ABSTRACT: The potential for release of ammonium (NH₄⁺) and/or urea from the pool of dissolved organic nitrogen (DON) was examined in samples taken from Lake Kinneret (Israel), the River Charente estuary and coastal water near Ile de Ré (French Atlantic coast). After prefiltration through 1.0 or 1.2 µm membranes to remove most of the microbiota with the exception of bacteria, water samples with or without supplements (40 µM) of various organic nitrogen compounds (arginine, glucosamine, guanine, hypoxanthine, lysine, ornithine or thymine) were incubated at in situ temperatures, in the dark, for 7 to 14 d. Concentrations of NH₄⁺ and urea were monitored during the incubation period. Increases of NH₄⁺ with time were observed in 8 out of 12 experiments with unsupplemented lake samples, and in a single trial with coastal water, but not with Charente estuary water. In some experiments, increases of urea concentrations were also observed. The addition of organic nitrogen compounds almost always led to NH₄⁺ increases in samples from all locations; guanine, hypoxanthine, arginine and, in the case of Charente water, glucosamine gave rise to urea. The addition of nitrification inhibitors (40 µM) at the start of some experiments gave inconsistent results, but in some cases appeared to increase the concentrations of NH₄⁺ with time. Taken together, the results of these experiments clearly indicate the potential in natural waters for degradation of DON pool constituents by indigenous bacteria and/or free dissolved enzymes to NH₄⁺ or urea; these in turn can be effectively exploited by the ambient microbiota. The breakdown of DON with the concomitant release of readily available compounds such as NH₄⁺ or urea could be an important process in the nitrogen nutrition of phytoplankton and bacteria.

KEY WORDS: DON · Decomposition · NH₄⁺ · Urea · Bacteria · Lake · Estuary

Published in AME Vol. 16, No. 3 (1999) on February 24
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999