Intergeneric And Spatial Differences In The Trophic Status Of Scleractinian Corals In The South China Sea And Their Ecological Significance

Under the background of climate warming and human activities,the frequency and intensity of global coral bleaching events are increasing.However,it remains unclear about the mechanisms that affect the resistance of corals to environmental stress and susceptibility to bleaching.The majority of scleractinian corals are“mixotrophic”organisms.They can obtain energy through photosynthesis of symbiotic zooxanthellae and nutrients ingested by the host.Exploring the trophic flexibility(i.e.,autotrophy/heterotrophy)of corals is helpful to understand the importance of the stability of energy supply on coral environmental adaptability and bleaching susceptibility.In this study,the intergeneric and spatial differences of corals trophic status were explored by using?¹³C of host tissues(?¹³C_h)and zooxanthellae(?¹³C_z),as well as the difference between?¹³C_hand?¹³C_z(?^{h-z 13}C),combining with zooxanthellae density,environmental parameters and ecological status.Furthermore,the potential effects of trophic flexibility on the environmental adaptability and susceptibility to bleaching of corals in the South China Sea were explored.Specifically,the conclusions are as follows:(1)The trophic status of different intergeneric corals in the Weizhou Island showed significant differences,which is closely related to zooxanthellae density.Both zooxanthellae density and?¹³C_zvalues of Porites,Favia and Favites are higher than those of Pavona and Acropora,indicating that autotrophic photosynthesis intensity of symbiotic zooxanthellae of Porites,Favia and Favites is higher than that of Pavona and Acropora.Besides,both?¹³C_h and?^{h-z 13}C values of Porites,Favia and Favites are lower than those of Pavona and Acropora,which indicates a higher contribution of heterotrophic predation to the coral host of Porites,Favia and Favites.Therefore,the relatively stress-tolerant Porites,Favia and Favites have higher trophic flexibility than vulnerable Pavona and Acropora.This is one of the significant factors that lead to the different bleaching susceptibility and environmental stress adaptability among corals of different species.(2)The zooxanthellae density of corals in different latitude reef areas(Sanya,Xisha and Nansha Islands)showed significant spatial differences,which is closely related to different environmental conditions.The higher nutrition level of Sanya is beneficial to the reproduction and growth of the zooxanthellae,while the higher Sea surface temperature(SST)and light intensity in the Xisha and Nansha Islands restrict the reproduction and development of zooxanthellae.The corals from Sanya have higher zooxanthellae density than those from the Xisha and Nansha Islands under the combined of environmental factors such as nutrient level,SST and light intensity.(3)The trophic status of corals in different latitude reef areas(Sanya,Xisha and Nansha Islands)showed significant spatial differences.Both zooxanthellae density and?¹³C_z values in the samples from Sanya are higher than those from the Xisha and Nansha Islands,indicating that corals from Sanya have higher photosynthetic rate and stronger autotrophic abilities.On the contrary,the relatively low?¹³C_h and?^{h-z 13}C values in the samples from the Xisha and Nansha Islands suggest that these corals have a higher heterotrophic ability than those from Sanya.Therefore,Scleractinian corals can improve their adaptability and tolerance to environmental stress by adjusting the relative intensity of autotrophic/heterotrophic under different environmental conditions.