Sedimentary Microbial Community Structure And Elemental Cycle Process In The Coexistence Area Of Cold Seeps And Hydrothermal Vents At The Okinawa Trough

As the most important part of the marine system,deep-sea is the largest ecological unit in the world.It has various extreme environments with different physical and chemical properties and different geological processes.In recent years,great progress has been made in the research of microbial taxa and biogeochemical processes in cold seeps or hydrothermal vents,which has greatly increased people’s understanding of the extreme environment of deep-sea.As the modern deep-sea extreme systems,both hydrothermal vents and cold seeps are the critical pathways between the lithosphere and exosphere for transfer and exchange of materials and energies.There are significant differences,but also many similarities between the two extreme ecosystems.Because hydrothermal plumes can change the environmental conditions of marine sediments at the spatiotemporal scale,especially the nutrient and metal content form a significant gradient with the distance from the hydrothermal vent,which in turn affects the composition of microorganisms and energy metabolism pathways in the sediments.If the cold seep and hydrothermal vent are spatially close and within the potential influence of hydrothermal plumes,then there may theoretically be a material and energy exchange between the two system,which in turn affects the microbial community structure and microbial-mediated geochemical cycle.Therefore,it has great significance to comprehensively understand the material cycle and energy flow of the deep-sea sediment by considering both cold seeps and hydrothermal vents into the seafloor extreme environmental system.The research on microbial community characteristics and biogeochemical cycles in cold seeps and hydrothermal vents coexistence systems from a broader perspective,which is expected to deepen the understanding of deep-sea extreme environments and microbial ecology.Based on this,this thesis focuses on the Okinawa Trough（OT）,there are abundant hydrothermal vents and cold seeps developed.Relying on the“Hai Yang Di Zhi Jiu Hao”scientific comprehensive research vessel,systematic sampling was carried out in the Minami-Ensei Knoll hydrothermal vent and the cold seep on the western slope of OT（the straight-line distance between the two system is about 50 kilometers）.With the help of 16S rDNA ampliconand metagenomic high throughput sequencing technology,the microbial communities and geochemical element cycles were analyzed and discussed in the coexistence zone sediments of the cold seep and hydrothermal vent.The following progress has been made:1、The intensity of seafloor methane seepage significantly affects the composition of microbial communities.In the sediments of cold seep on the western slope of OT,anaerobic methantrophy（ANME）archaea show a high degree of diversity,and together with sulfate reducing bacteria,they dominate the anaerobic oxidation of methane（AOM）;and the discovery of thermophilic microbial taxasuggests that microorganisms in cold seeps may communicate with hydrothermal vents.The Chao1,ACE,Shannon,and Simpson indices showed that the richness and diversity of bacteria and archaea in the samples of high methane seepage intensity region were higher than those in the low methane seepage intensity region.According to the intensity of methane seepage,dominant microbial taxa and surrounding environment characteristics in the sediment can assess the development stage of cold seeps.Among them,in the low-fluxes methane diffusion environment,the sulfate-methane transition zone（SMTZ）is deep,and the microbial-mediated degradation process of organic matter is dominated in the sediment above SMTZ.During the mature stage of cold seeps(R6-C2 site,methane seepage rate 1475 mmol m^-2 yr^-1),organic matter accumulates in large quantities.Then,as the intensity of methane seepage gradually decreases,the amount of organic matter in the sediment gradually decreases,and the cold seeps begin to enter the extinction stage(initiation phase;R5-C1 site,methane seepage rate 515 mmol m^-2 yr^-1).In fault areas or mud volcanoes,when methane is in high-fluxes,some of the methane passes through seafloor sediments and directly enters the seawater.In high-fluxes methane bubbling environments,SMTZ tends to be very shallow,AOM processes dominated in sediments,and cold seeps are in the active stage.ANME,δ-Proteobacteria,andε-Proteobacteriahave a relative high abundances and diversity in sediments with high methane seepage intensity(R6-C3 site,6450 mmol m^-2 yr^-1).Specifically,all known ANME types have been found in the cold seep sediments on the western slope of OT,including ANME-1b（the relative abundance is67%of the archaea community）,ANME-2c（8%）,ANME-1a（7%）,ANME-2b（3%）,ANME-2a（2%）and ANME-3（1%）.In general,ANME have their own ecological niche,and their coexistence in the same sampling site is the first reported,which is in line with the inference of the special environmental conditions of the coexistence of seafloor cold seeps and hydrothermal vents.In addition,thermophilic microorganisms are detected in the cold seepsediments,most likely from adjacent hydrothermal vens through fluid circulation.Taken together,these results indicate coupling or interaction between cold seep and hydrothermal vens in the OT.2、The microbial community