Dilution of 15N in Dunaliella tertiolecta by uptake of an unidentified compound following nitrate exhaustion

Abstract Nitrate (about 20 μM) was added as 15 NO 3 to a nitrate-limited continuous culture of Dunaliella tertiolecta at steady-state. Nitrate uptake was then estimated from the decrease in nitrate in the medium, the incorporation of 15 N into cells, and the increase in cellular nitrogen. Although the overall nitrogen budget over 5 h was balanced, there were large differences in estimates (up to a factor of five) of nitrate assimilation by the three methods on shorter time scale. After nitrate was exhausted from the medium, cellular nitrogen continued to increase while the 15 N content of the particulate matter decreased over the next 1.5 h. This indicated that an unidentified, unlabelled nitrogen form, which was neither nitrite, ammonium nor dissolved free amino acids, was being taken up by the cells, at rates comparable to those of nitrate. This phenomenon leads to an underestimation of new biomass production when assessed through 15 N incorporation into cells.