A sensitive NO 2؊ biosensor that is based on bacterial reduction of NO 2 ؊ to N 2 O and subsequent detection of the N 2 O by a built-in electrochemical N 2 O sensor was developed. Four different denitrifying organisms lacking NO 3 ؊ reductase activity were assessed for use in the biosensor. The relevant physiological aspects examined included denitrifying characteristics, growth rate, NO 2 ؊ tolerance, and temperature and salinity effects on the growth rate. Two organisms were successfully used in the biosensor. The preferred organism was Stenotrophomonas nitritireducens, which is an organism with a denitrifying pathway deficient in both NO 3 ؊ and N 2 O reductases. Alternatively Alcaligenes faecalis could be used when acetylene was added to inhibit its N 2 O reductase. The macroscale biosensors constructed exhibited a linear NO 2 ؊ response at concentrations up to 1 to 2 mM. The detection limit was around 1 M NO 2 ؊ , and the 90% response time was 0.5 to 3 min. The sensor signal was specific for NO 2 ؊ , and interference was observed only with NH 2 OH, NO, N 2 O, and H 2 S. The sensor signal was affected by changes in temperature and salinity, and calibration had to be performed in a system with a temperature and an ionic strength comparable to those of the medium analyzed. A broad range of water bodies could be analyzed with the biosensor, including freshwater systems, marine systems, and oxic-anoxic wastewaters. The NO 2 ؊ biosensor was successfully used for long-term online monitoring in wastewater. Microscale versions of the NO 2 ؊ biosensor were constructed and used to measure NO 2 ؊ profiles in marine sediment.
Nitrite (NO 2Ϫ ) has a central position in the global nitrogen (N) cycle and is involved in many important biological N transformations. With an intermediate oxidation state, NO 2 Ϫ acts as an electron donor in nitrification and serves as an electron acceptor in denitrification, dissimilative nitrite reduction to ammonium, and anammox.In natural ecosystems NO 2 Ϫ is of interest because of its toxicity for microorganisms and higher organisms (20,37). NO 2 Ϫ concentrations in natural bulk water bodies are usually very low, and in freshwater systems the average worldwide NO 2 Ϫ concentration has been estimated to be about 1 g of N liter Ϫ1 (ϳ0.07 M NO 2 Ϫ ) (28). Recently, however, there have been several reports of NO 2 Ϫ accumulation to concentrations of 5 to 140 M in European eutrophic rivers and estuaries (3,8,15,19,44). Studies have indicated that NO 2 Ϫ accumulations can be related to imbalances in NO 2 Ϫ production and consumption rates for either aerobic sediment processes (nitrification) or anaerobic sediment processes (denitrification and dissimilatory nitrate reduction to ammonia). In marine systems NO 2 Ϫ concentrations are normally negligible, but concentrations of 0.3 to 2.5 M have been reported near the oxic-anoxic boundary of stratified marine water bodies (5, 22). The distribution of NO 2 Ϫ in sediments is largely uncharacterized, except for a few studies that have documented NO 2 Ϫ a...
