title: Single-step nitrification models erroneously describe batch ammonia oxidation profiles when nitrite oxidation becomes rate limiting.
abstract: Nitrification involves the sequential biological oxidation of reduced nitrogen species such as ammonium-nitrogen (NH(4)(+)-N) to nitrite-nitrogen (NO(2)(-)-N) and nitrate-nitrogen (NO(3)(-)-N). The adequacy of modeling NH(4)(+)-N to NO(3)(-)-N oxidation as one composite biochemical reaction was examined at different relative dynamics of NH(4)(+)-N to NO(2)(-)-N and NO(2)(-)-N to NO(3)(-)-N oxidation. NH(4)(+)-N to NO(2)(-)-N oxidation and NO(2)(-)-N to NO(3)(-)-N oxidation by a mixed nitrifying consortium were uncoupled using selective inhibitors allylthiourea and sodium azide. The kinetic parameters of NH(4)(+)-N to NO(2)(-)-N oxidation (q(max,ns) and K(S,ns)) and NO(2)(-)-N to NO(3)(-)-N oxidation (q(max,nb) and K(S,nb)) were determined by a rapid extant respirometric technique. The stoichiometric coefficients relating nitrogen removal, oxygen uptake and biomass synthesis were derived from an electron balanced equation. NH(4)(+)-N to NO(2)(-)-N oxidation was not affected by NO(2)(-)-N concentrations up to 100 mg NO(2)(-)-N L(-1). NO(2)(-)-N to NO(3)(-)-N oxidation was noncompetitively inhibited by NH(4)(+)-N but was not inhibited by NO(3)(-)-N concentrations up to 250 mg NO(3)(-)-N L(-1). When NH(4)(+)-N to NO(2)(-)-N oxidation was the sole rate-limiting step, complete NH(4)(+)-N to NO(3)(-)-N oxidation was adequately modeled as one composite process. However, when NH(4)(+)-N to NO(2)(-)-N oxidation and NO(2)(-)-N to NO(3)(-)-N oxidation were both rate limiting, the estimated lumped kinetic parameter estimates describing NH(4)(+)-N to NO(3)(-)-N oxidation were unrealistically high and correlated. These findings indicate that the use of single-step models to describe batch NH(4)(+) oxidation yields erroneous kinetic parameters when NH(4)(+)-to-NO(2)(-) oxidation is not the sole rate-limiting process throughout the assay. Under such circumstances, it is necessary to quantify NH(4)(+)-N to NO(2)(-)-N oxidation and NO(2)(-)-N to NO(3)(-)-N oxidation, independently.
authors.affiliation: Environmental Engineering Program, University of Connecticut, Storrs, Connecticut 06269, USA.
authors.forename: K
authors.initials: K
authors.lastname: Chandran
authors.forename: B F
authors.initials: BF
authors.lastname: Smets
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D018929:Cell Culture Techniques / Q000379:methods; D006801:Humans; D008954:Models, Biological; D008962:Models, Theoretical; D009569:Nitric Oxide / Q000378:metabolism*; D010084:Oxidation-Reduction; D018384:Oxidative Stress
chemical_list: D009569:Nitric Oxide; D000641:Ammonia