Effect of u.v.-B irradiance on the response of 15N-nitrate uptake of Lauderia annulate and Synedra planctonica

The marine diatoms Lauderia annulata and Synedra planctonica harvested during exponential growth were exposed to different doses of u.v.-B (286, 439 and 710 J m −2 d −1 ) for 2 days. Uptake of 15 N-nitrate was estimated before, during and after u.v.-B radiation over 2 days. Exposure to high levels of u.v.-B (710 J m −2 d −1 ) caused irreversible damage at the second daily irradiance. Lauderia cells were less affected by u.v.-B stress than Synedra cells. 15 N-nitrate uptake was reduced under u.v.-B irradiance but could be reactivated within 1 day following exposure to a low dose (286 J m −2 d −1 ). Higher levels of u.v.-B radiation (710 J m −2 d −1 ) led to irreversible damage. The pattern of 15 N-incorporation into several amino acids of Lauderia varied after 2 days of u.v.-B radiation. 15 N enrichment of glutamine increased markedly after u.v.-B stress (717 J m −2 d −1 ) whereas I5 N excess of aspartic acid was significantly reduced. Results are discussed with reference to the u.v.-B damage of the nitrate transport system.