Ammonium assimilation pathways

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Zehr, J. P., and Falkowski, P. G. (1988). Pathway of ammonium assimilation in a marine diatom determined with the radiotracer N-13. J. Phycol. 24, 588-591. 

Summons, R. E., Boag, T. S., and Osmond, C. B. (1986). The effect of ammonium on photosynthesis and the pathway of ammonium assimilation in Gymnodinium microadriaticum in vitro and in symbiosis with tridacnid clams and corals proceedings of the royal society of London. Ser. B Biol. Sci. 227(1247), 147-159. 

Many other carboxylation reactions exist (Barton et al., 1991). For example, in methylo-trophic bacteria, formaldehyde and CO2 are combined to produce acetyl-CoA in the serine or hydroxypyruvate pathway. In contrast, the ribulose monophosphate cycle, which is another methylotrophic pathway of formaldehyde fixation, does not involve carboxylation steps. In addition to those described above, commonly found carboxylation reactions include those of pyruvate or phosphoenol pyruvate. In view of several relatively recent discoveries of novel CO2 assimilation pathways (e.g., the hydoxypro-pionate cycle and anaerobic ammonium oxidation) and growing interest in deep-subsurface microbiology, novel pathways of CO2 incorporation may be discovered in the near future. 

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 6 Microbial transformations of the nitrogen cycle. Pathways depicted are 1—N2 fixation 2—DIN assimilation 3—ammonium regeneration 4—nitrification 5—nitrate/nitrite reduction and 6—denitrification.

Assimilation of NH4. Studies on the assimilation of exogenous NH4" by N2- and NH4 -grown cultures were initiated to determine whether the pathways of assimilation of exogenous ammonium differed from the pathway operative during N2 fixation. 

---