammonia (equation 89). The formation of heme-nitrosyl intermediates has been detected in several cases,1515 while hydroxylamine is commonly thought to be an intermediate. Added hydroxylamine is rapidly reduced to ammonia. However, no intermediates are released, and ammonia is the only product... 

Molybdenum is essential to the formation and activity of assimila-tory nitrate reductases. Cells must assimilate molybdate from the environment, metabolize molybdenum in some manner to form active molybdenum cofactor, and then incorporate it into a large molecular weight protein so that it can perform a reversible redox reaction with nitrate. Investigations on the aqueous Mo (III) model systems for nitrate reduction and the coordination of molybdate by naturally produced phenolates will hopefully lead to an understanding of the complex process of molybdenum acquisitions by and molybdenum function in nitrate reductases. 

All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). 

Nitrate reduction and assimilation is a fundamental biological process in plants and various microorganisms. In this process nitrate is reduced ultimately to ammonia. Thus, as shown in Eq. (5), the reduction of nitrate to ammonia requires eight electron or hydrogen equivalents. 

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. 

Because NTR links the reduced and oxidized sides of the N cycle, it can be considered a central process that provides substrate to microbes that employ nitrate or nitrite as oxidant (see Chapter 5 by Ward, this volume Fig. 19.1, arrow 4). Like NH4, the products of NTR, N02, and NOs , may experience one of several possible fates, including (1) flux from the sediment, (2) assimilation within the sediment or at the sediment—water interface, or (3) reduction by one of three possible dissimilatory pathways DNF, dissimilatory nitrate reduction to ammonium (DNRA), or ANAM (Fig. 19.1, arrows 5, 6, and 7 Fig. 19.2). Uptake of NO by... 

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

Figure 12 Major reduction-oxidation reactions involving nitrogen. The reactions are numbered as follows (1) mineralization, (2) ammonium assimilation, (3) nitrification, (4) assimilatory or dissimilatory nitrate reduction, (5) ammonium oxidation, (6) nitrite oxidation, (7) assimilatory or dissimilatory nitrate reduction, (8) assimilatory or dissimilatory nitrite reduction, (9) denitrification, (10) chemodenitrification, (11) anaerobic ammonium oxidation, and (12) dinitrogen fixation (after Capone, 1991) (reproduced by permission of ASM Press from Microbial Production and Consumption of Greenhouse Gases Methane, Nitrogen Oxides, and Halomethanes, 1991).

Nitrate reduction studies have focused overwhelmingly on denitrification at the expense of other NO sinks such as dissimulatory NO reduction to NH4 (also known as DNRA or nitrate ammonification). The ecological implications of reducing NOJ to NH4, versus N2 are vastly different because NH4 is more readily retained in the ecosystem, and it is a form that is readily assimilated by biota. Thus, DNRA contributes to eutrophication by reducing the quantity of fixed nitrogen that is returned to the atmosphere as N2. 

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

Fig. 3.16 Generalized scheme of the role of bacteria in the carbon cycle and its coupling to the nitrogen and sulphur cycles (after Fenchel Jorgensen 1977 Jorgensen 1983a, b Parkes 1987 Fenchel Finlay 1995 Werne et al. 2002). For clarity, the forms of N, S and P liberated at each stage of mineralization are summarized on the left side of the diagram, where they contribute to the general mineral pools from which assimilation occurs. The nitrate reduction zone refers to the dissimilatory processes involved in denitrification.

The initial action of simazine and atrazine is the increase of nucleic acid synthesis. This increases protein synthesis and, thereby, the absorption of nitrate. However, nitrate reduction can occur only if sufficient carbohydrate is present for the formation of NADH. An increase in glucose catabolism increases the quantity of a-ketoglutaric acid. As a result of this assumed mechanism, nitrogen assimilation is increased at the expense of carbohydrates if there is not sufficient carbohydrate present, because the temperature is high and the light poor, or the nitrogen supply is good and, in this case, the. r-triazine effect is absent. 

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

Most plants can assimilate both reduced and oxidized forms of nitrogen, even though there is an energy cost in first nitrate reduction. 

Debouba, M., Maaroufi-Dghimi, H., Suzuki, A., Ghorbel, M.H., and Gouia, H. (2007). Changes in growth and activity of enzymes involved in nitrate reduction and ammonium assimilation in tomato seedlings in response to NaCl stress. Ann. Bot. (London) 99, 1143-1151. 

Oxygen content of the soil has no effect on anabolic nitrate reduction, as aerobes use nitrate as nutrient source and reduce it to ammonia during cell synthesis. The assimilative nitrate reductases are soluble proteins and are repressed by high ammonia levels. 

Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell (Payne, 1973). This nitrate assimilation can occur under both aerobic and anaerobic conditions. In other instances (Payne, 1973) nitrate serves as a terminal hydrogen acceptor under anaerobic conditions and this process is called nitrate respiration. In both cases the product of nitrate reduction is nitrite. The nitrate reductases from bacteria have been differentiated by Pichinoty and Piechaud (1968) into nitrate reductase A which is membrane bound and can reduce chlorate in addition to nitrate as a substrate and nitrate reductase B which is... 

Indications that the mitochondria regulate nitrate reduction in a direct manner are obtained from the observations that oxygen inhibits nitrite accumulation (Ferrari and Varner, 1970 Radin, 1973) and that uncouplers of the electron transport system permit nitrite accumulation under aerobic conditions (Ferrari and Varner, 1970). Canvin and Atkins (1974) and Atkins and Canvin (1975) reported that excised leaves under a dark aerobic gaseous environment do not assimilate into amino acids and that vac-... 

Figure 2 shows the possible sources of reductant for nitrate reduction and the postulated interrelations between the mitochondria and nitrate assimilation in green leaves in light or dark. 

How much of this scheme (Fig. 2) is applicable to nitrate assimilation in roots is not clear. Under aerobic conditions, roots treated with DNP or CCCP (carbonylcyanide /n-chlorophenylhydrazone) accumulated nitrite as measured by excretion into the medium (Lee, 1979). He concluded that a decrease in ATP was associated with an increase in nitrite accumulation and inferred that the decreases in nitrite reduction were responsible for increases in nitrite accumulation rather than decreases in nitrate reduction. The work of Guinn and Brinkhoflf (1970) and Lee (1979) indicate that the oxygen in the root environment is of major importance in regulating nitrate assimilation in roots. 

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