Studies On Cyanobacterial Community And Sheath Polysaccharides Metabolism And Their Gene Expression Patterns In Lake Taihu

Most lakes in China are in various degrees of eutrophication and the number of eutrophic lake is increasing.As a result,the high nutrient concentration coupled with climate change eventually led to rapid growth of algae and a frequent large-scale outbreak of cyanobacterial blooms.Cyanobacterial blooms experience a process during which the algal biomass in the water body gradually increases.In general,it undergoes processes of dormancy in winter,recruitment,growth and colony formation in spring,and then float to the water surface forming blooms in sunmer.During these processes,the formation and maintenance of cyanobacterial colonies which are mainly composed of capsule polysaccharides is essential for the formation of cyanobacterial blooms.In recent years,with the advent of the era of post-genomics,people have made rapid progress in the research of algaes and other microorganisms realms.Through metatranscriptomic studies,it is possible to clarify the response of microorganisms to environmental changes.In this study,finding suitable method for extracting total RNA from cyanobacterial colonies,metatranscriptome method was used to explore the mechanism of metabolism changes of cyanobacterial blooms at the transcriptional level in Meiliang Bay in Lake Taihu.Moreover,through the simulation experiment in the laboratory,differences in gene expression of the cyanobacterial bloom species Microcystis and Anabaena were explored.changes in gene patterns of cyanobacteria in the process of blooms were elucidated,and the expression of genes related to the metabolism of the capsule polysaccharides in the cyanobacterial colonies was also revealed.These studies provided clues to the developmental mechanisms of cyanobacterial blooms and the capsule polysaccharides from transcriptional point of vlew.The contents and results of this study are as follows:1.Changes in community structure and exopolysaccharide of cyanobacterial blooms in Meiliang Bay.16S high-throughput sequencing was used to study the community structure of cyanobacterial blooms(>120?m),the results showed that Anabaena was dominant from January to April,and accounting for 41.5%to 85.8%of all cyanobacteria.Microcystis was dominant from May to December,and accounting for 62.1%to 99.2%of all cyanobacteria.RDA results indicated that temperature and nitrate were the main driving environmental factors on the cyanobacterial communtiy structure.The content of capsule polysaccharides in the cyanobacterial colonies was significantly negatively correlated with water temperature and correlated with the community structure,there were significantly negatively correlated with the relative abundance of Microcystis and significantly positively correlationed with the relative abundance of Anabaena.The capsule polysaccharides of cyanobacterial colonies were mainly composed of seven monosaccharides,among which glucose and galactose were the highest.Eem(Excitation-emission Matrix)revealed that organic components in polysaccharides included three kinds such as protein-like,humic acid-like,and soluble microbial metabolite-like matters,the composition of which was relatively stable.2.Comparative study of RNA extraction methods from cyanobacterial colonies.Four methods were selected for RNA extraction from polysaccharides-rich samples,the FastRNA(?)Pro Blue Kit and Qiagen RNeasy Mini Kit kits,and the PGTX-bead and CTAB-bead methods.Through analysis of RNA extraction yield,purity,and gene expression levels of research,the results showed that Qiagen RNeasy Mini Kit and PGTX-bead methods were suitable methods for RNA extraction from Microcystis colonies.This study will use Qiagen RNeasy Mini Kit method to extract RNA.3.Analysis of gene expression patterns of cyanobacterial colonies in Taihu Lake.The gene expression patterns of cyanobacterial colonies(>120 ?m)was analyzed using the metatranscriptome method.The analysis result based on multi-dimensional scaling method divides cyanobacterial blooms monthly samples into three groups including,July and September(high-bloom period),February and March(non-bloom period),and the rest months(low-bloom period).Comparative analysis showed that the number of significantly overrepresented genes of cyanobacterial colonies were higher in the non?bloom period than in the low-bloom period,and the number of significantly overrepresented genes of cyanobacterial colonies were higher in the low-blooru period than in the high-bloom period.Most genes of cyanobacterial colonies were assigned to clustering-based subsystems,protein metabolism,carbohydrates and photosynthesis.RDA results indicated that temperature is the main environmental factor affecting the expression of cyanobacterial colonies.The expression of capsule polysaccharides-related genes were mainly assigned to the enzymes involved in exopolysaccharides biosynthesis,sialic acid metabolism and rhamnose metabolism.4.Gene expression patterns of two dominant species of cyanobacteria along with temperature fluctuation were investigated through simulated experiment in labratory.The results showed that the release amounts of capsule polysaccharides per cell of the two algaes increased under the low temperature conditions.The release amount of capsule polysaccharides from Anabaena colonies was much higher than that of the colonies of M.aeruginosa.Most transcriptome.genes of the two algal colonies were assigned to clustering-based subsystems,protein metabolism,carbohydrates,and photosynthesis.The expression of four genes in both algal colonies were significantly affected by temperature,but they were not completely the same.Among them,genes related to photosynthesis were the most affected by temperature,the lower the temperature and the lower the nulber of gene relative expression.The expression of caPsule polysaccharides-related genes in the two algae colonies were mainly assigned to the enzymes involved in exopolysaccharides biosyntihesis,sialic acid metabolism,and rhamnose metabolism.Temperature mainly affected the total expression of capsule polysaccharides-related genes,with the decrease of temperature showed that the total gene expression was up-regulated,but the relative expression of each capsule polysaccharides gene did not change significantly with temperature.