The antioxidant responses differ between Symbiodinium strains from different geographic origins

Studies conducted these last two decades have revealed that high sea surface temperatures accompanied by high levels of solar irradiance are responsible for an over production of reactive oxygen species (ROS) leading to the disruption of the symbiosis between cnidarians and their symbiotic Symbiodinium. But, all coral species do not show the same sensitivity to stress. In this context we examined how the antioxidant network of different Symbiodinium species responds to oxidative stress. We bypassed the various thermal and light tolerances existing among the genus Symbiodinium by using a chemical approach, i.e., a treatment with menadione. ROS produced during this oxidative burst reduced photosynthesis by 30 to 50% and significantly decreased the activity of superoxide dismutase. In addition, the low level of lipid peroxidation concomitantly with the decrease in the concentration of diatoxanthin and other carotenoids during the oxidative stress confirms their function of antioxidants and their role in the stabilization of membrane lipids. The analysis of the cellular damages also indicates that proteins were damaged and most likely eliminated by the ubiquitin-proteasome pathway. Finally, caspase-like activity decreased suggesting that cell death mechanisms are not initiated at the early stage of the stress. Although, the mechanisms at play seem to be the same, we found that the temperate Symbiodinium strain (A1) was less impacted by the treatment with menadione than the tropical strain (F1) suggesting that the variations observed are related to their geographic origin.