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Appl. Environ. Microbiol., Dec 1997, 4679-4685, Vol 63, No. 12
Copyright © 1997, American Society for Microbiology

Dissimilatory Nitrate Reduction in Anaerobic Sediments Leading to River Nitrite Accumulation

BHL Kelso, RV Smith, RJ Laughlin and SD Lennox
Department of Agricultural and Environmental Science, The Queen's University of Belfast, and Agricultural and Environmental Science Division, and Biometrics Division, The Department of Agriculture for Northern Ireland, Belfast, BT9 5PX, United Kingdom

Recent studies on Northern Ireland rivers have shown that summer nitrite (NO(inf2)(sup-)) concentrations greatly exceed the European Union guideline of 3 (mu)g of N liter(sup-1) for rivers supporting salmonid fisheries. In fast-flowing aerobic small streams, NO(inf2)(sup-) is thought to originate from nitrification, due to the retardation of Nitrobacter strains by the presence of free ammonia. Multiple regression analyses of NO(inf2)(sup-) concentrations against water quality variables of the six major rivers of the Lough Neagh catchment in Northern Ireland, however, suggested that the high NO(inf2)(sup-) concentrations found in the summer under warm, slow-flow conditions may result from the reduction of NO(inf3)(sup-). This hypothesis was supported by field observations of weekly changes in N species. Here, reduction of NO(inf3)(sup-) was observed to occur simultaneously with elevation of NO(inf2)(sup-) levels and subsequently NH(inf4)(sup+) levels, indicating that dissimilatory NO(inf3)(sup-) reduction to NH(inf4)(sup+) (DNRA) performed by fermentative bacteria (e.g., Aeromonas and Vibrio spp.) is responsible for NO(inf2)(sup-) accumulation in these large rivers. Mechanistic studies in which (sup15)N-labelled NO(inf3)(sup-) in sediment extracts was used provided further support for this hypothesis. Maximal concentrations of NO(inf2)(sup-) accumulation (up to 1.4 mg of N liter(sup-1)) were found in sediments deeper than 6 cm associated with a high concentration of metabolizable carbon and anaerobic conditions. The (sup15)N enrichment of the NO(inf2)(sup-) was comparable to that of the NO(inf3)(sup-) pool, indicating that the NO(inf2)(sup-) was predominantly NO(inf3)(sup-) derived. There is evidence which suggests that the high NO(inf2)(sup-) concentrations observed arose from the inhibition of the DNRA NO(inf2)(sup-) reductase system by NO(inf3)(sup-).


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