Distribution Of Polycyclic Aromatic Hydrocarbons In Coral Reefs And Response Of Symbiotic Symbiodiniaceae To Benzo[a]pyrene Stress

The coral reef is one of the marine ecosystems with the most abundant biodiversity and productivity in the world,and it has been attracted to the attention of the whole world.The Scleractinian corals are the fundamental coral reef builders,which have a symbiotic relationship with endosymbiotic dinoflagellates of Symbiodiniaceae.This point has far-reaching significance for the sustainable development of the coral reef ecosystem.In recent decades,coral bleaching frequently and extensively occurs under the effect of global climate change and human activities.It results in increased degradation of the reef and the decrease of marine diversity,even a serious influence on the marine ecosystem's health.Polycyclic aromatic hydrocarbons(PAHs)are a universal presence in the marine environment,which distribution in environmental medium and affecting marine organisms are considered as the focus.Hainan has the most abundant coral diversity with the most extensive coral distribution in China,whereas it is threatened by the increasing degradation of coral reefs.Therefore,this study analyzed the distribution characteristics and sources for PAHs in coral reefs of Hainan,and clarified the response mechanism and adaptation strategies of corals endosymbiotic dinoflagellates of Symbiodiniaceae under PAHs stress.It provided the important reference basis for the coral response to environmental stresses and ecological response of endosymbiosis regulation researches,meanwhile,it provides important theoretical and data support for the protection and restoration of the coral reef ecosystem.The main research results are as follows.(1)This study clarified the PAHs composition and sources in reefs and compared the interspecific differences for the PAHs enrichment ability of corals through the analysis of PAHs distribution characteristics in the environmental medium and corals in coral reefs at Hainan.Gas chromatography/mass spectrometry(GC-MS)was used to detect the total polycyclic aromatic hydrocarbons(?PAHs)among environmental medium(seawater and sediments)and fourteen coral species in the distribution areas of coral reefs at Hainan.The average concentration of?PAHs in the corals was significantly higher than that in the seawater and sediments.Compared with other regions at home and abroad,the overall PAHs pollution level at Hainan reefs was low to medium level.The PAHs concentration in the coastal coral reefs was higher than that in the Xisha reefs.Except for Yinyu Island and Jinqing Island,low molecular weight polyaromatic hydrocarbons(LPAHs)were dominated in these coral reefs with petroleum source and pyrolysis sources.In addition,the PAHs levels were different among various coral species,as well as the bio enrichment ability for PAHs.Scleractinian corals in Hainan reefs have higher bio enrichment efficiency for the high molecular weight polyaromatic hydrocarbons(HPAHs)in the environmental medium.Porites lutea,Galaxea fascicularis,and Pavona varians had stronger bio enrichment of PAHs in seawater,and G.fascicularis and Porites rus had strong bio enrichment of PAHs in sediments.(2)According to the analysis of the correlation between the Symbiodiniaceae community constitution and environmental factors in different coral reefs at Sanya,the distribution characteristics and interspecific differences of Symbiodiniaceae community structure and diversity were revealed,as well as the main environmental factors affecting the Symbiodiniaceae community of corals.Using high throughput sequencing technology to sequenced gene of symbiotic Symbiodiniaceae among various dominant coral species in March and July at Sanya,the results showed that spatiotemporal and interspecific differences were present in the Symbiodiniaceae diversity.The diversity was higher in July than it in March,and the highest was in Wuzhizhou Island and Luhuitou Island,the higher were found in Acropora digitifera and Acropora hyacinthus.Eight clades and 158 subclades of Symbidiniaceae were detected in all collected coral samples.Symbiodiniaceae in Scleractinian corals at Sanya reefs were dominated by Clade C/D and subclade C15/D1,its community structure was found the spatiotemporal and host inter-species differences.During March and July,the dominant population in Favites flexuosa and G.fascicularis was replaced,whereas the replacement not occurred in Acopora digitifera and Acopora hyacinthus.Environmental factors including seawater temperature,salinity,p H,NO₂-^-N and NO₃^--N were mainly affecting the Symbiodiniaceae community structure and distribution in Sanya reefs.There were interspecific differences for the correlation between Symbiodiniaceae and environmental factors,and most of the subclades of clade C(such as C1,C1231,etc.)were significantly negatively correlated with nutrients and PAHs in the environmental medium,indicating that Symbiodiniaceae clade C was sensitive to environmental changes and PAHs pollution.(3)According to the analysis for physiology and the community constitution variation in corals symbiotic Symbiodiniaceae under Benzo[a]pyrene(Ba P)stress,to explore the toxicity effect of Ba P and Symbiodiniaceae population adaptation.Through the determination of Symbiodiniaceae chlorophyll content and cell density indexes under Ba P stress,the occurrence of an obvious decrease in two indexes showed the time-effect relationship under Ba P stress.Additionally,the real-time quantitative technique and high-throughput sequencing technology were applied to the determination and the relative abundance analysis of Symbiodiniaceae under Ba P stress.The Symbiodiniaceae relative abundance was discovered that clade D relative abundance increased with the time-effect relationship,as well as subclades D1?D1a and A1.2,whereas clade C relative abundance decreased with the time-effect relationship,as well as subclades C1 and C232,indicating that these species of Symbiodiniaceae could be sensitive for Ba P stress.(4)This study preliminarily discussed the key functional genres of Symbiodiniaceae transcriptome response to the Ba P stress base on the screening of differential gene and analysis of the differential gene function and participated in the biological process.Through applying the high-throughput transcriptome sequencing technology to sequence the transcriptome of coral symbiotic Symbiodiniaceae under Ba P stress,and conducting the bioinformatics analysis,it was found that the nucleic acid metabolism,photosynthesis,oxidative phosphorylation and a series of biological processes in the symbiotic Symbiodiniaceae were affected under Ba P stress.The key genes of nucleic acid metabolism under Ba P stress had significant changes,the DNA replication and RNA transcription were affected,even may cause DNA damage,which affected the nucleic acid metabolism.In order to maintain normal metabolism of nucleic acid process,symbiotic Symbiodiniaceae may be formed the protection mechanism through the highly expression of related enzymes.Ba P stress reduced the expression of photosynthesis-related genes in symbiotic Symbiodiniaceae,especially in the light reaction.Moreover,Ba P stress led to decreased gene expression of oxidative phosphorylation related enzymes,and inhibited the production of ATP,weaken oxidative phosphorylation function,and affected the energy metabolism of Symbiodiniaceae,thus resulting in the dysfunction of biological functions.These were a disadvantage to maintain the symbiotic relationship between corals and Symbiodiniaceae.