Microbial Community Assembly,Succession And Ecological Function In The Yellow River Delta Wetland

The Yellow River,which is the second largest river in China,is regarded as the world's largest contributor of fluvial sediment load to the ocean.Approximately 30-40%of the sediment transported by the Yellow River is deposited in the estuary,forming the coastal estuarine wetland of the Yellow River Delta.The coastal estuarine wetland is the intersection area of marine ecosystem,river ecosystem and terrestrial ecosystem,and has important ecological functions.The study about the Yellow River Delta wetland is related to the ecological protection and high-quality development of the Yellow River basin.In this thesis,we selected the Yellow River Delta wetland as the research object,combined with the 16S rRNA gene amplicon,metagenomic and metatranscriptomic sequencing and metabolomic analysis,aimed to explore the shifts in the soil,water and sediment bacterial community assembly,succession and ecological function along with two main dynamic process(i.e.,colonization of vegetation and the Yellow River into the sea)in the coastal estuarine wetland of the Yellow River Delta.We collected soil samples from five plots(barren land,wetlands with sparse Suaeda salsa,wetlands with dense S.salsa and wetlands with dense Phragmites australis)in the Yellow River Delta along the vegetation transition zone on December 25,2017.We found that the richness and diversity of soil bacteria increased with vegetation.Results of the Principal coordinate analysis(PCoA)and hierarchical cluster analysis showed that the bare wetland samples were a single group,other vegetation wetland samples were another group,suggesting that bacterial community structure in the bare wetland significantly different from vegetation wetland.The bare land displayed an extremely high abundance of Cyanobacteria as a monospecies genus,its abundance accounted for 17.98%of the total bacteria abundance,in which a single Cyanobacteria OTU accounted for 13.27%of the total abundance.However,a Gemmatimonadetes genus was predominant as multiple species in all the vegetative wetlands,its abundance in different vegetation wetlands ranged from 8.21%to 10.58%.These results indicated that the species and composition patterns of dominant bacteria in bare wetland and vegetation wetland were significantly different.Correlation analysis between taxonomic composition and environmental factors showed that pH,soil water content and salinity contributed the most to the composition of soil bacterial community in wetland.In general,the taxonomic composition of the community determines the ecological function of the community.Therefore,we further analyzed the differences in the ecological function of soil microorganisms in bare wetland and vegetation wetland using soil metatranscriptomic sequencing.The results showed that expression of the genes related to photosynthesis was enriched exclusively in bare land.Genes involved in biological organic carbon metabolism(for example,purine metabolism,pyruvate metabolism,nicotinate and nicotinamide metabolism)and the cycling of main elements(carbon,nitrogen,sulphur and phosphorous)were highly expressed in vegetative wetlands,which was consistent with the MAG analysis of Gemmatimonadetes and soil metabolomic results(such as 2',3'-cyclic GMP and Trigonelline).The Cyanobacteria monospecies is thus responsible for the carbon sink in the infertile wetlands,and the genus plays a crucial role in ecosystematic functions in vegetative wetlands.Our results highlight that the soil microbial populations play a role in ecosystematic functions for wetlands and that vegetation is the determinant for the population and functional shifts in the coastal estuarine wetland of the Yellow River Delta.Then,we collected 162 water and sediment samples from fixed sites in the Yellow River estuary in summer and winter 2019 and filtered the water samples to separate the particle-associated bacteria(PA,3.0 ?m)from the free-living bacteria(FL,0.22 ?m).The salinity of the water samples ranged from 0.5 PSU to 30.1 PSU,and that of the sediment samples ranged from 0.4 g/kg to 16.5 g/kg.The richness and diversity of benthic(sediment,S)bacteria were much higher than the PA bacteria,which was significantly higher than the FL bacteria(p<0.05),however,there was no significant difference in bacterial richness and diversity for the same environmental type(PA,FL and S)between summer and winter(p>0.05).The planktonic bacterial communities could be easily classified into three distinct groups along the water salinity gradient:the freshwater group,the brackish water group and the seawater group,but the