The Variation Of Marine Microbial Community Structure And Its Ecological Functions Following A Phaeocystis Bloom,Highlighted By Omics Techniques

Since 2011,large-scale algal blooms caused by Phaeocystis globosa have occurred annually in the Beibu Gulf,Guangxi Province,China,and have resulted in significant negative impacts to local industry,aquaculture and safety of nuclear power cold sources.During bloom,the environmental conditions including the availability of nutrients were significantly different with the non-bloom conditions,which affects the entire ecosystem,including marine microbes.And marine microbes form a feedback mechanism for the growth of phytoplankton,which play fundamental roles in the developmental processes of algal blooms.The detailed ecological functions of microbes during extinction P.globosa bloom still need to be investigated comprehensively.Thus,in the present study,we focused on two P.globosa bloom events in the Beibu Gulf from March to June 2017 and January to February 2018.Coupled metagenomic and metatranscriptomic sequencing derived from environmental DNA and RNA were applied to elucidate the biogeographic distribution pattern and community construction of microbes,and analyze the structural characteristics and transcriptional expression of active microbial communities from the blooming period to the recession period of P.globosa bloom.The result revealed the ecological responses of the microbial community along with subsiding of the P.globosa bloom.The major findings were listed as following:First,with the assistance of 16S amplicon technique,the microbial structure showed significant variation among samples from different sites（bloom and non-bloom sites）,depth（surface and bottom）,fraction（free-living and particle-attached）,and time（outbreak,recession,and extinction period）.The bacterial abundance,microbial OTUs richness,and Shannon diversity in the bloom area were significantly higher than those in the non-bloom area,and increased significantly in April and then decreased in June.The microbial structure was significantly affected by the abundance of P.globosa and environmental parameters such as salinity,DO,Si O₃^2-,and PO₄^3-.The similarities of microbial community decreased with spatial distance,indicating that dispersal-related processes drive the biogeographic distribution of microbes.The variation of microbial communities was mostly attributed to environmental selection,spatial distance,and the abundance of P.globosa successively.Second,the co-occurrence networks of microbial communities in bloom and non-bloom waters differed in terms of structure and composition,and the bloom network had more links and closer relationships between genera than the non-bloom network.The top five genera identified as keystone taxa in the bloom network were OM60 clade,Sulfitobacter,Oleibacter,Altererythrobacter,and Psychrobacter.Network analysis indicated that microbial communities had non-random pattern and driven by high environmental selection and low competitive effect among a few species.Third,during the blooming and recession periods of P.globosa,total and active microbial community composition were assessed by integrated metagenomic and metatranscriptomic approach.Our results found the abundance and diversity of the total and active communities increased along with the decline of the P.globosa bloom.Taxonomic analysis uncovered the active communities experienced distinctly different metabolic conditions across the bloom onset and collapse stages.The transcriptional activity of microbial groups,such as Erythrobacter and Candidatus puniceispirillum increased significantly during the recession periods.The transcriptionally active taxa of Vibrio,which correlated with most functional genes were enriched in blooming period.The active microbial structure was significantly affected by the abundance of P.globosa,DO and PO₄^3-content,indicating that the microbes respond to changes in water environment of bloom at the level of transcriptional activity.Among them,the transcript abundance of Nitrospina,Litoreibacter,and Candidatus puniceispirillum were significantly negatively correlated with the abundance of P.globosa.Fourth,the transcriptional analyses indicated the blooming and recession periods of P.globosa had significantly different metabolic potentials.The number of differentially expressed genes mapped to central metabolic pathways varied across communities.In the nutrient assimilation pathways,the uptake of L-amino acid by microbes increased significantly during the bloom period,while the utilize and transport of inorganic phosphorus,organic substances,such as phosphate,glutamine,monosaccharides and polysaccharides,increased during the recession period.Motility and quorum sensing were enhanced during the recession period,while the secretion of extracellular protein virulence factors in the Type II secretory system was highly transcribed in the whole period.The results of the study systematically depicted the spatiotemporal dynamics of microbial community composition in both FL and PA assemblages from blooming to subsiding of P.globosa blooms.And compared the composition differences among total microbial community（DNA level）,active community（RNA level）,and functional metabolism-related community（differentially expressed gene-related microbes）the blooming and recession periods of P.globosa.The integrated field investigation and in-depth analysis of molecular data preliminary revealed the underlying mechanism of microbial absorption and utilization of inorganic and organic nutrients,and the changes of microbial key cell processes in response to blooming and recession of the P.globosa bloom.It provides a scientific basis for further revealing the organic degradation of algae source and the circulation process of key source elements after P.globosa bloom,as well as the environmental adaptation mechanism and ecological function of microbial community.