A method for investigating the heterotrophic assimilation of ammonium and nitrate through planktonic organisms

Abstract

The kinetics of assimilation of ammonium and nitrate is investigated for various predetermined concentrations of water samples extracted from the upper subsurface of the littoral zone of a small reservoir (Lake IAG) in São Paulo, Brazil ( 23º39’ S and 46º37’ W). Five concentrations were predetermined for added ammonium: 300; 370; 440; 650; 1280 µg.N.L^-1 and nitrate: 340; 590; 890; 1090; 1590 µg.N.L^-1. The samples were maintained under dark and oxygenated at a controlled temperature of 20^ºC ± 1^ºC, for the period of 9 days for ammonium and 11 days for nitrate. Once a day the concentrations of ammonium and nitrate in the flasks were analyzed colorimetric method. The highest uptake rate occurred at the initial concentrations of 300 µg.N.L^-1 (k = 0,23 day^-1) and 340 µg.N.L^-1 (k = 0,25 day^-1 ) for ammonium and nitrate respectively. These results indicate that the latter concentrations are close to the ideal conditions for assimilation of ammonium and nitrate by the microorganisms in the system. It is suggested therefore that these are the concentrations at which incubation processes should be started. In addition, concentrations above 500 µg.N.L^-1 for ammonium and 600 µg.N.L^-1 for nitrate are likely to cause inhibition of the assimilation process. The data do not follow a simple Michaelis-Menten equation, probably because some inhibition in the assimilation occurred.