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 N. On the other hand, seasonal data and depth profile studies, using NO , showed that denitrification was the principal route of dissimilatory NO 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 0-2 cm horizon in Tay estuary mud-flats where maximum rates of NO respiration were also recorded [28-56 μg N d (g dry wt sediment)]. Autotrophic nitrification rates in Tay estuary sediments showed a distinct seasonality, highest rates [0.93 μg 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 NH and NO oxidizing bacteria followed a similar pattern of distribution. Heterotrophic nitrification appears to play an insignificant role in Tay estuary sediments.


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