| Studying on microbial ecology and function in hot springs is of great significance to understanding of early life on Earth and its involvement in element cycls(e.g.,C,N,S,Fe).The microbial community compositions(MCC)in hot springs are commonly linked with ecological functions.Studies on the MCC and their response to environmental and spatial factors avail to understand microbial response to extreme conditions and to prospect microbial resources in hot springs.On the basis of the contribution of biomass and biodiversity to the whole microbial community,microbial community consisted of abundant(relative abundance>1%)and rare sub-communities(relative abundance<0.01%),with the former contributing major biomass but minor biodiversity and the latter contributing minor biomass but major biodiversity.In the meanwhile,the rare sub-community is an important component of the whole microbial community,and it plays an important role in ecological fuctions and maintaining the stability of the whole ecosystem.However,previous studies on MCC of Tibetan hot springs were mainly focused on the abundant community and their response to some environmental factors(e.g.,Temperature),while little is known about the composition and function of abundant and rare sub-communities and their response to environmental conditions in hot springs.In addition,the soluble Fe(II)and Fe(III)-containing minerals are the most important electron and energy recourses for microbes in geothermal environments.However,few studies have been performed on the iron redox processes driven by microbes and resulting influences on the mineral transformation in geothermal sediments.The Tibetan geothermal zone(especially the southern area)is one of the most active geothermal areas in the world,hosts thousands of hot springs with diverse geochemical parameters,and receives less interference from human activity.Thus the Tibetan geothermal zone is an ideal place to study the microbial ecology and function in hot springs.With the use of integrated technologies of high-throughput sequencing(Illumina MiSeq),geochemistry and statistics,phylogenectic composition and ecological functions of total microbial community and abundant,intermediate and rare sub-communities were studied in sixty-six Tibetan hot springs,which cover a range of temperature(32–86~oC),pH(3.0–9.5),salinity(0.13–1.32 g/L),and pairwise geographic distance(0–610 km).Subsequently,enrichment,isolation and phylogenetic identification were performed for thermophilic anaerobic iron-oxidizing and–reducing bacteria in some selected hot springs,followed by systematical characterizations of iron redox processes and the resulting minerals.The main results were summarized as follows:1.Microbial community composition,ecological function and their environmental response in Tibetan hot springs1)The abundant and rare biospheres exhibited different community compositions:the abundant sub-communities were mainly composed of Chloroflexi,Proteobacteria,Deinococcus-Thermus,Aquificae,Bacteroidetes and Firmicutes.In contrast,the rare sub-communities mainly consisted of most newly proposed or candidate phyla of Dictyoglomi,Hydrogenedentes,Atribacteria,Hadesarchaea,Aminicenantes,Microgenomates,Calescamantes,Omnitrophica,Altiarchaeales and Chlamydiae.However,the abundant and rare sub-communities shared some common phyla(e.g.,Crenarchaeota,Bathyarchaeota,Chlorobi),indicating that their abundant/rare status could transform to each other when enniroments change.Such mechanisms could maintain the sustainable stability of the entire ecosystem.2)The abundant,intermediate and rare communities were responsed differently to the environmental and spatial factors.The abundant sub-community was mainly affected by temperature and total orgnic carbon content,and the intermediate sub-community was mainly affected by pH and salinity,while the rare sub-community was mainly affected by DOC and pH.Such different response to environmental factors among the abundant,intermediate and rare sub-communities could be attributable to their different growth rate and activity.Spatial factors showed more contribution to the shaping of the intermediate and rare communities than to abundant sub-community,suggesting that the abundant taxa were more easily dispersed than their rare counterparts among hot springs.3)The dominant ecological functions and their responses to environmental factors differed between the abundant and rare sub-communities,indicating that they occupied different ecological niches in hot spring ecosystems.The ecological function prediction results showed that microbial functional groups involved in C,N,S cycles were widespread in the Tibetan hot springs,that some unique ecological functions were detected in the abundant(e.g.,ammonia oxidation),and the rare sub-communities(e.g.,organic matter degradation),and that the rare sub-community may play more important roles in organic matter degradation than their abundant counterparts in hot springs.2.Mcirobial iron reduction/oxidation processes in Tibetan hot springs and their geological significance1)ANDFOM in the Tibetan hot springs were maily present in hyperthermophilic hot springs(>80~oC)(QZM-1,QZM-2,QZM-16),and they were mainly affiliated with the phyla of Betaproteobacteria,Alphaproteobacteria and Firmicutes.The resulting minerals in ANDFO process showed similar mineral composition(magnetite,siderite),crystal morphology(euhedral rhombic hexahedron and/or dodecahedron)and size(1.5–2?m,larger than its counterparts(usually<1?m)formed by chemical reactions)in BIFs.Considering the similarity between the environmental conditions of hot springs(high temperature,low dissolved oxygen,etc.)and the Precambrian Earth when BIFs formed,it can be speculated that ANDFOM might play important roles in the formation of BIFs.2)The culturable dissimilatory iron reducing bacteria(DIRB)in the Tibetan hot springs were mainly composed of genera of Thermonicola,Carboxydocella,Carboxydothermus,Desulfotomaculum,Thermodesulfobacterium,Thermanaerothrix,Thermodesulfovibrio,Methyloversatilis,Thermotoga,and Peptococcaceae bacterium Ri50.Most of genera were ever reported to contain dissimilatory iron reducing bacteria,while the genus Methyloversatilis was first time to be able to dissimilatorily reduce iron.Through detecting of the Fe(III)-reducing kinetics and the newly produced Fe(III)minerals of four pure isolates of DIRB QZM1201,QZM1202,QZM2101and GD504,we found that iron recution extent and secondary minerals were distinct under different bacterial strains,election donors(actate vs.lactate),and electron acceptors(FeOOH vs.NAu-2).With lactate as the electron donor,strain QZM1201(16S rRNA gene showing 99.71%identity with Thermincola carboxydiphila)and QZM1202(showing 97.21%identity with Carboxydocella thermautotrophica)reduced FeOOH with the formation of vivianite desposit;QZM1201,QZM1202,QZM2101(showing 100.00%identity Methyloversatilis universalis)and GD504(showing 99.50%identity with Desulfotomaculum australicum)reduced NAu-2 with nontronite illitization,and the resulting reduction extents were comparable to those by mesophilic Fe(III)-reducers in the presentce of eletron shutters.This also suggested that the DIRB played important roles in Fe cycles and the formation of secondery minerals in hot spring sediments. |
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