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References of the abstract :
Coral bleaching has been studied for decades to unravel the physiological mechanisms behind this phenomenon. It is mainly hypothesized to begin with a burst in Reactive Oxygen Species (ROS), but the following steps have not been completely resolved. As ROS are mostly produced in chloroplastic and mitochondrial membranes-associated electron transport chains, the role of oxidized fatty acids/oxylipins is worth exploring. Such compounds have already been identified as the signaling molecules following ROS after exposure to some abiotic stresses in plants. Here, we did not observe ROS increase after the addition of 2,4-Decadienal/2,4-DD (an oxidation product of -6 polyunsaturated fatty acids) neither in Breviolum minutum nor in Exaiptasia pallida H2, which tends to confirm the appearance of oxylipins downstream and not upstream of ROS. During the bleaching cascade, after a heat stress for example, an increase in NO has been measured in anemones. Here, we observed that 60 µM 2,4-DD doubled NO content in both symbiotic and bleached E. pallida. A similar NO increase was observed in cultured B. minutum although requiring a higher concentration of 500 µM. Thus, the animal is more sensitive to oxylipins than its microalgae. Oxylipins are known to play a role in the immune system and this is why genes encoding for their enzymatic production are downregulated in healthy symbiotic anemones. Given our preliminary results, and the ability of oxidized fatty acids to cross membranes, we will explore the following hypothesis for the bleaching cascade: oxylipins production by ROS with spontaneous/non-enzymatic reactions in Symbiodiniaceae, mainly with photosynthesis, membranes crossing, and immune response triggering in the animal. Alongside this potential role in bleaching, preconditioning experiments are ongoing to determine if small doses of oxylipins can enhance the defenses of anemones against heat stress, as already shown in plants.
