M. Pujopay et al., EXCRETION OF DISSOLVED ORGANIC NITROGEN BY PHYTOPLANKTON ASSESSED BY WET OXIDATION AND N-15 TRACER PROCEDURES, Marine ecology. Progress series, 153, 1997, pp. 99-111
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.