Nitrite accumulation

Denitrification can be affected by free ammonia, but this inhibition does not appear up to 300 to 400 mg/L NH3.46 This high concentration can justify that no inhibition of the denitrification process has been reported for this kind of wastewater.3-4 Eiroa and colleagues37 observed that nitrate was eliminated much faster at higher initial urea concentrations. However, they also found an increase of nitrite accumulation, which was later removed, due to high urea concentrations. 

Nitrification seems limited to a number of autotrophic bacteria. The dominant genus that is capable of oxidizing ammonia to nitrite in soils is Nitmsomonas, and the dominant genus capable of oxidizing nitrite to nitrate is Nitrobacter. Normally, the two processes are closely connected and nitrite accumulation does not occur. Nitrifying bacteria are chemolithotrophs that utilize the energy derived from nitrification to assimilate C02. 

Kelso B. H. L., Smith R. V., Laughlin R. J., and Lennox S. D. (1997) Dissimilatory nitrate reduction in anaerobic sediments leading to river nitrite accumulation. Appl. Environ. Microbiol. 63, 4679-4685. 

Fig. 2. Cometabolic interaction between toluene and o-xylene and corresponding nitrate reduction and nitrite accumulation.

X 10 M), or dexamethasone (DEX) (1 / A1) on nitrite accumulation. The effect of 100 nM spermine or DEX on nitrite accumulation was also studied when these agents were given 6 hr after LPS (post). Data are expressed as the means SEM of n = 9 wells from three experimental days. and , Significant (P < 0.05 and P < 0.01, respectively) differences in the presence of various inhibitors compared to the control, or between groups as indicated. 

Nitrite Nitrite is an intermediate in oxidative and reductive processes such as nitrification and denitrification. It also serves as a substrate for anammox and can be assimilated by phytoplankton. Only when these processes become uncoupled, such as at the base of the euphotic zone and in oxygen minimum zones, does nitrite accumulate to significant levels (0.1-10 (iM). In some oxygen-deficient regions of the water column in the eastern tropical North Pacific, eastern tropical South Pacific, and the Arabian Sea, nitrite can represent as much as 25% of the pool of nitrogen oxides (NOs -F NO2-). 

The importance of chemical oxidation of nitrites cannot be judged by whether nitrites accumulate in quantity in soils. Actually, we know that the more favorable the conditions are for nitrites to accumulate, the less likelihood there is of chemical nitrification so long as the soil environment remains constant and the nitrites remain where formed. They are, however, free to move with the soil water and if the movement is into a soil of pH 5 or less there may be a rapid chemical oxidation to nitrates. For this reason it is important to know to what extent, and under what conditions, nitrites do accumulate. Many investigators have observed such accumulation but only a few representative observations need to be given. 

FIGURE 8.37 Ammonium oxidation under ideal conditions where very little nitrite accumulates as an intermediate product. Second step of nitrification is inhibited resulting in accumulation of nitrite. 

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-... 

Canvin and Woo (1979) reported that under certain conditions Antimycin A (mitochondrial ATP site II inhibitor) was more effective in enhancing nitrite accumulation (75% of anaerobic control) by leaf discs than either amytal or rotenone (ATP site I inhibitors). In plant mitochondria, the malate dehydrogenase located on the outside of the inner membrane is capable of utilizing external NADH. They infer that in leaves under dark aerobic conditions, the mitochondria effectively compete with nitrate reductase for cytoplasmic NADH. Confirmation of this competition was afforded by a reconstituted system consisting of mitochondria, nitrate reductase, nitrate and NADH or NAD, malate, and malate dehydrogenase (Reed and Hageman, 1977). Nitrite production under aerobic conditions was 10% that observed under anaerobiosis. 

Other experimental data indicate that the oxygen is affecting nitrite accumulation by increasing nitrite reduction. When conditions are optimized for entry of rotenone (mitochondrial site I inhibitor) into leaf secticHis the leaves under aerobic conditicHis accumulated one-half as much nitrite as those without rotenone under anaerobic conditions (Hageman et al., 1980b Reed and Hageman, 1977). The rotenone treatment reduced respiration by... 

Fig. 1. The inhibitory effects of oxygen on nitrite accumulation by excised wheat (subsp. Centurk) leaves incubated in a gaseous medium in the dark. Nitrogen and compressed air were mixed with ball-type flow-meters and oxygen concentration verified by an oxygen electrode. Accumulated nitrite was extracted by boiling 0.1M phosphate pH 7.5. Data plotted in the insert were calculated for tne 15- to 30-min incubation period (from Hageman et at., 1980b).

Because rotenone does not block mitochondrial ATP site II (site of entry of cytoplasmic NADH) these data infer that the nitrite rather than nitrate reduction is affected. Nitrite accumulated in excised leaves (gaseous milieu) under nitrogen is rapidly lost upon transfer to air (Jones and Sheard, 1978). When only the base of excised leaves are placed in a small amount of water or water containing DNF or FCCP (carbonyl-chlorideptri-fluoromethoxyphenylhydrazone) nitrite accumulation is unaffected by the treatments, under anaerobic conditions. Upon transfer to air, leaves treated with DNP or FCCP show relatively small amounts of loss of nitrite in comparison with controls (Hageman et al., 1980b). 

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. 

They suggested that the active pool could be located in the cytoplasm and the storage pool in the vacuole. Using comparable procedures, pool sizes of nitrate could not be measured in wheat leaves (Hageman et al., 1979a,b). These studies indicated that loss of nitrate reductase activity, diminishing supplies of reductant and other factors rather than lack of nitrate were responsible for the cessation of nitrite accumulation (used to indicate active pool size). It was concluded that the amount of nitrate in the cytoplasm was low and that nitrate was readily available from the apoplast (including the xylem elements). 

N2 or nitrite, was accumulated via the oxidation of aceto-HX and gly-HX by "OH at neutral and alkaline pH and oxidation of SAHA by N3 or Br2 at alkaline pH. Nitrite was not produced even after exposure of the irradiated solutions to oxygen. The lack of nitrite accumulation under anoxia excludes the reaction of NO with HNO (reactions 6—8) ruling out any release of NO along with HNO. The yields of HNO at neutral pH were determined to be 9 1% and 25 5% for gly-HX and aceto-HX, respectively, and 17 4% for SAHA at alkaline pH. 

Rhee, S.K., J.J. Lee, and S.T. Lee (1997). Nitrite accumulation in a sequencing batch reactor during the aerobic phase of biological nitrogen removal. Biotechnol. Lett. 19, 195-198. 

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