EXCRETION OF DISSOLVED ORGANIC NITROGEN BY PHYTOPLANKTON ASSESSED BY WET OXIDATION AND N-15 TRACER PROCEDURES

The release of DON in diatom (Phaeodactylum tricornutum) and Chlorophy cea (Dunaliella tertiolecta) batch cultures was examined by both the w et oxidation procedure and the newly improved N-15 tracer technique fo r simultaneous isotope-ratio analysis of inorganic and organic forms o f nitrogen. Cultures were conducted under different initial nitrate/ph osphate ratios and under light and nutrient stressed conditions. Exper imental results remained lower than previous and recent evaluations an d showed that less than 10% of the nitrate uptake was released or excr eted as DON. Rates of DON release were around 10.4 to 13.3 nmol N l(-1 ) h(-1). Taking into account potential stress-induced DON losses, resu lts tend to confirm that these products were not artefactual and had r esulted from phytoplankton excretion. These values of DON release enab led us to completely or nearly completely balance the nitrogen budgets in our experiments. However, since DON originating from phytoplankton constitutes an important resource for some organisms, processes other than direct active excretion (such as grazing, cell death or virus in fections) must be the main ones leading to significant release of thes e biodegradable compounds and might explain the high levels of DON rel ease sometimes observed during experimental studies or in the natural environment.