Cyanobacterial harmful algal blooms (CyanoHABs) are a global concern and pose various hazards to ecosystems and humans. Periphyton is a mixture of heterotrophic and photoautotrophic microorganisms, which effectively mitigated CyanoHABs. However, how periphyton affects free-living (FL) and particle-attached (PA) bacteria and how FL and PA bacterial communities contribute to the control of CyanoHABs remains unclear. Herein, we first characterized the response of FL and PA bacterial communities to periphyton in an outdoor mesocosm setting (1000 L) using high-throughput 16S rRNA gene sequencing data. Pre-incubation of periphyton before inoculation enhanced control efficiency: (1) Periphyton reduced the planktonic chlorophyll-a concentration by 38.6% on day 11; (2) Microcystis and Dolichospermum cell density decreased by 70.8% and 94.8%, respectively. The dissimilarity in microbial interaction networks between the control and treatment was 92.2%, indicating that periphyton changed the bacterial interactions. In response to periphyton, the PA bacterial community exhibited higher stability than the FL bacterial community. Periphyton treatment likely favored the growth of organic matter-degrading bacteria. In addition, Saprospiraceae, Aeromonas, Rhodobacteraceae, and Brevundimonas, which have potential inhibitory effects on cyanobacteria, were enriched in the mesocosm treatment. In summary, our findings suggest that periphyton may mitigate CyanoHABs by altering bacterial communities.

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