benthic samples showed no such pattern.The CCA showed that salinity explained 34.2%,39.3%and 10.1%of the variation in the PA,FL and S community compositions in summer and 43.7%,37%and 8.6%of that in winter,these values are much higher than those for the environmental factors TOC,TN and pH.Mantel test showed the differences in the planktonic bacterial community composition had a close correlation with salinity in both the summer and winter samples(summer/winter-PA:Pearson's r=0.8978/0.9360,summer/winter-FL:0.9273/0.8740,p<0.01 summer/winter-S:Pearson's r=0.5805/0.5777,p<0.05).These results showed that salinity had a stronger effect on planktonic than on benthic microbial communities.The dominant planktonic bacteria changed more distinctly than the dominant benthic bacteria with changes in salinity.The dominant planktonic bacteria disappeared or flourished with the change of salinity,while the dominant benthic bacteria did not.The winter planktonic bacteria had almost no relationship with the summer planktonic bacteria,but approximately 65.19%of the winter benthic bacteria came from the summer sediments.The planktonic bacteria in the brackish water came mainly from seawater,which was confirmed in the laboratory,whereas the benthic bacteria were weakly affected by salinity,which appeared to be a mixture of the bacteria from riverine and oceanic sediments.Benthic bacterial community assembly in the sediments was mainly controlled by homogeneous selection(summer:72.08%,winter:82.91%)and almost unaffected by changes in salinity,the dominant assemblage processes for planktonic bacteria changed dramatically along the salinity gradient,from homogeneous selection in freshwater to drift in seawater.These results indicate that among the analyzed environmental factors,salinity is the key driver of estuarine microbial succession and it is more important in shaping planktonic than benthic bacterial communities in the Yellow River estuary.In order to further analyze the ecological function and environmental adaptability of planktonic and benthic bacteria in the Yellow River estuary,we selected representative samples for metagenomic(9 S,9 PA and 9 FL)and metatranscriptomic(water samples with the most dramatic changes in salinity,4 PA and 4 FL)sequencing analysis.Metagenomic analysis showed that FL group held the highest abundance(metabolic potential)of some metabolic pathways directly related to elemental cycling,such as carbon-associated aerobic respiration,nitrogen-associated nitrogen assimilation and sulphur-associated sulfur mineralization.274 nonredundant MAGs were recovered from metagenomic' sequences using the binning,including 168 Proteobacteria,55 Actinobacteria and 36 Bacteroidetes.Metatranscriptomic analysis showed that genes related to osmotic pressure regulation had higher activity in planktonic FL bacteria than planktonic PA bacteria,and it is worth mentioning that these genes showed the highest expression activity in brackish water,suggesting that changes of osmotic pressure(from freshwater to brackish water or from seawater to brackish water)induced gene expression.These results indicate that planktonic bacteria FL,planktonic bacteria PA and benthic bacteria have significantly different metabolic potential of maj or elements,and planktonic bacteria FL has a more obvious response to salinity changes than planktonic bacteria PA,especially the expression of genes related to bacterial osmotic pressure regulation.These results indicate that FL group has a stronger response to salinity changes than PA group.In conclusion,our results reveal that planktonic bacteria FL,planktonic bacteria PA,and benthic bacteria had significantly different responses to salinity changes,including community assembly,source tracking,abundance changes of dominant taxa and environmental adaptability during the major natural process of freshwater river flowing into the sea.In this thesis,we selected the Yellow River Delta wetland,where environmental factors changed dramatically,and analyzed the changes of soil bacteria with vegetation colonization,revealed the decisive role of vegetation on soil microbial composition and ecological function of wetlands.Planktonic bacteria and benthic bacteria along with the salinity were also explored.We found that the planktonic bacteria and benthic bacteria had significantly different responses to salinity changes,including community assembly,source tracking and ecological functions.These results extended our understandings of the microbiome in the Yellow River Delta and provided theoretical guidance for ecological protection and high-quality development in the Yellow River Delta and even the Yellow River basin.