Investigation Of Wasting Disease And The Diversity And Spatiotemporal Distribution Of Potential Pathogenic Bacteria Labyrinthula In Seagrass Bed In Hainan

In recent decades,due to the impact of global climate change and human activities,the global seagrass bed ecosystem has shown an obvious trend of degradation.Seagrass disease problems caused by the above factors are also contributing to seagrass bed degradation,such as wasting diseases caused by Labyrinthula protists,i.e.,periodic severe death events in seagrass meadows.However,to date,wasting disease infection in seagrass beds in China has not been reported.In this study,Thalassia Hemprichii,the dominant population in Hainan,was selected as the research object,and the severity of wasting disease in 6 Thalassia Hemprichii seagrass beds of the 3 coastal sea areas affected by eutrophication in Wenchang City was investigated.At the same time,seven environmental factors related to seagrass growth were selected to study the relationship between infection degree and main environmental pollution factors.In addition,the diversity characteristics of Labyrinthula,the pathogen of wasting disease,were also analyzed.According to the results of disease investigation,wasting disease was found in all the three regions,and the wasting index was significantly different in the regional mean and two independent months(F=7.680,P<0.05;F=3.837,P<0.05;F=4.651,P<0.05),and the infection degree was as follows:Changpi Port>Gangdong Village>Dongjiao Coconut Forest.The detection results of environmental factors in different regions/different periods showed that at least 4 of the 7 environmental factors were different,and the main factors were p H,chlorophyll a,inorganic phosphorus and inorganic nitrogen.The principal component analysis showed that the inorganic phosphorus,inorganic nitrogen and chlorophyll a related to eutrophication all had a great influence on the consumption index in August or December.Pearson correlation analysis for two independent months was as follows:in August,salinity and inorganic nitrogen were significantly correlated with the wasting index(R~2=0.309,P<0.05;R~2=0.284,P<0.05);There was a significant correlation between temperature and wasting index in December(R~2=0.252,P<0.05).In this study,Labyrinthula was successfully isolated from tropical Thalassia Hemprichii disinfected leaves for the first time,and spindle cells of Labyrinthula and two common cell colonies were observed by inverted microscope:dense and branching colonies.A total of 82 valid Labyrinthula sequences were obtained from 6 loci,and the similarity of Labyrinthula sequences for each loci was clustered between 65%and80%.Phylogenetic tree showed that 36 Labyrinthula sequences were closely related to Labyrinthula zosterae with pathogenicity confirmed,which indicated that the corresponding Labyrinthula sequences might be pathogenicity.The remaining 46 sequences were not in the same cladus with the known Labyrinthula sequences in the NCBI database and were far from each other,indicating that there might be new species or strains of Labyrinthula in the study area.High-throughput sequencing results showed that there were 79 OTU of Labyrinthula in18 leaf samples,including 5 Labyrinthula species,including 5 Labyrinthula species in Changpi Port,2 Labyrinthula species in Gangdong Village,and only 1 Labyrinthula species in Dongjiao Coconuts Forest.Labyrinthula exists in both healthy and disinfected leaves,and the number of Labyrinthula in disinfected leaves is far greater than that in healthy leaves,but wasting disease only appears in disinfected leaves,indicating that Labyrinthula may be a conditional pathogen.By comparing the species composition of the disease samples from three regions,we found that the relative abundance of Labyrinthula from large to small was Changpi Port(2.28%),Gangdong Village(2.11%)and Dongjiao Coconut Forest(0.32%).Correlation analysis of environmental factors and samples showed that inorganic nitrogen had a significant effect on the distribution of species(R~2=0.3519,P<0.05).To sum up,the diversity of Labyrinthula in the three regions is as follows:Changpi Port>Gangdong Village>Dongjiao Coconut Forest.This study showed that wasting diseases were seriously infected in seagrass beds in Hainan Province,and environmental factors related to eutrophication had a significant effect on the occurrence of wasting diseases.In addition,the diversity of Labyrinthula in Hainan seagrass beds is high,and there may be some new species adapted to the tropical environment or different tropical host plants.Therefore,long-term effective monitoring of seagrass beds is needed.