Measurement of Nitrous Oxide Reductase Activity in Aquatic Sediments

Abstract

Denitrification in aquatic sediments was measured by an N 2 O reductase assay. Sediments consumed small added quantities of N 2 O over short periods (a few hours). In experiments with sediment slurries, N 2 O reductase activity was inhibited by O 2 , C 2 H 2 , heat treatment, and by high levels of nitrate (1 mM) or sulfide (10 mM). However, ambient levels of nitrate (<100 μM) did not influence activity, and moderate levels (about 150 μM) induced only a short lag before reductase activity began. Moderate levels of sulfide (<1 mM) had no effect on N 2 O reductase activity. Nitrous oxide reductase displayed Michaelis-Menten kinetics in sediments from freshwater ( K m = 2.17 μM), estuarine ( K m = 14.5 μM), and alkaline-saline ( K m = 501 μM) environments. An in situ assay was devised in which a solution of N 2 O was injected into sealed glass cores containing intact sediment. Two estimates of net rates of denitrification in San Francisco Bay under approximated in situ conditions were 0.009 and 0.041 mmol of N 2 O per m 2 per h. Addition of chlorate to inhibit denitrification in these intact-core experiments (to estimate gross rates of N 2 O consumption) resulted in approximately a 14% upward revision of estimates of net rates. These results were comparable to an in situ estimate of 0.022 mmol of N 2 O per m 2 per h made with the acetylene block assay.