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 Chlorophycea (Dunaliella tertiolecta) batch cultures was examined by both the wet oxidation procedure and the newly improved N-15 tracer technique for simultaneous isotope-ratio analysis of inorganic and organic forms of nitrogen. Cultures were conducted under different initial nitrate/phosphate ratios and under light and nutrient stressed conditions. Experimental results remained lower than previous and recent evaluations and showed that less than 10% of the nitrate uptake was released or excreted 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, results tend to confirm that these products were not artefactual and had resulted from phytoplankton excretion. These values of DON release enabled 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 infections) must be the main ones leading to significant release of these biodegradable compounds and might explain the high levels of DON release sometimes observed during experimental studies or in the natural environment.