composition and function of marine sediments are habitat-specific distribution in different stations of the coexistence zone of cold seep and hydrothermal vent in OT.The relative abundance of thermophilic microorganisms in submarine fracture zones sediments with high heat flow values increased.Adjacent to the hydrothermal vent,the nitrogen cycling process mediated by microorganisms was significantly enriched,while near the cold seep,the sulfate reduction was strong.The results show that cold seeps and hydrothermal activity are important materialsources of microbial-mediated biogeochemical cycling in marine sediments.In the subsurface sediment samples of five stations（GC-2,-4,-5,-6,-8）in the coexistence zone of the cold seep and hydrothermal ventin OT,bacterial abundance was high and archaea were low.The relatively abundant taxamainly includedα-,γ-,δ-Proteobacteria,and it has different distribution characteristics.Major microbial groups at GC-2 site（cold seep on the western slope of the OT）are sulfate reducing bacteria（SRB）.Due to the characteristics of high sulfate at the deep-sea sediment-water interface and its anaerobic environment,abundant SRB and their strong metabolic capacity promote the rapid circulation of sulfur.The GC-6 station is located in the seafloor fault zone and therefore has a higher heat flow value,and the relative abundance of thermophilic microorganisms is higher than that of other stations.Whereas the microbial community structure in the adjacent Minami-Ensei Knoll hydrothermal vent（GC-8 station）sediments was mainly affected by environmental factors NO₂^-（0.0023 mg/L）and sandy content（16.5%,“permeable”sediments）.The dominant microbial taxa areα-Proteobacteria and Planctomycetes,with oxidation of low-valent sulfur compounds and anaerobic ammonia oxidation as the main metabolic types.In addition,through microbial functional gene analysis,it was found that in the aerobic-anaerobic interface of“permeable”marine sediments,ammonia oxidation,sulfur oxidation coupled with nitrate reduction and other processes promoted the cycle of nitrogen.Candidatus Methylomirabilis oxyfera,which has a unique nitrite-driven methane oxidation capacity,is also distributed in this sedimentary layer utilizing NO₂^-and methane.The OT is close to the East China Sea shelf,have accumulated thicker layers of organic-rich Quaternary sediments.Therefore,the hydrothermal fluid carries a large number of reducing materials（ammonium,sulfide,metal ions）and the surrounding oxidized cold seawater mixed with each other,can form a hydrothermal plume of several hundred meters to several thousand meters.The input of exogenous materials derived from hydrothermal fluids has an important impact on the community structure of microorganisms in sediments and their mediated elemental cycling processes.3、The interaction of seafloor cold seeps and hydrothermal fluids affected the occurrence of metal-dependent anaerobic oxidation of methane.The co-occurs of cold seeps and hydrothermal vents in OT provides a good model for the coupling of marine element circulation process.In anaerobic marine sediments,sulfate(SO₄^2-)is the main electron acceptor of AOM（Sulfate-dependent anaerobic oxidation of methane,Sulfate-AOM）.Most of the methane is intercepted at SMTZ.However,methane concentrations in surface sediments remained high in active cold seeps（GC-2 station,353μL/L）.In addition to the presence of a large number of SRB in the GC-2 station,many iron-reducing bacteria（e.g.,Geobacter）have been detected.By detecting and calculating the metal pools and fractions of hydrothermal Fe(f_hydrothermal)in the cold seepand hydrothermal vent coexistence zone of OT,it can be preliminarily confirmed that the activemetal material carried by the hydrothermal plume from the spreading center of the back-arc basin can precipitate into the cold seep of western slope.Here,the dynamic change of fluid in the cold seep of western slope formed the upward migration of fluid.In the uppermost oxidation zone,aerobic oxidation of methane and reducing iron occurs.In the SMTZ below it,methane in the seeping fluid and sulfate that normally diffuses downward occur Sulfate-AOM.However,due to the addition of a large number of hydrothermal iron oxides,the iron-dependent anaerobic oxidation of methane（Fe-AOM）may be generated at the same time.In this study,hydrothermal plumes from the spreading center of the OT can provide additional energy and nutrients to the cold seep sediments in the semi-enclosed topography and the unique coexistence of cold seeps and hydrothermal vents,and organically link the two ecosystems through biogeochemical cycle coupling.Microbial-mediated methane formation and oxidation has huge implications for the storage,conversion or release of methane on the seafloor and even on the global carbon cycle.The coexistence system of cold seeps and hydrothermal vents in the OT provides a good model for the study and exploration of special metabolic types.The results of the study provide scientific evidence for a comprehensive understanding of the microbial communities in the extreme environment of the OT hydrothermal vents and cold seeps coexistence system and the biogeochemical cycles in which they participate.The conceptual model of seafloor cold seeps and hydrothermal vents fluid interaction has important scientific implications for material sources and energy flows in the deep biosphere.