Response of Laminaria saccharina (phaeophyta) growth and photosynthesis to simultaneous ultraviolet radiation and nitrogen limitation

The brown macroalga Laminaria saccharina (L.) J. V. Lamour. was grown in large outdoor tanks at 50% ambient solar radiation for 3-4 weeks in July and August of 2000, 2001, and 2002, in either ambient or nitrogen (N)-enriched seawater and in either ambient light [PAR + ultraviolet radiation (UVR)] or ambient light minus UVR. Growth, N-content, photosynthetic pigments, and RUBISCO content increased in N-enriched seawater, indicating N-limitation. UVR inhibited growth, but this inhibition was ameliorated by N-enrichment. The response of growth to UVR could not be explained by changes in respiration and photosynthesis. Gross light-saturated photosynthesis (P-max) remained unaffected by UVR but was significantly higher under N-enrichment, as was dark respiration (R-d). UVR had no effect on pigments or N content. However, RUBISCO contents were low in the presence of UVR, reflecting the overall change in soluble cellular protein. Overall, our data indicate that the response to UVR in L. saccharina depends on other environmental factors, such as N, and these effects need to be considered when evaluating the response of macroalgae to increased UVR.