Bacterial Nitrification

We should also learn which organisms and allelochemicals adversely affect the microbial symbionts. Some have already been shown to suppress growth of bacterial nitrogen fixers and nitrifiers as well as mycorrhizal fungi. Compounds that inhibit nitrification could prove to be important agriculturally. 

The nitrogen cycle entails formation of ammonia by bacterial fixation of N2, nitrification of ammonia to nitrate by soil organisms, conversion of nitrate to ammonia by higher plants, synthesis of amino acids from ammonia by all organisms, and conversion of nitrate to N2 by denitrifying soil bacteria. 

These corrected values for the pKA of HNO (>11) and reduction potential of NO (< —0.7 V) demonstrate that HNO, rather than NO, is the predominant species in neutral solution and indicate that NO cannot be easily converted to NO- by simple outer-sphere electron transfer (Scheme 6), unlike the O2/O2 redox couple. The different potentials and concentrations of NO and O2 in cellular or physiological systems suggest that NO is essentially inert to reduction to NO in mammalian biology. Note that certain processes in bacteria are suggested to have sufficient potentials to reduce NO (165, 166), which may have some importance both to normal bacterial physiology, including nitrification and denitrification, and to antibacterial and pathogenic responses. 

Seventeen genera of facultative anaerobic bacteria (e.g., Pseudomonas and Bacillus) can perform denitrification under anaerobic or low-oxygen conditions, where they use NO3- as an electron acceptor during anaerobic respiration (Jaffe, 2000). In fact, in many estuaries, denitrification is limited by the availability of NC>3 (Koike and Sprensen, 1988 Cornwell et al., 1999). Sources of NC>3 and NC>2 for denitrification are from diffusive inputs from the overlying water column and nitrification in the sediments (Jenkins and Kemp, 1984). The activity of other bacterial processes under anoxic conditions has been shown to affect the activity of denitrifying bacteria. For example, SO42- reduction occurs in anoxic sediments whereby SC>42 is reduced to sulfide (Morse et al., 1992)—more... 

Probably these anomalies of the OM parameters might be connected with the process of chemosynthesis, and the layer of bacterial chemosynthesis should play an important role in the formation of the vertical distribution of nutrient species there. The results of measurements of the dark CO2 fixation [78,79] usually reveal the primary maximum of chemosynthesis (about 0.4-2.0 jiM d x) in a 20-30-m layer below the hydrogen sulfide boundary. The less pronounced secondary maximum is observed about 5-10 m shallower than the hydrogen sulfide boundary and is likely to be connected with nitrification [78]. 

The rate of bacterial growth and hence the rate of nitrification is both temperature and pH dependent. Maximum bacterial activity occurs at about 28°C and a pH of about 8, Below a temperature of about 2°C, the reaction is very slow (Fig. 8.7). Below pH 5.5, the nitrifying bacteria decrease their activity, and below pH 4.5 the nitrification process is severely restricted lack of oxygen also inhibits nitrification. As noted above, oxidation of NH to NO is an enzyme-driven reaction and commonly the Km (see Chapter 7) under optimum conditions is observed to be somewhere around 2.5 mM. Some Km values below 2.5 mM are observed under high pH values when a large fraction of the ammonium is in the NH3 form. 

Bianchi, M., Bonn, P., and Fehatra (1994). Bacterial nitrification and denitrification rates in the Rhone River plume (northwestern Mediterranean Sea). Marine Ecology-Progress Series 103, 197-202. 

Welsh, D. T., and CastadeUi, G. (2004). Bacterial nitrification activity direcdy associated with isolated benthic marine animals. Marine Biology 144, 1029—1037. 

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