Community Structures And Functions Of Bacteria Associated With Blooming Seaweeds In The Yellow Sea

Golden tides and green tides are marine ecological phenomena mainly caused by fulminant proliferation of brown seaweeds Sargassum or green seaweeds Ulva respectively,which have shown a significant uptrend worldwide in recent years,especially in coastal China.Since 2007,green tide has occurred in the Yellow Sea for successive years;at the end of 2016,large scale golden tide firstly broke out in the Yellow Sea;in 2017,the green tide and the golden tide even co-occurred in this sea area.Genetic analysis showed that,either the golden tide or the green tide in the Yellow Sea were dominated by a specific genotype of single species,clearly suggesting that the outbreaks of these two seaweeds have biological causes.Algae-associated bacteria were extensively interacted with their host,and were closely involved in those important biological processes such as growth and morphogenesis of the host.Revealing the structure and function of the bacterial communities associated with the fulminating seaweed,which would provide new cognition for elucidation of the “biological” mechanism of seaweed blooms.In this paper,the samples of floating Sargassum horneri were collected at different sites in 2017,and the samples of floating Ulva prolifera were collected in different years and sites.Both the epiphytic and endophytic bacteria from two kinds of floating seaweeds were prepared by silica sand vortex oscillating method.In order to effectively avoid the interference generated from the 16 S rDNA encoded in genomes of algal organelles,several pairs of primers were analyzed.Based on Illumina Miseq highthroughput sequencing,the community structure characteristics of bacteria associated with two blooming seaweeds were analyzed,and the PICRUSt was used to predict flora function.The results were as following:1)Community structures and functions of the bacteria associated with the floating S.horneri.Molecular identification results showed that the golden tide seaweed in the Yellow Sea was consisted by a single species of S.horneri.In 2017,both the brown thalli in the relatively high-nutrition sea area and the yellow thalli in the relatively low-nutrition sea area were collected,respectively.The chlorophyll contents of the former were significantly higher than that of latter,suggesting that the yellow thalli were under nutritional stress.Analysis of the structure and functions of bacterial communities showed that,compared with the environmental background,bacterial communities of S.horneri were differed from that of seawater regardless of host status,indicating the floating S.horneri in two status all had obvious selectivity to their associated bacteria,but the flora compositions were extremely significant differed between brown and yellow thalli.The bacterial communities of brown S.horneri exhibited clearly tissue specific distribution tendency,which in yellow thalli was unconspicuous,indicating that host status affected the assemblage of associated bacteria.Studies found that the dominant functions of bacterial communities in brown and yellow thalli both were nitrogen and iron transport,suggesting that the community assembly of bacteria associated with floating S.horneri was based on function rather than taxonomy.Moreover,this study firstly both revealed the composition of endophytic and epiphytic bacteria in the floating S.horneri.The composition of dominant endophytic bacteria was relatively stable both come from Bacillus and Propionibacterium regardless of host status,whereas the dominant epiphytic bacteria varied obviously.The epiphytic bacteria of yellow thalli were mainly from Flavobacterium and Paracoccus,and that of brown thalli were mainly from Kocuria,Bacteroides and Pseudomonas.The results of PICRUSt functional prediction found that the dominant functional proteins were mainly derived from epiphytic bacteria.Compared with the brown thalli,the epiphytic bacteria of the yellow thalli contained much higher abundance of nitrogen and iron transporters.Therefore,we speculated the yellow thalli in a worse physiological state under the influence of environmental conditions,was the key factor to ultimately determine the assembly of the associated bacterial communities through affect the intensity of functional requirements.2)Community structures and functions of the bacteria associated with the floating U.proliferaThe drifting Ulva prolifera were collected in 2007,2016,2017,2018,and from relatively high-nutrition or relatively low-nutrition sea areas.The results of flora structure and function analysis showed that,compared with the environmental background,the floating U.prolifera samples also have obvious selectivity to its associated bacteria,but the composition of the bacterial community was significantly varied with interannual and sampling sites.Studies found that the dominant functions of the bacterial community were consistent,i.e.nitrogen transport,suggesting that the assemblage of bacterial communities associated with floating U.prolifera was also based on functions rather than taxonomy.Moreover,this study revealed the composition characteristics of endophytic and epiphytic bacteria of floating U.prolifera for the first time,and found a single OTU from Cupriavidus was stable in floating U.prolifera over the years.The result of PICRUSt functional prediction showed that the nitrogen transport function was mainly from the endophytes of floating U.prolifera.3)Comparative analysis of the bacterial communities associated with the floating S.horneri and floating U.prolifera.Compared with the bacterial composition of environmental samples,floating S.horneri and floating U.prolifera both has obvious selectivity for its-associated bacteria,under the influence of environmental conditions and physiological status of host,the composition of associated bacteria were changed obviously,but the dominant functions remain consistent,indicating that the associated bacterial community assemblage of two kinds of blooming seaweeds were based on function rather than taxonomy.Nitrogen transport was predicted as the dominant function in floating S.horneri and floating U.prolifera,suggesting that the associated bacteria participated to host outbreak.Additionally,both the S.horneri and the U.prolifera were detected to harbor stable endophytic strains,and even consistent in the same dominant OTU of Loktanella.However,we found a vast majority of dominant bacterial composition was differed between two seaweed;the dominant functions of bacterial communities associated with floating S.horneri were mainly from epiphytic bacteria,while in the floating U.prolifera were from endophytes;meanwhile,compared with floating U.prolifera,the iron transport function was also dominant in the floating S.horneri.This study preliminary revealed the community structure,function and dynamics of bacterial communities associated with two blooming seaweeds in the Yellow Sea,and carried out a comparative analysis.The above scientific knowledge will provide important clues for the verification of bacterial function during the seaweed bloom in the field,and also will guide the isolation and culture of important strains,and will construct an algae-bacteria interaction test system subsequently,which would provide new insights for studying the outbreak mechanism of seaweed blooms from a biological perspective.