| Marine microbial flagellate is a class of highly heterogeneous group of marine protists in global marine ecosystem with high biodiversity and multiple trophic modes.It occupies a broad niche in the global marine ecosystem and plays a key role in the biogeographical cycles.However,their spatiotemporal distributions of diversity and community structure,as well as the patterns and processes that drive the community assembly are still poorly characterized.In this study,we used high-throughput sequencing technology combined with traditional microscopic observation to investigate the diversity,community composition,and biogeographic pattern of microbial flagellates in three subtropical-tropical marine regions in China.For different size ranges(pico and nano flagellates)and different ecological groups(dominant and rare sub-communities),their biogeographical patterns and community assembly mechanisms showed different characteristics in different spatial scales(local and regional).The main findings are as follows:(1)Good consistency of the potential relationship between the environmental conditions and microbial flagellates’communities was reflected by their spatiotemporal patterns in three subtropical-tropical marine regions in China.The microbial flagellates’ communities in the East China Sea showed significant differences between the Spring and Autumn,the surface and bottom layers,as well as the coastal and shelf areas.In the Taiwan Strait,the difference of the microbial flagellates’ communities between seasons(Spring and Summer)was greater than that between years(2018 and 2019).The difference between the northern and southern areas was also significant,while the differences between the water layers and among different habitats(coastal,shelf,and slope-basin area)were weak or insignificant.As for the similarity of the microbial flagellates’ communities in the northeastern South China Sea in summer,it displayed depth-decay effect with significant differences among three water layers(surface,DCM,and bottom).In the regional scale,we found that the distribution patterns of the pico-and nano-flagellate communities in the subtropical-tropical waters were largely affected by the complex movement of the multiple water masses.They also showed obvious seasonal differences and depth gradients.However,the differences of the communities among different sea areas were relatively small,which reflected the connections between water masses to a certain extent.(2)The abundance and diversity of microbial flagellates in the studies area displayed certain temporal and spatial distribution characteristics,which had significant correlations with physical and chemical factors,food concentrations,and the abundance of nano-flagellates themselves.The latitude horizontal distribution of the richness was mainly affected by temperature,with decreasing trend with increasing latitude.At the longitude level,the richness increased from the nearshore to the offshore area.The vertical distribution of the diversity displayed high values above the euphotic layer,with higher diversity in the bottom layer than surface layer probably due to sufficient competition and niche differentiation under relatively stable environmental conditions and habitat heterogeneity among different patches in the former.As for the seasonal variation,the diversity in the Summer was usually higher than that in the Spring or Autumn.In addition,we found a significantly negative correlation between the diversity and the abundance of the nano-flagellate community,which also reflected by an inverse relationship of the correlations between both of the diversity and the abundance with various environmental factors.This indicated that the environmental factors may indirectly affect the diversity of the flagellate communities by affecting their abundance.(3)Regarding to the species composition of the microbial flagellates’ community,we found that different taxa groups had relatively fixed size.For example,Marine Alveolata(MALVs),Stramenopiles,Aupusozoa,and Archaeplastida were mostly belong to pico-sized flagellates,while Dinophyceae,Cercozoa,and Choanoflagellida were mostly nano-flagellates.The relative richness(OTUs numbers)and relative abundance(reads numbers)of various taxa were different,with different proportions in the dominant and rare groups.Alveolata was the most abundant and diverse taxa in the studied area,mostly belonging to the dominant group.Hacrobia was the second largest group with high relative richness,with higher proportion of the rare taxa than dominant taxa.Cercoazoa and Choanoflagellida were the main groups of the Rhizaria and Opisthokonta,respectively,mostly belonging to rare taxa with high relative richness.Stramenopiles also hold high species richness and were highly distributed in rare taxa.In addition,different taxa responded differently to the environmental conditions,which led to certain spatiotemporal distributions,representing the niche differentiation of these taxa.(4)The non-random geographic distribution of the microbial flagellates’community in the studied area was related to specific environmental characteristics and spatial factors.The potential correlation between the microbial flagellates’community and environmental factors depends on the degree to which the biological community matches or adapts to the changing environmental conditions.The geographic distances that represent spatial factors have impact on the dispersal of the biological community,resulting in spatial difference of the community.At the same time,multiple interactions(such as environmental factors and spatial factors)jointly determine the community assembly under different habitat conditions due to the correlations between various factors.The importance of these factors will change with the changes in spatial scale,target groups,and seasons.In the local scale(East China Sea,Taiwan Strait,and South China Sea),the deterministic process played a leading role in the construction of flagellate communities.However,in the whole regional scale,the dynamic construction of flagellate community in different water masses was mainly affected by the dispersal limitation due to spatial distance,while the environmental factors were still correlated with the community composition.Large-sized nano-flagellates displayed relatively weaker dispersal ability than pico-flagellates,which were more susceptible to dispersal limitation,so that they showed more significant correlations with spatial factors.As for the dominant and rare taxa groups of the microbial flagellates’ community,their diversity and community composition were totally different.However,they both had certain biogeographic distribution patterns and showed distance-decay effect in response with environmental and spatial factors.Specifically,the dominant taxa in each sea area tended to have higher correlations with environmental factors,and were able to be widely distributed with the transportation of ocean currents.On the contrast,the biogeographic distributions of rare taxa were limited to specific habitats due to dispersal limitation or more complex mechanisms of community assembly,such as interactions among taxa within the community.The influence of spatial factors on the microbial flagellates’ communities was greater in Spring than in Summer and Autumn(excluding the samples from the South China Sea in summer),which may be related to the monsoon exchange and water masses movement.In Spring,the monsoon exchange frequently,and the movement of water masses is complex and multi-directed,while the water masses in Summer are relatively simple but with higher speed and flux than those in Spring.This makes the dispersal limitation effect of the water masses in Spring stronger,while the biological communities in water masses in Summer were easier to spread.(5)Finally,the co-occurrence network of the community also plays an important role in the microbial flagellates’ community assemblage.In this study,we found that almost all the significant correlations(|r|>0.6,P<0.05)in each community were positive,which indicated coexistence or cooperation among different species to maintain the stability of the community structure.The degree of modularity of the microbial flagellates’ communities was high,indicating complex interacted network structure and obvious "small world" properties in each area.This phenomenon was more obvious in the whole regional area probably due to high degree of environmental heterogeneity.The network structure of the community became looser with increasing number of sub-modules.This indicated that the network relationship within the community would become weaker as the spatial distance increased,while the stability of the community would correspondingly decrease.Higher degree of connections of rare sub-community compared with dominant sub-community in the biotic associations among or within modules indicated the importance of rare biosphere which contributed a large part in the whole community.Meanwhile,rare taxa also showed a higher proportion in the composition of the key stones.All together,these results showed that rare biosphere could not only serve as a "seed bank" to provide ecological buffer and functional redundancy,but also had great biological activity that played an irreplaceable role in maintaining the stability of the ecosystem.In summary,our extensive and systematic sequencing revealed spatiotemporal patterns of the diversity and community structure of microbial flagellates in various habitats of the subtropical-tropical areas in China.The biogeographical patterns and ecological driving mechanisms of the community assembly were also explored.The findings contribute to improve our understanding of the diversity,community structure,and community assembly mechanism of marine microbial flagellates,and to promote the research of marine flagellates from the phenomenon and the role to the process and the mechanism.Future research can combine the functional characteristics of specific species(such as functional genomics,transcriptomics)to further evaluate the importance of various ecological processes in determining community assembly and maintaining ecosystem biodiversity. |
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