Activation of endogenous tolerance to bleaching stress by high salinity in cloned endosymbiotic dinoflagellates from corals

Background: Large-scale coral bleaching events have become increasingly frequent in recent years. This process occurs when corals are exposed to high temperatures and intense light stress, leading to an overproduction of reactive oxygen species (ROS) by their endosymbiotic dinoflagellates. The ROS buildup prompts corals to expel these symbiotic microalgae, resulting in the corals' discoloration. Reducing ROS production and enhancing detoxification processes in these microalgae are crucial to prevent the collapse of coral reef ecosystems. However, research into the cell physiology and genetics of coral symbiotic dinoflagellates has been hindered by challenges associated with cloning these microalgae.

Results: A procedure for cloning coral symbiotic dinoflagellates was developed in this study. Several species of coral symbionts were successfully cloned, with two of them further characterized. Experiments with the two species isolated from Turbinaria sp. showed that damage from light intensity at 340 μmol photons/m²/s was more severe than from high temperature at 36 °C. Additionally, preincubation in high salinity conditions activated their endogenous tolerance to bleaching stress. Pretreatment at 50 ppt salinity reduced the percentage of cells stained for ROS by 59% and 64% in the two species under bleaching stress compared to those incubated at 30 ppt. Furthermore, their Fv'/Fm' during the recovery period showed a significant improvement compared to the controls.

Conclusions: These findings suggest that intense light plays a more important role than high temperatures in coral bleaching by enhancing ROS generation in the symbiotic dinoflagellates. The findings also suggest the genomes of coral symbiotic dinoflagellates have undergone evolutionary processes to develop mechanisms, regulated by gene expression, to mitigate damages caused by high temperature and high light stress. Understanding this gene expression regulation could contribute to strengthening corals' resilience against the impact of global climate change.