2:45 PM - 3:00 PM
[ACG50-04] Modeling coral bleaching based on dynamics of zooxanthellae population and reactive oxygen species inside a coral polyp
Keywords:coral bleaching, numerical simulation, zooxanthellae, reactive oxygen species
Coral bleaching is a phenomenon in which corals expel/digest a large amount of their symbiotic algae (zooxanthellae), and it is caused by some stresses, e.g., thermal stress. In the summer of 2016, mass bleaching induced by higher seawater temperature and resultant mass mortality had catastrophically damaged coral communities on many coral reefs all over the world. Such mass bleaching events will likely occur more frequently in near future due to global warming. But the reason and mechanism of the bleaching are still unclear. Therefore, for projecting near future status of coral communities precisely, it is important to elucidate the bleaching mechanism and to develop a numerical simulation model.
It is observed that corals expel zooxanthellae even under normal thermal conditions (e.g. Hoegh-Guldberg et al., 1987). The number of zooxanthella cells increases due to reproduction, but the zooxanthellae density in the coral tissue is kept around the order of 106 cells cm-2 under normal thermal conditions. Therefore, it is considered that the zooxanthella density of ~106 cells cm-2 is optimal and coral is controlling the density to be an optimal value by expelling zooxanthellae. Now, how is the coral determining the optimal value of zooxanthella density? Zooxanthellae produce photosynthate which is an important energy source for corals, but these also produce reactive oxygen species (ROSs), which damage coral cells, through their photosynthesis (e.g. Weis 2008). It is considered that corals basically want to keep the density of zooxanthellae as high as possible for improving photosynthate availability. But when the zooxanthella density increases, the concentrations of harmful ROSs also increase in the coral cells because of zooxanthellae ROS production. Therefore, coral may control zooxanthella density for keeping ROS concentration within tolerable levels by expelling/digesting zooxanthellae. Additionally, it is reported that the production rate of ROS increases with increasing light intensity and temperature (e.g. Saragosti et al. 2010; McGinty et al., 2012). When temperature increases, ROS release rate per individual zooxanthella cell also increases, then the ROS concentration increases. Thus, corals have to decrease zooxanthellae density for keeping the ROS concentration at tolerable levels. This is our hypothesis for the coral bleaching mechanism. In this sense, the bleaching action might be an emergency measure of corals.
Based on this hypothesis, coral bleaching model was developed based on the coral polyp model (Nakamura et al., 2013) by incorporating both ROS dynamics and zooxanthella population dynamics. The ROS dynamics includes light and temperature dependent ROS release process and detoxification of ROS by antioxidant substances, and the zooxanthella population dynamics includes processes of reproduction, mortality, and expelling/digesting rates that depend on the ROS concentration in the coral cell. These dynamic processes are linked with coral internal environments reproduced by the coral polyp model.
Results of simulated 30 day incubation experiments under different temperature conditions by the bleaching model well reproduced coral bleaching phenomenon dependent on temperature. Moreover, it is notable that the simulation result under a higher incubation temperature for first 5 days followed by incubation under normal temperature for 25 days well reproduced recovery process following bleaching process. It is one of very unique features of this model.
Moreover, the bleaching model was coupled with a hydrodynamic-biogeochemical model based on the Regional Ocean Modeling System (ROMS; Shchepetkin and McWilliams 2005), and the coupled model system was applied to the Shiraho coral reef, Ishigaki Island, Japan. From these results, it was confirmed that the zooxanthella density decreases with increasing offshore temperature, and clear spatial variation was confirmed that coincided with spatial variation of water temperature inside the reef.
It is observed that corals expel zooxanthellae even under normal thermal conditions (e.g. Hoegh-Guldberg et al., 1987). The number of zooxanthella cells increases due to reproduction, but the zooxanthellae density in the coral tissue is kept around the order of 106 cells cm-2 under normal thermal conditions. Therefore, it is considered that the zooxanthella density of ~106 cells cm-2 is optimal and coral is controlling the density to be an optimal value by expelling zooxanthellae. Now, how is the coral determining the optimal value of zooxanthella density? Zooxanthellae produce photosynthate which is an important energy source for corals, but these also produce reactive oxygen species (ROSs), which damage coral cells, through their photosynthesis (e.g. Weis 2008). It is considered that corals basically want to keep the density of zooxanthellae as high as possible for improving photosynthate availability. But when the zooxanthella density increases, the concentrations of harmful ROSs also increase in the coral cells because of zooxanthellae ROS production. Therefore, coral may control zooxanthella density for keeping ROS concentration within tolerable levels by expelling/digesting zooxanthellae. Additionally, it is reported that the production rate of ROS increases with increasing light intensity and temperature (e.g. Saragosti et al. 2010; McGinty et al., 2012). When temperature increases, ROS release rate per individual zooxanthella cell also increases, then the ROS concentration increases. Thus, corals have to decrease zooxanthellae density for keeping the ROS concentration at tolerable levels. This is our hypothesis for the coral bleaching mechanism. In this sense, the bleaching action might be an emergency measure of corals.
Based on this hypothesis, coral bleaching model was developed based on the coral polyp model (Nakamura et al., 2013) by incorporating both ROS dynamics and zooxanthella population dynamics. The ROS dynamics includes light and temperature dependent ROS release process and detoxification of ROS by antioxidant substances, and the zooxanthella population dynamics includes processes of reproduction, mortality, and expelling/digesting rates that depend on the ROS concentration in the coral cell. These dynamic processes are linked with coral internal environments reproduced by the coral polyp model.
Results of simulated 30 day incubation experiments under different temperature conditions by the bleaching model well reproduced coral bleaching phenomenon dependent on temperature. Moreover, it is notable that the simulation result under a higher incubation temperature for first 5 days followed by incubation under normal temperature for 25 days well reproduced recovery process following bleaching process. It is one of very unique features of this model.
Moreover, the bleaching model was coupled with a hydrodynamic-biogeochemical model based on the Regional Ocean Modeling System (ROMS; Shchepetkin and McWilliams 2005), and the coupled model system was applied to the Shiraho coral reef, Ishigaki Island, Japan. From these results, it was confirmed that the zooxanthella density decreases with increasing offshore temperature, and clear spatial variation was confirmed that coincided with spatial variation of water temperature inside the reef.