Seasonal variation in total phenolic content of Dictyopteris polypodioides (Dictyotaceae) and Cystoseira amentacea (Sargassaceae) from the Sicilian coast

Phlorotannins are polyphenolic secondary metabolites found in almost all brown algae that function as defense against grazers, pathogens and epiphytes but are also involved in photoprotection mechanisms. These compounds, produced in the Golgi apparatus, are accumulated in cytoplasm, within vesicules called physodes, or bound to the cell wall. The concentration of phlorotannins dif-fers within and between species, shows geographical variations but may be also affected by abiotic or biotic factors. Aims of this study were to evaluate: (i) the temporal variation of total phenolic content in two brown algae, Dictyopteris polypodioides and Cystoseira amentacea, living respec-tively in the upper infralittoral zone and in the infralittoral fringe, and (ii) its relationship with depth. Thalli of both algae were collected from the north-western coast of Sicily. Results showed significant differences in total phenol content between D. polypodioides and C. amentacea with a higher value observed in the first one. Both species showed a seasonal pattern of total phenolic contents and differences in the period of their maximum production were also ob-served between the two species. In D. polypodioides the peak was observed during winter and au-tumn (0.95 and 0.81% DW) whereas during spring and summer in C. amentacea (0.17 and 0.57% DW). The phenol concentration resulted negatively correlated with the air temperature and the me-dium solar radiation in D. polypodioides whereas the correlation was positive in C. amentacea. Even though seaweeds living on the rocky intertidal habitats, subjected to large variations of envi-ronmental conditions, should invest more energy in defences, D. polypodioides showed a phenolic content higher than C. amentacea. Our results suggest that phenol content and seaweed zonation are not interdependent and that sheet-like algae such as D. polypodioides, resulting more attractive than thick leathery ones to herbivores, produce larger concentrations of these compounds in order to deter feeding by herbivores.