IMPACT OF UV-B RADIATION ON UPTAKE OF N-15-AMMONIA AND N-15-NITRATE BY PHYTOPLANKTON OF THE WADDEN SEA

Natural marine phvtoplankton populations from the German Wadden Sea and unialgal cultures of the haptophycean Phaeocystis pouchetii were tested in 1989 under controlled UV-B stress conditions. Assimilation of N-15-nitrate in phytoplankton consisting mainly of P. pouchetii, or in pure cultures of this alga. was found to be very sensitive to enhanced UV-B dosage in comparison N-15-ammonia uptake. In contrast, in phytoplankton samples containing Ceratium spp., Coscinodiscus sp., Noctiluca sp. or others, rate of (NO3-)-N-15 uptake was higher and only slightly affected by UV-B irradiance compared to the P. pouchetii sample. UV-B inhibitory effect on uptake of inorganic nitrogen by P. pouchetii was more pronounced under strong white-light conditions and after a UV-B pre-illumination period of several hours than under low white light. Pools of glutamine and alanine decreased after UV-B exposure. Results are discussed with reference to the damaging effects of white light and UV-B on nitrogen metabolism.