where the atomic weight of nitrogen = 14, the molecular weight of NH3 = 17, and the formula weight of NO3— = 62.
Example 13.3 The drinking water standard for nitrate-N is 10 mg/L. How much is this when it is expressed as nitrate?
Answer The formula weight for nitrate = 1(14) + 3(16) = 62 g/mol:
In addition to the oxidation and reduction reactions discussed in more detail below, several other reactions are important in the nitrogen cycle. One of these is the conversion of organic nitrogen to ammonia. As with any change from an organic form to an inorganic one, this process can be referred to as mineralization. However, a more specific name, ammonification, is often used in this case. The reverse reaction, assimilation, refers to the uptake of an element for incorporation in cell material. Ammonia assimilation does not represent a change in the oxidation state of the nitrogen. However, nitrate assimilation (as discussed below in Section 13.2.1) represents a reduction and requires energy.
Another important nitrogen transformation without oxidation or reduction included in Figure 13.18 is the acid-base reaction between ammonia (nonionized) and ammonium (a cation):
When speaking of the ammonia-N concentration in water, it is usually the total ammonia-N, or NH4+-N + NH3-N that is meant, as this is what is measured by all of the commonly used analytical methods. The pKa value (—log Ka) at 25°C for this reaction, at which half of the total ammonia would be in each form, is about 9.3:
Since acid-base reactions are fairly rapid, at neutral pH values, only a small percentage of the total ammonia is expected to be present as NH3 (0.50% at pH 7, 4.8% at pH 8).
Example 13.4 The pH of a water sample at 25°C is 7.5. If the measured total ammonia-N concentration is 3.0 mg/L, what is the concentration of nonionized NH3-N? Answer Let [NT] = [NH3] + [NH+]; then
13.2.1 Nitrogen Reduction
Important nitrogen reduction reactions include nitrogen fixation, dissimilatory nitrate reduction (including denitrification), and nitrate assimilation.
Nitrogen Fixation Although a gaseous sea of N2 blankets Earth, it offers nothing in the way of nutritional value except to a select few prokaryotes. The trivalent bond (N=N) of this molecule is simply too strong for normal metabolic cleavage. Indeed, one of the names that Antonie Lavoisier originally considered in the late eighteenth century for this recently discovered gas was "azote," meaning lifeless or inert.
Since the vast majority of life-forms are unable to utilize nitrogen gas, a critical step in the global cycling of nitrogen is its fixation. Nitrogen fixation is the reduction of N2 to organic or ammonia nitrogen. As indicated in Figure 13.18, some nitrogen is also fixed abiotically in natural systems through oxidation reactions involving combustion, photolysis, or electricity (lightning).
A variety of nitrogen fixers are known, scattered among a number of taxa, but all are prokaryotes. They are often separated into free-living vs. symbiotic forms, although this is not a phylogenetic approach. Although Azotobacter and Beijerinckia are probably the best known of the free-living, nitrogen-fixing aerobes among the proteobacteria, other examples are found among species of Klebsiella (but only when growing under anaerobic conditions) and Citrobacter (members of the Enterobacteriaceae family), Methylomonas (a methanotroph), and Thiobacillus (a sulfur-oxidizing autotroph). There are also aerobic gram positives (e.g., a few Bacillus and the actinomycete Streptomyces) and many cyano-bacteria, such as Anabaena (see Figure 10.20) and Nostoc. Free-living anaerobic nitrogen fixers include members of the gram-positive spore-formers Clostridium and Desulfotoma-culum (a sulfate-reducer); the sulfate-reducing proteobacteria Desulfovibrio; several phototrophs (Table 10.4), such as Chlorobium (green sulfur bacteria), Chromatium (purple sulfur), Rhodospirillum (purple nonsulfur), and Heliobacterium (another gram-positive spore-former); and a few methanogens, such as Methanococcus, which are archaea.
The best known symbiotic nitrogen fixers are rhizobia, proteobacteria such as Rhizo-bium associated with leguminous plants (e.g., beans, peas, soybeans, peanuts, alfalfa, and clover). Within the rhizospere (root zone in the soil), these bacteria infect the roots of the plant, forming nodules (knoblike growths). Root nodules help the host plant by providing fixed nitrogen (often, the limiting nutrient for plants), while providing organic substrates (often, the limiting nutrient for heterotrophs) produced by the plant for the bacteria. The actinomycete Frankia also produces root nodules, most commonly in nonleguminous woody plants such as the alder and bayberry.
Other nitrogen-fixing bacteria form associations with plants without nodule formation. Clostridium and Desulfovibrio, for example, grow in the root zone of eelgrass, a shallow saltwater plant, and Azotobacter and Azospirillum grow in the rhizosphere of some grasses, including corn. Nitrogen-fixing cyanobacteria may also form symbiotic relationships with fungi, as in some lichens, as well as with plants such as the water fern Azolla (used agriculturally to fix nitrogen in rice paddies). This great diversity of organisms and interactions further demonstrates the great adaptability of life!
Nitrogen fixation requires an uncommonly high metabolic investment of energy and reducing power to break molecular nitrogen's strong trivalent bond (almost twice the energy of oxygen's double bond). The conversion of a single molecule of N2 into 2 mol of reduced nitrogen requires eight protons, eight electrons, and between 16 and 24 ATP (Figure 13.19).
The highly specialized nitrogenase enzyme complex, which includes dinitrogenase, employs cofactors containing iron and usually molybdenum or, occasionally, vanadium. Other iron-containing enzymes and ATP are needed to transfer the required electrons to the nitrogenase system. The reducing power (electrons) must come, in turn, from an electron-donating substrate (usually organic, although some nitrogen fixers can use sulfide, hydrogen, or carbon monoxide). Although the conversion of two nitrogen atoms from an oxidation state of 0 to —3 requires only six electrons, the process actually requires eight, producing a molecule of hydrogen gas (H2).
There is some variation in the enzymes among different species, but even the nitro-genases of aerobic bacteria are typically highly sensitive to irreversible inactivation by oxygen. How, then, can these aerobes fix nitrogen? Obviously, they must have means of preventing the contact of oxygen with the enzymes. One approach, taken by Klebsiella, which is a facultative anaerobe, is to fix nitrogen only under anaerobic conditions. Other
Nitrogen Oxidation State
Dinitrogen N2 o
Nitrogen Oxidation State
Dinitrogen N2 o
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