Dissimilatory Nitrate Reduction and Nitrification in Estuarine Sediments

Abstract

Enumeration of populations of nitrate respiring bacteria in estuarine sediments of the River Tay, Scotland, showed that bacteria capable of dissimilating NO, to NH,+ predominated over those denitrifying NO, to N2. On the other hand, seasonal data and depth profile studies, using 5N03, showed that denitrification was the principal route of dissimilatory NOT reduction (78-90% of NO, respired), with maximum rates of both processes occurring in the summer. Population densities of both populations of NO; respiring bacteria were highest in the 0-2cm horizon in Tay estuary mud-flats where maximum rates of NO, respiration were also recorded l28.56 pg N d-(g dry wt sediment)-' 1. Autotrophic nitrification rates in Tay estuary sediments showed a distinct seasonality, highest rates C0.93 pg N d-' (g dry wt sediment)-'] occurring during the summer. Nitrification rates declined rapidly with sediment depth and were not detectable below the oxidized zone (3 cm). Population densities of autotrophic NHJ and NO, oxidizing bacteria followed a similar pattern of distribution. Heterotrophic nitrification appears to play an insignificant role in Tay estuary sediments.