Microbial Nitrate Reduction in Freshwater Sediments

Abstract

Nitrate reductase activity was 3-4 orders of magnitude greater in freshwater sediments than in the overlying water. Viable (most probable number) counts of denitrifiers provided sufficient resolution to distinguish between anoxic and surface waters, and between these and sediments, but did not correlate with differences between or within sediment cores. Activity within the sediment depended on the electrode potential (Eh) profile, which in turn was related to the degree of turbulence and O2 concentration in the overlying water. Sediments from the littoral zone or those in contact with oxygenated water were oxidizing to a depth of 5-10 mm and the Eh then decreased rapidly. In these sediments nitrate reductase activity was often at its maximum at a depth of 10-15 mm, on the Eh gradient, and coincided with a mean Eh value of 210 mV. Under reducing conditions the Eh gradient moved upwards and nitrate reductase activity was greatest at the sediment-water interface. These observations were supported by analyses of the N2 gas content of the sediments. Inhibition of the enzymes with chlorate indicated that approximately 60% of the activity was dissimilatory in sediments where the Eh was greater than 100 mV, and that this proportion increased to more than 90% when the Eh fell below 50 mV. Although the evidence was not conclusive, there was also some indication that nitrate reductase activity in aerobic surface sediments was greater in the larger (> 250 .mu.m) particle size fraction, which suggested that these particles might act as microsites for nitrate respiration.