Characterization And Identification For Key Morphological Features Of Causative Species Of The Yellow Sea Green Tide

Since 2007,the green tide of Ulva prolifera in the Yellow Sea has become a normalized marine disaster.Many studies have shown that green algae attached on Pyropia yezoensis aquaculture rafts in Subei shola provide an important initial biomass for the green tide.The molecular identification results show that the green algae attached on the rafts are composed of several species,which have species succession over time,and the final proportion of U.prolifera may be related to the scale of the green tide in that year.Therefore,timely and accurate monitoring of the composition characteristics of raft-associated green algae has important sense to prediction and early warning of green tide.Because of the large area of rafting area,the spatial diversity of species composition,especially the short time in the early stage of green tide,it is essential to establish a species identification method based on morphological characteristics and suitable for a large number of samples on site.In addition,the Yellow Sea green tide is dominated by the floating ecotype of U.prolifera,and it is generally believed that it has a highly branched phenotype,which may give it significant interspecific competitive advantage.However,since insufficiency of indexes to the characteristic of branch phenotypic,whether branching phenotype and its plasticity are a significant difference between attached U.prolifera and floating ecotype is still unknown.In view of the above problems,several stations were set up in P.yezoensis aquaculture rafts in Subei shola to collect samples with multiple voyages in this study.Based on the results of molecular identification,key morphological characteristics of common dominant raft-associated species were observed,compared and statistically analyzed.According to the branching phenotype of U.prolifera,the differences of branching degree and plasticity between attached U.prolifera and floating ecotype were compared by widely collecting samples and using new quantitative indexes.The following important results are obtained:(1)The molecular identification of 529 samples of raft-associated green algae in Subei shola was carried out,and five dominant species were detected: U.linza,U.prolifera,U.arago?nsis,U.compressa and Blidingia sp.,which were consistent with previous studies.Importantly,U.arago?nsis was firstly identified as a new record in China,which has been mis-identified as U.flexuosa for long time.(2)Through morphological observation,measurement and comparative analysis,a key based on morphological characteristics to above-mentioned five species identification is proposed: first,these five species were divided into high-branching(HB)group and low-branching(LB)group by the top branching order.In LB group,Blidingia sp.was distinguished by length-to-width ratio(LWR),U.linza and U.arago?nsis were distinguished by the number of pyrenoid.In HB group,U.prolifera and U.compressa were identified by cell arrangement and the shape of junction between primary branch and main axis.In order to maximize convenience of on-site application,a practical accuracy test was designed to evaluate the application of the top branching order plus LWR of the algae to distinguish the green seaweeds.The results showed that average accuracy was 81.3%,and identification accuracy of U.prolifera was 89.1%,which could meet the requirements of field application.(3)Two quantitative indicators,i.e.the top branching order and the intensity of primary branches,were used to characterize the branching phenotypes of U.prolifera.The comparative analysis results showed that attached U.prolifera generally has primary branch or no branching,while the top branching order might reach five in the floating ecotype.The intensity of primary branches in the floating ecotype was 5.8times higher than that of attached populations,indicating a significant difference in the branching phenotypes between floating ecotypes and attached U.prolifera.In addition,the plasticity of branching phenotypes was also proved much stronger in the floating ecotype than attached populations under both natural and artificial cultivation conditions.These results suggest that,the highly branched phenotype is an important adaptive feature of the floating ecotypes,and the strong plasticity of the branched phenotype may help them adapt to the changeable marine environment.The above results will provide an important methodological basis for the early monitoring and scale prediction of green tide in the Yellow Sea,and provide scientific basis and necessary morphological evaluation indexes for further study of branching mechanism in U.prolifera.