Bhl. Kelso et al., DISSIMILATORY NITRATE REDUCTION IN ANAEROBIC SEDIMENTS LEADING TO RIVER NITRITE ACCUMULATION, Applied and environmental microbiology, 63(12), 1997, pp. 4679-4685
Recent studies on Northern Ireland rivers have shown that summer nitri
te (NO2-) concentrations greatly exceed the European Union guideline o
f 3 mu g of N liter(-1) for rivers supporting salmonid fisheries. In f
ast-bowing aerobic small streams, NO2- is thought to originate from ni
trification, due to the retardation of Nitrobacter strains by the pres
ence of free ammonia. Multiple regression analyses of NO2- concentrati
ons against water quality variables of the six major rivers of the Lou
gh Neagh catchment in Northern Ireland, however, suggested that the hi
gh NO2- concentrations found in the summer under warm, slow-how condit
ions may result from the reduction of NO3-. This hypothesis was suppor
ted by field observations of weekly changes in N species. Here, reduct
ion of NO3- was observed to occur simultaneously with elevation of NO2
- levels and subsequently NH4+ levels, indicating that dissimilatory N
O3- reduction to NH4+ (DNRA) performed by fermentative bacteria (e.g.,
Aeromonas and Vibrio spp.) is responsible for NO2- accumulation in th
ese large rivers. Mechanistic studies in which N-15-labelled NO3- in s
ediment extracts was used provided further support for this hypothesis
. Maximal concentrations of NO2- accumulation (up to 1.4 mg of N liter
(-1)) were found in sediments deeper than 6 cm associated with a high
concentration of metabolizable carbon and anaerobic conditions. The N-
15 enrichment of the NO2- was comparable to that of the NO3- pool, ind
icating that the NO2- was predominantly NO3- derived. There is evidenc
e which suggests that the high NO2- concentrations observed arose from
the inhibition of the DNRA NO2- reductase system by NO3-.