DILUTION OF N-15 IN DUNALIELLA-TERTIOLECTA BY UPTAKE OF AN UNIDENTIFIED COMPOUND FOLLOWING NITRATE EXHAUSTION

Nitrate (about 20 mu M) was added as (NO3)-N-15 to a nitrate-limited c ontinuous culture of Dunaliella tertiolecta at steady-state. Nitrate u ptake was then estimated from the decrease in nitrate in the medium, t he incorporation of N-15 into cells, and the increase in cellular nitr ogen. Although the overall nitrogen budget over 5 h was balanced, ther e were large differences in estimates (up to a factor of five) of nitr ate assimilation by the three methods on shorter time scale. After nit rate was exhausted from the medium, cellular nitrogen continued to inc rease while the N-15 content of the particulate matter decreased over the next 1.5 h. This indicated that an unidentified, unlabelled nitrog en form, which was neither nitrite, ammonium nor dissolved free amino acids, was being taken up by the cells, at rates comparable to those o f nitrate. This phenomenon leads to an underestimation of new biomass production when assessed through N-15 incorporation into cells. ((C) A cademie des sciences / Elsevier, Paris.).