Extrinsic factors influencing phlorotannin production in the brown alga Ascophyllum nodosum

Many brown seaweeds contain high concentrations of phlorotannins (polyphenolics) that show extensive phenotypic variation, which can be generated by variation in a number of biotic and abiotic extrinsic factors. Extrinsically induced changes in the phlorotannin content of brown macroalgae can have significant consequences for secondary production in nearshore marine communities since they will affect the palatability and degradability of a dominating type of macroalgal tissue. In the present study the separate and interactive effects of natural herbivory and 3 abiotic factors on phlorotannin production in the brown alga Ascophyllum nodosum were tested in 2 different induction experiments. In the first experiment the hypothesis that grazing by the gastropod Littorina obtusata, nitrogen enrichment, and enhanced or ambient W-B radiation (UVBR) can affect the phlorotannin content of A. nodosum was tested, and in the second experiment the effects of ambient UVBR and a variable water level (desiccation) were examined. In the first experiment it was found that 3 wk of grazing by L, obtusata caused a significant increase in the phlorotannin content of the algae and this effect was independent of the nitrogen and UVBR treatments which had no effects on phlorotannin levels. This is one of few demonstrated examples of induced chemical responses in seaweeds caused by natural herbivory. Measurements of biomass changes and tissue nitrogen content of the experimental algae showed that the nitrogen uptake of the algae was unaffected by herbivory and UVBR, and that the growth of A. nodosum plants was nitrogen-limited but not affected by UVBR. In the second experiment there was a significantly higher phlorotannin content in algae exposed to natural UVBR, compared to no UVBR, after 7 wk, but not after 2 or 4 wk. Furthermore, there was a significant interaction effect between the UVBR and the water level treatment. The phlorotannin content was higher in algae exposed to a variable water regime compared to continuously submerged algae when UVBR was filtered out, but not under ambient UVBR. In summary, the results of this study support the induced defence model (IDM), but not the carbon-nutrient balance model (CNBM), in explaining intraspecific variation in the phlorotannin content of A. nodosum. The results also support the previously proposed hypothesis that phlorotannins play a role as inducible screens against harmful UV radiation.