| SAR92 clade is an important oligotrophic marine Gammaproteobacteria(OMG)group that widely spreads in the global ocean.SAR92 is an important marine carbohydrate metabolizing bacterium,and plays an essential role in substance cycle and energy metabolism in the ocean.There is a potential interaction between SAR92 and marine phytoplankton,especially during phytoplankton bloom,SAR92 can grow rapidly and becomes the dominant group,which means that phytoplankton may be an important biological factor affecting the growth of SAR92 in the ocean.Furthermore,expression of CAZyme genes increases significantly during phytoplankton bloom,suggesting that SAR92 has the potential ability to degrade and utilize polysaccharides,and phytoplankton-derived polysaccharides may be an important factor in inducing the growth of SAR92.Marine phytoplankton is the main producer of polysaccharides,which can produce more than 15 Gt polysaccharides every year.The degradation of marine polysaccharides requires a variety of corresponding enzymes,and only a few groups of marine bacteria can carry out effective polysaccharide metabolism,and most of them have PULs.Due to the complex structure of polysaccharides,the degradation of polysaccharides requires highly specific enzymes to cleave glycosidic bonds,gradually break the polysaccharides and release monosaccharides,which are then utilized by bacteria.Meanwhile,the polysaccharide metabolism of bacterial groups is specific.The existence of these bacteria rapidly degrades a large number of polysaccharides in the surface seawater,which is an important part of high molecular weight organic carbon cycle in the ocean.S AR92,as a bacterium with the ability of polysaccharide metabolism,its specificity of polysaccharide metabolism in the ocean is still unknown.The study of polysaccharide metabolism strategy of SAR92 can help us to understand the polysaccharide metabolism of SAR92 in different sea areas and the specific response of SAR92 to phytoplankton polysaccharides.It has revealed the role of SAR92 in the degradation of marine polysaccharides,so as to comprehensively understand the biogeochemical cycle of marine high molecular weight organic carbon.In this dissertation,the environmental factors,relative abundance of SAR92,phytoplankton abundance,polysaccharides metabolism related functional gene abundance from different marine areas were extracted by the bioinformatics method,and the artificial seawater medium to grow SAR92 was developed.Combined with qPCR and quantitative proteomics,the relationship between SAR92 and phytoplankton in the global ocean was revealed.The specific metabolic ability of SAR92 to phytoplanktonderived polysaccharides and the molecular mechanism involved in polysaccharides metabolism were elucidated,and the ecological strategy for SAR92 to be widely distributed in the global ocean was discussed.The main results are as follows:1.SAR92 was widely distributed in the surface layer of different sea areas in the global ocean,but its abundance was significantly different.Among the physicochemical factors,the inorganic nutrients of nitrogen,phosphorus and silicon had a significant positive correlation with SAR92,while the temperature showed a significant negative correlation with S AR92;only a few phytoplankton groups had correlation with SAR92,among which dinoflagellates showed a significant negative correlation with SAR92,while diatoms had a significant positive correlation with SAR92.Diatoms and SAR92 showed the similar habitat,especially in the coastal Antarctic,where diatoms and SAR92 occupied a certain advantage,suggesting that there might exist tight interaction between them.The CAZyme belonged to SAR92 was widely distributed in the global ocean,indicating that polysaccharide metabolism was an important ecological strategy of SAR92 to adapt diverse environments.The expression level of β-glucan metabolism genes in SAR92 was higher,suggesting that diatom-derived polysaccharides might be an important factor driving SAR92 growth;2.The model SAR92 strain HTCC2207 exhibited specificity for polysaccharides and only laminarin and xylan could be used efficiently,for HTCC2207 possessed multiple laminarin and xylan PULs.HTCC2207 could express the endohydrolases GH10 or GH16,GH17,GH81 and GH158 to hydrolyze xylan or laminarin to oligosaccharides,then express exohydrolases GH3,GH17 and GH30 to hydrolyze oligosaccharides to corresponding monosaccharides and transport them into cytoplasm to participate in glycolysis.The expression of CAZyme was not only induced by related polysaccharides,but also was induced by monosaccharide components.The metabolic regulation mechanism of laminarin was quite different from that of xylan.The metabolism of laminarin required more GH enzymes,and the abundance of flagellum synthesis related proteins in the laminarin treatment was also greatly increased compared with the xylan treatment,indicating that the predatory and motile ability of HTCC2207 cells were more active in the laminarin treatment;3.The response of HTCC2207 to the exudate and lysate of different phytoplankton species was significantly different.The exudate and lysate of three different dinoflagellates species could not promote the growth of HTCC2207,indicating that dinoflagellates could not provide carbon source available for HTCC2207.However,the exudate only from a diatom species Chaetoceros sp.could promote the growth of HTCC2207,indicating that this species could secrete carbon source available for SAR92.All three diatom lysates significantly promoted the growth of HTCC2207,indicating that carbon sources which can be used by HTCC2207 were ubiquitous in diatoms.In addition,the sugar in the diatom lysate was significantly consumed during the growth of HTCC2207,indicating that the sugar from diatom was one of the carbon sources that HTCC2207 could use.Furthermore,GH family genes specifically in response to laminarin were significantly expressed in HTCC2207 cells with the addition of diatom lysate in consistent with the response of these genes to laminarin addition,indicating that laminarin in the diatom lysate could be efficiently utilized by HTCC2207.The expression levels of several specific GH genes in the diatom lysate were higher than that in the laminarin treatment at 2 h,and then decreased significantly at 4 h,indicating that laminarin in the diatom lysate was preferentially metabolized by HTCC2207.The global distribution of laminarin specific GH genes also indicated that laminarin metabolism might be one of the important ecological strategies of SAR92 in the ocean.In general,this dissertation investigated polysaccharide metabolism strategy of SAR92,clarified the environmental factors affecting the distribution of SAR92 and the relationship between phytoplankton and SAR92,and revealed the specific polysaccharide metabolism ability of SAR92.The key ecological strategies and mechanisms of this group in marine environment were unveiled,which provided a model for future study on the metabolic mechanism and niche of marine unculturable bacteria. |
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