Summary：

Interactions between bacteria and phytoplankton during bloom events are essential for both partners,which impacts their physiology, alters ambient chemistry and shapes ecosystem diversity. Here, we investigated the community structure and metabolic activities of free-living bacterioplankton in different blooming phases of a dinoflagellate Prorocentrum donghaiense using a metaproteomic approach. The Fibrobacteres-Chlorobi- Bacteroidetes group, Rhodo-bacteraceae, SAR11 and SAR86 clades contributed largely to the bacterial community in the middle-blooming phase while the Pseudoalteromonadaceae exclusively dominated in the late-blooming phase. Transpor ters and membrane proteins, especially TonB-dependent receptors were highly abundant in both blooming phases. Proteins involved in carbon metabolism, energy metabolism and stress response were frequently detected in the middle-blooming phase while proteins participating in proteolysis and central carbon metabolism were abundant in the late-blooming phase. Beta-glucosidase with putative algicidal capability was identified from the Pseudoal-teromonadaceae only in the late-blooming phase,suggesting an active role of this group in lysing P.donghaiense cells. Our results indicated that diverse substrate utilization strategies and different capabili-ties for environmental adaptation among bacteria shaped their distinct niches in different bloom phases, and certain bacterial species from the Pseu-doalteromonadaceae might be crucial for the termination of a dinoflagellate bloom

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