Physiological Response Of Endosymbiotic Dinoflagellates Cladocopium Goreaui To Eutrophication And Microplastic Stress

The symbiotic relationship between Symbiodiniaceae and scleractinian coral is the basis of coral reef ecosystems around the world.Nowadays,coral reef ecosystems have been threatening by eutrophication and microplastics due to increasing human marine activities.How symbiotic Symbiodiniaceae respond to these environmental changes is the key to the balance and health of coral reef ecosystems,and has become one of the hot spots in the field of coral research.The present study used endosymbiotic Symbiodiniaceae Cladocopium goreaui as the research object to explore its physiological response mechanisms to eutrophication and microplastic pollution.Nitrogen nutrition was restored to normal level in the medium after nitrogen-restricted culture to simulate eutrophication.After the recovery of nitrogen nutrition,the density and chlorophyll a content of C.goreaui increased significantly at 84 and 72 h respectively.Its cell size,nitric oxide synthase activity and caspase3 activation level decreased significantly at 72 h.No significant changes in Fv/Fm and superoxide dismutase and catalase activities were detected during the whole nitrogen recovery period.In addition,the carbon content and the carbon/nitrogen ratio in algal cells decreased significantly at 84 h after the recovery of nitrogen nutrition.After the exposure of polystyrene microplastic(Diameter 1.0?m,9.0×10⁹particles L^-1),the density and cell size of C.goreaui decreased significantly at 7 d and 6-7 d,respectively.Chlorophyll a content increased significantly at 7 d of exposure,Fv/Fm did not change significantly during the entire exposure period.We observed significant increases in SOD activity and Caspase3 activation level,significant decrease in glutathione S-transferase activity.Transcriptomic analysis revealed 191 significantly upregulated and71 significantly downregulated genes at 7 d after microplastic exposure.Fifteen GO terms were overrepresented for these significantly upregulated genes,which were grouped into four categories including transmembrane ion transport,substrate-specific transmembrane transporter activity,calcium ion binding,and calcium-dependent cysteine-type endopeptidase activity.It was also found that thirteen of the significantly upregulated genes encode metal ion transporter,and five light-harvesting protein genes were among the significantly downregulated genes.The research results revealed the physiological regulation mechanism of eutrophication and microplastic exposure on endosymbiotic Symbiodiniaceae C.goreaui.Eutrophication can promote the growth of Symbiodiniaceae by increasing the content of chlorophyll and inhibiting apoptosis,while microplastics can suppress detoxification activity,nutrient uptake,and photosynthesis,elevate oxidative stress,and raise the apoptosis level through upregulating ion transport and apoptotic enzymes of Symbiodiniaceae to repress its growth.This research is helpful to understand the physiological changes of symbiotic Symbiodiniaceae under environmental stress,lay a foundation for the research of scleractinian corals,and provide a theoretical basis for the conservation of coral reef ecosystem.