Influence of light and nitrogen on the phlorotannin content of the brown seaweeds Ascophyllum nodosum and Fucus vesiculosus

Phlorotannins, C-based defence compounds in brown seaweeds, show a high deg ree of spatial and temporal variation within seaweed species. One important model explaining this variation is the Carbon Nutrient Balance Model (CNBM ), which states that the relative supply of carbon and limiting nutrients w ill determine the level of defend compounds in plants. Nitrogen is often co nsidered to be the limiting nutrient for marine macroalgal growth and the C NBM thus predicts that when the carbon:nitrogen ratio is high, photosynthet ically fixed carbon will be allocated to production of phlorotannins. In th e present study, we evaluated the effects of light (i.e. carbon) and nitrog en on the phlorotannin content of two intertidal brown seaweeds, Ascophyllu m nodosum and Fucus vesiculosus. This was done in an observational field st udy, as well as in a manipulative experiment where plants from habitats wit h different light regimes were subjected to different nitrogen and light tr eatments, and their phlorotannin content was measured after 14 days. The re sults showed that then was a negative relationship between tissue nitrogen and phlorotannin content in natural populations of F. vesiculosus, but not in A. nodosum. In the short term, the phlorotannin content in both algal sp ecies was not affected by changes in nitrogen availability. Exposure to sun light had a positive effect on the phlorotannin content in natural populati ons of both algal species but. in the manipulative experiment, only F. vesi culosus showed a rapid response to changes in light intensities. Plants sub jected to sunlight contained higher phlorotannin content than shaded plants . In conclusion, the results imply that nitrogen availability explains some of the natural variation in the phlorotannin content of F: vesiculosus, bu t the light environment has greater importance than nitrogen availability i n predicting the phlorotannin content of each species.