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Dissolved inorganic carbon (DIC) is an important source of carbon in aquatic ecosystems,especially under conditions of increased frequency of cyanobacterial bloom.However,the importance of bacteria in direct or indirect utilization of DIC has been widely overlooked in eutrophic freshwater.To identify the functional bacteria that can actively utilize DIC in eutrophic freshwater during cyanobacterial bloom,stable-isotope probing (SIP) experiments were conducted on eutrophic river water with or without inoculation with cyanobacteria (Microcystis aeruginosa).Our 16S rRNA sequencing results revealed the significance of Betaproteobacteria,with similar relative abundance as Alphaproteobacteria,in the active assimilation of H13CO3-into their DNA directly or indirectly,which include autotrophic genera Betaproteobacterial ammonia-oxidizing bacteria.Other bacterial groups containing autotrophic members,e.g.Planctomycetes and Nitrospira,also presented higher abundance among free-living bacteria in water without cyanobacteria.Microcystis aggregates showed a preference for some specific bacterial members that may utilize H13CO3-metabolized by Microcystis as organic matter,e.g.Bacteroidetes (Cytophagales,Sphingobacteriales),and microcystindegrading bacteria Betaproteobacteria (Paucibacter/Burkholderiaceae).This study provides some valuable information regarding the functional bacteria that can actively utilize DIC in eutrophic freshwater.