Denitrification mechanisms

Figure 4.1 shows that NOs" is the stable form of nitrogen over the usual range of pe + pH in aerobic environments. The fact that most of the N2 in the atmosphere has not been converted to NO3 therefore indicates that the biological mediation of this conversion in both directions is inefficient. Hence NO3 reduction to N2 occurs by indirect mechanisms involving intermediaries. Dissimilatory reduction of N03 (i.e. where the nitrogen oxide serves as an electron acceptor for the cell s metabolism but the N reduced is not used by the microbes involved) potentially occurs by two processes denitrification. 

Shearer, G., and Kohl, D. H. (1988). Nitrogen isotope fractionation and "O exchange in relation to the mechanism of denitrification of nitrite by Pseudomonas stutzeri. J. Biol. Chem. 263, 13231-13245. 

The understanding of four aspects of the lower stratosphere must be improved. The first is the processes of heterogeneous conversion. Does it occur in three hours or in three days Which type of aerosols or particles are most favorable for conversion and what are the conditions required for their formation How do the particles form, and what are their compositions and structures What are the most important mechanisms for denitrification These issues can be settled only by a combination of laboratory and stratospheric measurements. 

Nitrogen is returned to its atmospheric form by the action of denitrifying bacteria such as Pseudomonas thiobacillus and Micrococcus denitriflcans. The process is referred to as denitrification and represents the major mechanism of nitrogen loss in the overall nitrogen cycle whereby various forms of nitrogen in the soil revert to the N2 form. The reactions and their energetics are given below ... 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

Cohen, Y., and Gordon, L. I. (1978). Nitrous oxide in the oxygen minimum of the eastern tropical North Pacific Evidence for its consumption during denitrification and possible mechanisms for its... 

Up until a few years ago the general understanding was that in the world s oxygen deficient zones, heterotrophic denitrification was the sole mechanism... 

The isotopic fractionation is easily seen in 8 N03 and 8 N2 distributions in the major open-ocean denitrification zones (Altabet et al., 1999 Brandes et al., 1998 Cline and Kaplan, 1975). Typical open ocean values ofsub-euphotic zone nitrate are about 5%o (Lehmann et al., 2005 Sigman et al., 2000 Wu et al., 1997) but within the ODZ they climb to upwards of 15%o. Concomitant with this increase is a decrease in the 8 N2 from about 0.6%o to 0.2%o (Fig. 6.15). The large enrichment of N-N03 and the mirror image decrease in N-N2 is undoubtedly due to fractionation during denitrification. It is also possible to derive a fraction factor, , from the isotope distributions in the ODZ if one makes some assumption about the amount of nitrate that has been removed by denitrification, i.e., the nitrate deficit. For the eastern tropical North Pacific Brandes et al. (1998) assumed a Raleigh fraction mechanism and both open (advection-reaction) and closed (diffusion-reaction) systems to derive fractionation factors from the data, in Fig. 6.15. (Raleigh fractionation 8 N03 = where 8 N03 is the isotopic composition... 

Figure 13.13 illustrates the major fluxes across the boundaries of the basin, as well as sinks (denitrification) and new sources (N2 fixation) of reactive N. The important physical reservoirs included in the budget are estuaries, continental shelves, and the open ocean, each undergoing additions and removals of N by various mechanisms. The estuaries receive N from rivers (540 x 10 mol year ), about half of which (250 X 10 molyear ) is lost from the system via denitrification (Seitzinger, 1988). The bulk of the remainder passes through the system to the shelves, with a small amount lost to the estuarine sediments. Major rivers do not have the conditions... 

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