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Most marine macroalgae live attached to a substrate in coastal areas characterized by a very variable environment mainly due to the tide cycle which is more or less important depending on the geographical situation. Macroalgae spread over coastal zones according to their resistance to several abiotic and biotic factors. Some species are thus present on intertidal zones and are outside the water for a more or less long time each day; on the other hand, others are always underwater if they are living in the subtidal zone or in pools in which abiotic parameters can be strongly modified during a tide cycle. During these alternating immersion/emersion phases, macroalgae deal then with important modifications of many abiotic parameters, such as light, temperature, salinity, and biotic parameters, such as grazing, fouling, pathogens. To counteract these important variations, intertidal macroalgae develop resistance mechanisms such as the synthesis of special compounds, like photoprotectors, osmolytes, antifouling molecules as examples. In this chapter, after a presentation of the main environmental parameters facing intertidal seaweeds in temperate environment, some adaptation/protection mechanisms of these algae are presented, with an emphasis on brown (structural species whose biomass is very high on intertidal zones) and red seaweeds (whose diversity is very high).

International audience; To cope with the biotic and abiotic stresses experienced within their environment, marine macroalgae have developed certain defence mechanisms including the synthesis of photo-protective molecules against light and particularly harmful UV radiation. The aim of this study was to screen selected red algae, a highly diverse phylogenetic group, for the production of photo-protective molecules. The pigment content and composition (Le. chlorophyll-a, phycobiliproteins and carotenoids) and the composition of mycosporine-like amino acids (MAAs) were studied in 40 species of red macroalgae collected in Brittany (France), at two distinct periods (i.e. February and July 2017). A high inter-specific variability was demonstrated in terms of pigment content and MAA composition. Twenty-three potential MAAs were detected by HPLC, and six were identified by LC-MS (i.e. shinorine, palythine, asterina-330, porphyra-334, usurijene and palythene). This is the first study to report on the composition of pigments and MAAs in a diverse group of red seaweeds from Brittany, including some species for which the MAA composition has never been studied before. Nevertheless, the results suggested that some species of red algae are more likely to cope with high levels of light radiation since those species such as Bostrychia scorpioides, Porphyra dioica, Gracilaria vermiculophylla and Vertebrata lanosa are living in environments exposed to higher levels of irradiation, and had various MAAs in addition to their photo-protective pigments.