Community Structure And Spatio-Temporal Distribution Of Bacteria In The Rhizosphere Of Zostera Japonica In The Yellow Sea And Bohai Sea

Seagrass bed is an important component of blue carbon.Due to the influence of human and natural factors,seagrass beds have been largely degenerated worldwide,especially in China.Structurally-rational microbial community will enhance the adaptability of plants in the environment,while the imbalance structure will lead to the decline of seagrasses.However,it is not well understood that the diversity and community structure of bacteria in the rhizosphere sediments of seagrass beds,especially that the succession of rhizosphere microorganisms with the time and space are still unclear.In this study,vegetated and unvegetated sediments were collected from Kenli district of Dongying,Changhai county of Dalian and Rongcheng City of Weihai,in spring?April?,summer?August?,autumn?October?and winter?January?from 2017 to 2018.The composition and structure of bacterial community were determined by 16S rRNA high-throughput sequencing technology.The results showed that the bacteria in rhizosphere were abundant than the non-rhizosphere,among which Proteobacteria?42.8%?,Bacteroides?17.6%?,Actinobacteria?8.1%?and Cyanobacteria?7%?were the predominant taxa.On the one hand,the successions of dominant groups changed seasonally.Actinobacteria,Chloroflexi,Bacteroidetes,Gemmatimonadetes were the major phylum of spring,summer,autumn and winter respectively.On the other hand,community composition was also affected by regional and sample types?with or without seagrasses colonization?.For example,relative abundances of phylum Patescibacteria occupied high proportion in Dongying,and Chloroflexi and Proteobacteria were one of the abundant groups in vegetated and unvegetated samples,respectively.Alpha diversity analysis showed that species richness and diversities were strongly influenced by the time and space.Indices Shannon,Simpson,Chao1 and OTU richness were the lowest in spring but the highest in winter.The regional comparisons of alpha diversity index showed that Dongying>Weihai>Dalian.Furthermore,the correlation analysis between alpha diversity with environmental factors showed that TOC,TOC/TON,SO₄^2-,V,Mn were the key factors shaping the alpha diversity of bacteria in the rhizosphere of seagrasses,indicated that the growth or apoptosis of seagrasses with seasons may affect the content of organic matter in sediments,as well as the succession of bacterial communities.Compared to seasonal effects,Beta diversity analysis showed that regional and rhizosphere effects posed a greater influence on the bacterial community structure of seagrass sediments.Significant differences were found among the three regions on spatial scale?P=0.001?,no matter seagrasses vegetated or not.The bacterial community structure of the sediments in Dongying were significantly different from those of in Weihai and Dalian.A possibility is that the bacterial community structure may related to the texture of sediments.There is sandy sediment in Weihai and Dalian,but muddy sediment in Dongying.The difference between vegetated and unvegetated of samples was significant?P=0.001?.However,part of seasonal comparisons was not significant,such as summer vs.autumn?P=0.141?and autumn vs.winter?P=0.055?.The physical and chemical properties of samples showed that organic matter,heavy metals,NO₃^-and NO₂^-were generally higher in summer.The smallest particle size and the highest content of heavy metals in Dongying.Compared to unvegetated sediments,contents of organic matter,NH₄⁺,NO₂^-and heavy metals?except Mn and As?were higher and particle size was smaller in vegetated sediments.Combining RDA analysis of bacterial communities and environmental factors assay,silt,TOC,NH₄⁺and heavy metals?As,V?were likely the most important factors shaping community structure.We speculated that sediment size may play a key role in community structure due to that it may be absorb nutrients,heavy metals and so on.This also explained why the community structure of bacteria in Dongying was unique when compared to Weihai and Dalian.The functional prediction of bacterial community in rhizosphere sediments of seagrasses was conducted by PICRUSt analysis.The results showed that the abundance of nitrate reduction gene and assimilating sulfate reduction gene were the lowest in spring.The relative abundance of dissimilating nitrate reduction gene,denitrifying reduction gene and nitrogen fixation gene was highest in Dongying.The abundance of dissimilating sulfate gene and nitrogen fixation related gene in vegetated was significantly higher than that in unvegetated,this confirmed the key role of sulfate respiration and biological nitrogen fixation in seagrasses rhizosphere.Summary,this study has described the diversity and structure of bacteria in the ecosystem of Zostera japonica of the Yellow Sea and Bohai Sea in China.It was found that the seasonal,regional and rhizosphere effect played important roles in shaping the community structure of bacteria in the rhizosphere of seagrasses.