Composition And Metabolic Characterisitcs Of Bacterial Community In Different Marine Phytoplankton Bloom Regimes

In marine ecosystem,the ecological relationship between phytoplankton and bacteria is very complex and important.The interactions between marine phytoplankton and bacteria play essential roles in substance cycle,energy exchange,information transfer as well as structure and function shaping of marine ecosystem.Under some specific conditions,abnormal proliferation or assemblage of phytoplankton cells can form blooms.Phytoplankton blooms can disturb marine environment and change bacterial community structures and their metabolic activities,which will affect bloom process and biogeochemical cycles.At present,most marine phytoplankton and bacteria are still difficult to cultivate.The existing research methods are also very limited.Our knowledge of bacterial composition,metabolic activity and ecological effects under different phytoplankton blooms are still poorly understood,and systematic in studies are still lack.Previous studies are mostly focused on a single specific algal bloom event,but there are few studies on the mixed algal bloom caused by multiple algal species or the bloom caused by the algae with special cell morphology.This dissertation applied a metaproteomic approach to investigate the bacterial community composition and metabolic activities under different phytoplankton bloom regimes.The expressions of high abundant and specific bacterial proteins and their biological processes under different phytoplankton bloom regimes were identified.Combined with the in situ environmental factors,the major bacterial groups and their metabolic characteristics under different phytoplankton blooms were detected.The molecular response mechanisms of the major bacterial groups to different phytoplankton bloom regimes were unviled.The main results of this study were as follows:(1)The composition and metabolic characterisitcs of free-living bacterial community were significantly different in three bloom regimes caused by the mixed dinoflagellate bloom of Prorocentrum donghaiense and Karenia mikimotoi.In the P.donghaiens dominated bloom regime,Pseudoalteromonas and Vitrio occupied a high proportion of the bacterial community;in the K.mikimotoi dominated bloom regime,Alteromonas was the dominant bacterial group,and it also occupied a certain proportion in other bloom regimes;in the bloom dissipation regime,the abundance of CytophagaFlavobacteria-Bacteroides(CFB)group and marine Roseobacter clade(MRC)increased significantly.The major bacterial groups in different bloom regimes had different substrate utilization strategies.Pseudoalteromonas and Alteromonas mainly transported substrates such as arabinose through the TonB-dependent receptor(TBDR)and expressed abundant lyases such as peptidases,proteases and glycoside hydrolases;Vibrio and MRC mainly transported substrates such as amino acid through the ATP binding cassette(ABC)transporters;while CFB group used high molecular substances via highly expressing RagB/SusD proteins in the bloom dissipation regime.The protein expressions of the major bacterial groups involved in the biogeochemical cycle of elements in different bloom regimes were also different.In addition,diverse expressions of chemotactic proteins,quorum sensing proteins,signal transduction and recognition proteins,flagellin,and cell adhesion-related proteins from the major bacterial groups in different bloom regimes might be an important mechanism for their rapid response to bloom regime shifts.(2)The composition and metabolic activities of free-living bacterial community presented significant difference in the non-bloom stage,colony blooming stage and late colony bloom stage of Phaeocystis globosa.In the non-bloom stage,Cyanobacteria was the dominant bacterial group mainly composing of Synechococcus.MRC dominated the bacterial community during the colony blooming stage with a large number of P.globosa colonies.The abundance of CFB group increased significantly in the late colony bloom stage.During the colony blooming stage,a large number of P.globosa colonies might cause significant changes of in situ environmental substrates and reduce the readily available substrates in the surrounding water.During this bloom stage,the expression of bacterial transporters decreased,and the expression of key proteins involved in carbon restriction response and gluconeogenesis pathway increased.Some bacterial groups that can use special substrates were more adapted to this bloom stage.For example,the MRC could utilize aromatic compounds through theβ-ketoadipate pathway and further utilize acetate to meet their carbon source requirement.(3)Trichodesmium bloom in the oligotrophic sea area had little effect on the in situ bacterial community.In the bloom and non-bloom regime,the dominant heterotrophic bacterial groups from the particle-attached and free-living size were similar.Alteromonas occupied a certain proportion in both regimes and more actively in the bloom regime.The metabolism of Trichodesmium was very active in both freeliving and particle-attached size fractions in the bloom regime.While the autotrophic bacteria Prochlorococcus in the free-living and particle-attached size fractions were metabolically active in the non-bloom regime.Alternomonas might interact with dominant autotrophic cyanobacteria in both the bloom and non-bloom regimes.High expression of catalase from Alteromonas might contribute to the removal of oxygen free radicals in Prochlorococcus in the non-bloom regime,while its alkaline phosphatase and iron transport related proteins were highly expressed in the bloom regime which could promote the utilization of the essential elements required by Trichodesmium.In two different regimes,the dominant autotrophic bacterial groups,Trichodesmium and Prochlorococcus,showed significant differences in highabundance proteins.Trichodesmium highly expressed carbon assimilation and photosynthesis-related proteins in the bloom regime,while high expression of free radical removing enzymes and transporters from Prochlorococcus in the non-bloom regime were observed.These results indicated that different autotrophic bacterial groups had unique metabolic characteristics in the oligotrophic environments.(4)Some bacterial groups could adapt to multiple phytoplankton bloom regimes and showed growth advantages,such as Alteromonas and MRC.While some other bacterial groups were more adapted to a specific bloom regime,such as Pseudoalteromonas.Blooms caused by phytoplankton species that can form special cell morphology might have an impact on the in situ substrates.Some bacterial groups that can use unique substrates were more likely to have an advantage in such bloom regimes.The metabolism of the major bacterial groups in different phytoplankton bloom regimes was different,and they exhibited unique metabolic activities.This might be the result of the long-term co-evolution of certain bacterial groups with specific phytoplankton bloom species,which helped them quickly respond to the specific bloom regime and became the dominant species to occupy different ecological niches.