INTERACTIONS BETWEEN NUTRITIONAL-STATUS AND LONG-TERM RESPONSES TO ULTRAVIOLET-B RADIATION STRESS IN A MARINE DIATOM

Influences of nutritional status on the photoinhibitory effects of ult raviolet-B radiation (UVBR: 290 to 320 nm) on the specific growth rate s (mu(obs) and biomass of Phaeodactylum tricornutum were determined us ing nutrient-replete batch cultures and nutrient-limited continuous cu ltures. P. tricornutum cultures were exposed to UVBR doses representat ive of current mid-latitude and ozone-depletion intensities. Specific growth rates and biomass were inhibited from 2 to 16% by UVBR during n utrient-replete growth. However, no effect of UVBR on mu(obs) or bioma ss was detectable when nutrient limitation exceeded the potential for limitation by UVBR. Thus, a competitive interaction appears to occur b etween macronutrient stress and UVBR stress, such that mu(obs) and bio mass will be determined by the most limiting factor. Our results sugge st that measurable decreases in phytoplankton mu(obs) and biomass from UVBR are most likely in nutrient-rich areas of the ocean, while these parameters may not be appropriate for measuring UVBR stress in region s of nutrient limitation.