Seasonal Changes In Energy Supply Modes Of Scleractinian Corals And Their Ecological Significance

Coral reef ecosystems are currently one of the most biodiverse ecosystems on Earth,but in the context of global warming,coral reefs are under constant threat of thermal bleaching.From the point of view of energy supply,there is a large loss of symbiotic zooxanthellae in the bleached corals,which leads to a reduction in the energy provided by corals through the transfer of symbiotic zooxanthellae photosynthesis.However,it is unclear how corals maintain a stable energy supply in the face of reduced zooxanthellae.In this study,89 coral samples were collected from the Xisha Islands in the South China Sea in the spring（March）and summer（June）of 2020,including four genera:Favia（N=27）and Porites（N=18）,which are relatively tolerant to the environment;Acropora（N=21）and Pocillopora（N=23）,which are relatively fragile.The physiological parameters（zooxanthellae density,chlorophyll a content,tissue biomass,lipid content and lipid proportion）were measured,combined with stable carbon isotope indexes which can indicate coral autotrophic ability（photosynthesis）and heterotrophic ability（feeding）.The seasonal variation of energy supply model of reef corals in the South China Sea and its ecological significance were discussed.The main conclusions were as follows:（1）The physiological parameters of corals have seasonal changes,and their photoautotrophic capacity will respond to the seasonal changes of physiological parameters,which will lead to changes in the energy supply model of corals.Symbiotic zooxanthellae density（ZD）and photosynthetic intensity(δ¹³C_z values)of corals in all four genera decreased in summer.In this case,corals can maintain daily metabolic energy（DME）requirements by switching from autotrophic to heterotrophic nutrition.（2）Lipid reserves maybe an important component of the coral energy supply system in addition to the energy provided by autotrophy and heterotrophy.When autotrophy decreases in summer due to high temperatures,corals need to adjust their nutritional patterns to maintain energy supply,i.e.heterotrophic feeding intensity increases.In addition,corals can also provide energy by consuming lipid reserves to maintain a stable energy supply.In this case,the corals with relatively strong heterotrophic capacity（the massive coral Favia and Porites）were less dependent on lipids（lipids decreased by only 2.13%and 2.46%,respectively,in summer）;the corals with relatively weak heterotrophic capacity（the branching coral Acropora and Pocillopora）were more dependent on consuming their own lipid reserves to maintain the stability of their energy supply（lipids decreased by19.90%and 17.19%,respectively,in summer）.Massive coral Favia and Porites have higher energy supply stability.（3）Corals can maintain the stability of their energy supply by flexibly adjusting their nutritional patterns and depleting lipid reserves,which may be one of the most important factors for improving the environmental stress tolerance and bleaching resistance of corals.