Abstract:The impact of UV‐B irradiation on unialgal cultures of the temperate marine diatomOdontella sinensisand of the AntarcticOdontella weissflogiiwas tested under controlled laboratory conditions. Uptake rates of inorganic nitrogen byOdontella sinensiswere more affected by UV‐B radiation than those ofOdontella weissflogii. Utilization of15N‐ammonium was reduced after 3 h of UV‐B exposure. Values of Kmfrom several marine phytoplankton species were estimated from algae not exposed to UV‐B and from those after 3 h of UV‐B radiation. Calculation of Lineweaver‐Burk plot showed, under UV‐B stress, a noncompetitive inhibition for15NH+4uptake and a competitive inhibitory effect for15NO−3byOdontella weissflogii. The damage to15NH+4uptake by UV‐B was more pronounced under red than blue light; a contradictory result was obtained for15NO−3utilization byOdontella weissflogii. Reduction of the pigments by UV‐B under white light was dependent on the exposure time; a strong depression of the contents of chlorophyllide a, chlorophyll c, diatoxanthin and the fucoxanthins was found. Contents of the chlorophylls are markedly affected by UV‐B under red light whereas the chlorophyll a concentration is enhanced in blue and green light. UV‐B exposure in conjunction with blue and green light led to a reduction of the protein content and an increase in the amino acid contents. The pattern of pool sizes of free amino acids varied after UV‐B exposure. UV‐A irradiance had no effect. The possible targets of UV‐B irradiation on the uptake system of inorganic nitrogen are discussed in detail. Adaptation to the environmental conditions, e.g. via synthesis of UV stress proteins or mycosporine‐lik