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Microbial Nitrate Reduction in Freshwater Sediments

Summary: Nitrate reductase activity was three to four 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 (E
h) profile, which in turn was related to the degree of turbulence and oxygen concentration in the overlying water. Sediments from the littoral zone or those in contact with oxygenated water were oxidizing to a depth of 5 to 10 mm and the E
h then decreased rapidly. In these sediments nitrate reductase activity was often at its maximum at a depth of 10 to 15 mm, on the E
h gradient, and coincided with a mean E
h value of 210 mV. Under reducing conditions the E
h gradient moved upwards and nitrate reductase activity was greatest at the sediment-water interface. These observations were supported by analyses of the nitrogen gas content of the sediments. Inhibition of the enzymes with chlorate indicated that approximately 60 % of the activity was dissimilatory in sediments where the E
h was greater than +100 mV, and that this proportion increased to more than 90% when the E
h 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 μm) particle size fraction, which suggested that these particles might act as microsites for nitrate respiration.