Community Structure And Activity Potential Of The Prokaryotes In The Hadal Sediments Of The Typical Trenches In The Pacific Ocean

The deep ocean harbors around 75% of prokaryotic biomass and more than half of the prokaryotic production of the global ocean,and is a key site for organic carbon remineralization and long-term carbon storage in the biosphere.Research efforts in the past decades have achieved dramatic advances on understandings of diverse deep-sea habitats,such as seamounts,hydrothermal vents and cold seeps.However,the hadal zone with the water depth over 6,000 m,is seldomly touched due to the great technological challenges.Organic matter content and microbial carbon turnover rates are significantly higher at hadal trench sediments compared with abyssal plain site,making the hadal trenches “hot spots” for microbial colonization and organic matter deposition in the deep ocean.The microbial driven degradation of sedimentary organic carbon in hadal trenches are important for organic carbon mineralization and carbon biogeochemical cycle in the deep ocean.However,the knowledge of microbial communities in hadal sediments is still very limited.Majority of the existing studies about hadal microbial communities were based on DNA,which detected the bulk community and their overall functional structures.It is still not known what kind of microbes are alive and potentially the major contributor of microbial activities in the hadal zone,what are the relative activity of different microbial taxa in the alive community.In this Msc thesis,we explored the abundance,species composition and spatial variations of the bulk and active prokaryotic community in sediment cores taken from the deepest points of two representative hadal trenches(Mariana Trench,MT;and Mussau Trench,MST),using high-throughput sequencing on amplicons of 16 S r RNA gene(r DNA)and 16 S r RNA,respectively.The objectives are to(1)determine the diversity and compositions of bulk and potentially active prokaryotic communities;(2)test the novelty of the hadal prokaryotic lineages;(3)compare the species composition and spatial variations of the potentially active prokaryotic communities in both trenches;and(4)study the relative activities and interactions of potentially active prokaryotic taxa in sediments of the hadal trenches.This is the first study to assess the novelty of prokaryotic communities in hadal sediments by comparing the detected 16 S r RNA and r DNA sequences to those present in the SILVA and NCBI databases.Our results showed that averagely 32.30% of OTUs from the r DNA and r RNA libraries of both trenches were identified as novel OTUs at 97% identity level.When using 99% identity as the threshold,novel OTUs averagely account for 64.08% of total OTUs from the two sediment samples.The results suggest that the hadal trenches may be a reservoir of new microbial species,supporting the hypothesis that topographical isolation and extreme environmental conditions lead to new species in the hadal zone.Beta diversity analysis of microbial community showed that there were significant differences between species composition of the r DNA and r RNA libraries from sediments of hadal trenches,and the differences between molecular markers(r DNA vs.r RNA)were even larger than those between different depths,different trenches,or different sampling times.Meanwhile,correlations between r RNA and r DNA frequencies of different taxa were very weak.The most abundant prokaryotic lineages in the bulk communities of trench sediments generally showed their low relative abundance in r RNA libraries,suggesting that the potential activity of an OTU in hadal sediment does not follow its relative abundance in the bulk community and high abundance taxa may not be important for ecological functions in the hadal sediments.To our knowledge,this is the first study to explore co-occurrence network and modular patterns of potentially active sediment prokaryotic communities in hadal trenches.The topology of the network exhibited high level of modularity and close interactions between potentially active taxa,suggesting that highly diversified microniches and structured microbial communities exist in sediment of the hadal trenches.We successfully identified high activity taxa and keystone taxa which were mainly belonging to classes/phyla Gemmatimonadetes,Actinobacteria,SAR202 clade,JG30-KF-CM66 and Alphaproteobacteria.Among them,Gemmatimonadetes and Chloroflexi have been showed as the top phyla of the potentially active communities and many members of the two phyla were keystone taxa in interactive network,suggesting their significance in mediating the activity and functions of trench prokaryotic communities.In addition,several potential hydrocarbon degrading bacteria taxa Planomicrobium,Xanthomonadales,Anaerolineaceae as well as SAR202 clade were found to show high activity potentials and played keystone roles in the co-occurrence network.Such finding,combined with the abundant alkane content in sediment,leading us to postulate that alkane degradation might be an important pathway of carbon turnover in the sediment of the hadal trenches.Besides,the vertical and annual variation of prokaryotic total communities and potential active communities were studied by comparing the Mariana Trench sediment samples.Our results showed that there were significantly difference between top 10 cm below seafloor(cmbsf)and bottom 10 cmbsf in abundance of the potentially active prokaryotic communities.JG30-KF-CM66 clade,SAR202 clade,Anaerolineaceae,Actinobacteria and Phycisphaeraceae were rapidly decreased below 10 cmbsf.Furthermore,there were significantly annual differences between potentially active microbial communities and little variations between bulk communities.The relative abundance of Dehalococcoidia,Planctomycetacia and Hydrogenedentia were dramatically increase with time-scale.This study determined for the first time the species of potentially active microbial communities,the proportion of new microbial lineages and their interactive networks in the hadal trench sediments.The results of this research paper are important for understanding the microbial processes occurring in the hadal zone,and for laying a foundation for revealing the biogeochemical cycling mechanism in the deep sea.