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Appl Environ Microbiol. 1987 February; 53(2): 217-223
Copyright © 1987, American Society for Microbiology. All Rights Reserved.

Estimation of Bacterial Nitrate Reduction Rates at In Situ Concentrations in Freshwater Sediments

Limnological Institute, Vijverhof Laboratory, 3631 AC Nieuwersluis, The Netherlands

ABSTRACT

A method was developed to follow bacterial nitrate reduction in freshwater sediments by using common high-performance liquid chromatographic equipment. The low detection limit (14 pmol) of the method enabled us to study concentration profiles and reaction kinetics under natural conditions. Significant nitrate concentrations (1 to 27 µM) were observed in the sediment of Lake Vechten during the nonstratified period; the concentration profiles showed a successive depletion of oxygen, nitrate, and sulfate with depth. The profiles were restricted to the upper 3 cm of the sediment which is rich in organics and loosely structured. Nitrate reduction in the sediment-water interface followed first-order reaction kinetics at in situ concentrations. Remarkably high potential nitrate-reducing activity was observed in the part of the sediment in which nitrate did not diffuse. This activity was also observed throughout the whole year. Estimates of K_m varied between 17 and 100 µM and V_max varied between 7.2 and 36 µmol cm^–3 day^–1 for samples taken at different depths. The diffusion coefficient of nitrate ([10 ± 0.4] x 10^–6 cm² s^–1) across the sediment-water interface was estimated by a constant-source technique and applied to a mathematical model to estimate the net nitrate reduction during the nonstratified period. In this period, observed nitrate reduction rates by the model, 0.2 to 0.4 mmol m^–2 day^–1, were lower than those found for oxygen (27 mmol m^–2 day^–1) and sulfate (0.4 mmol m^–2 day^–1). During the summer stratification, nitrate was absent in the sediment and reduction could not be estimated by the model.

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