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Abstract :
[en] In symbiotic cnidarians, living in symbiosis with photosynthetic dinoflagellates of the Symbiodiniaceae family causes daily local hyperoxia and promotes the generation of Reactive Oxygen Species (ROS) that, under some particular environmental conditions, may cause the breakdown of the symbiotic interaction (i.e. bleaching). Despite much effort, there are still large gaps in our understanding of ROS signaling and how the antioxidant network is modulated in Symbiodiniaceae. It is known from plants and green algae that the nature of ROS signaling depends on the chemical identity of ROS. Therefore, to understand the mechanisms by which cells sense and respond to oxidative stress, it is necessary to investigate responses to individual ROS. Under oxidative stress, oxidants such as free radicals attack polyunsaturated fatty acids (PUFAs) and carotenoids containing C-C double bonds. During this study we determined signatures that discriminate between 1O2 and hydroxyl radical-dependent lipid and carotenoid oxidation. First, we characterized the fatty acid and the carotenoid compositions in cultured and freshly isolated Symbiodinium microadriaticum by using chromatographic methods. Then we oxidized the predominant PUFAs and carotenoids in vitro by exposing them to photosynthesizer and light to generate 1O2, and H2O2 in presence of Fe3+ to generate hydroxyl radical. Finally, knowing the pattern of carotenoid and lipid oxidation products produced in vitro by 1O2 or free radicals, we looked for those oxidation products in cultured and symbiotic S. microadriaticum cells subjected to various stressful conditions such as conditions involving endogenously produced 1O2 or superoxide ion/H2O2 ; exposure to high light intensity; and high light exposure combined to elevated temperature incubation. This work also aimed to find conditions in which cells of S. microadriaticum could be preconditioned or acclimatized by ROS. To this end, different prooxidant molecules were tested.
