Lack of phlorotannin induction in the brown seaweed Ascophyllum nodosum in response to increased copper concentrations

Heavy metal resistance in plants and algae may involve an internal detoxification mechanism, where toxic metal ions are chelated to substances that are stored in special compartments within the cells. Phlorotannins (brown algal polyphenolics) are strong chelators to heavy metals in solution and are found in special compartments (physodes) within the cells, and have therefore been suggested to function as an internal detoxification mechanism in brown seaweeds. Phlorotannins are also believed to function as a chemical defence against herbivory and UV radiation, and the production of phlorotannins has been shown to be induced by increased intensities of these factors. In the present study, we investigated the possible role of phlorotannins as an inducible and internal detoxification mechanism in the brown seaweed Ascophyllum nodosum (L.) Le Jol., in response to increased copper (Cu) concentrations in the surrounding water. The phlorotannin content, tissue Cu content and growth of Cu-exposed seaweeds from areas with different levels of contamination were compared in an observational field study and 2 manipulative induction experiments. The phlorotannin content and growth of the seaweeds were not affected by high Cu concentrations, although A. nodosum plants exposed to high Cu levels accumulated high concentrations of Cu in their tissues. The accumulated Cu was not specifically associated with phlorotannins, since only a small part of the total tissue Cu content of the seaweeds was found in the extracted phlorotannin fractions. The results of the present study show that phlorotannin production in A. nodosum is not induced by high Cu concentrations and that, although plants accumulate high levels of Cu in the tissues, binding of Cu to phlorotannins is probably not an important internal detoxification mechanism in A, nodosum. Since plants were not negatively affected by elevated Cu levels in terms of growth, substances other than phlorotannins are probably more important in the metal resistance mechanism of A. nodosum.