Uptake of 15N-Ammonium and 15N-Nitrate by Antarctic Diatoms: Dependence on the Daytime and Effect of UV-B Radiation

Abstract

The antarctic diatoms Rhizosolenia truncata Karsten and Thalassiosira antarctica Comber, grown at a temperature of 0°C, a light intensity of 50 ¼mol m -2 s -1 and a light/dark regime of 12: 12 h were used during exponential growth for 15 N-experiments in the light and dark period. Maximal uptake rates of 15 N-ammonia and 15 N-nitrate of both species were found in the light period whereas low uptake rates were observed in the dark, only. Assimilation of both nitrogenous compounds by both species were 4-fold higher at a final concentration of 100 ¼M compared to 10 ¼M. Uptake of 15 N-nitrate was slightly lower in the presence of ammonia whereas 15 N-ammonia assimilation was not affected by nitrate. Uptake rates of both nitrogen compounds reached maximal values at + 6 °C whereas a low 15 N-enrichment has been measured in cells adapted to ± 0 °C and + 10°C. The aim of our study was to obtain information on the daily variation of the nitrogen uptake by Antarctic phytoplankton. In another series of experiments the effect of UV-B radiation on uptake rates of Rhizosolenia truncata Karsten and Coscinodiscus spec. was tested during the exposure time. A reduction in uptake of ammonium and nitrate in dependence on the UV-B dose and the exposure time was found. Results are discussed with reference to the possible target of UVB irradiance.