Combined Addition of Microalgae and Probiotic Enhances Bacterial Community Network Stability, Water Quality, and Fish...

Microalgae and probiotics are increasingly considered a promising approach for addressing environmental problems in freshwater aquaculture. However, how this approach affects the resident free-living (FL) and particle-associated (PA) bacterial communities that are essential in biogeochemical processes remains unclear. Here, we established a 60-day mesocosm aquaculture system for Micropterus salmoides to examine the effects of microalgal and probiotic addition on fish growth, water quality, and resident bacterial communities. Four treatments were included: Chlorella vulgaris addition (6.25 × 10⁶ cells/L), Bacillus subtilis addition (7.5 × 10⁷ CFU/L), their combined addition, and a control. Our results indicated that the combined addition significantly improved fish growth performance and reduced total and inorganic nitrogen concentrations than single additions. Moreover, the combined addition enhanced nitrogen- and carbon-related metabolic functions in FL fraction and increased host-associated functions in PA community. Taxa assigned with the classes Bacilli, Bacteroidia, Gammaproteobacteria, and Alphaproteobacteria showed predominant explanations for variations in nitrogen and phosphorus. Furthermore, the combined addition enhanced potential interactions among taxa across lifestyles and strengthened the bacterial community network complexity and stability. These findings provide new insights into the microbial mechanisms underlying microalgae-probiotic synergism and offer a scientific basis for sustainable microbial management in freshwater aquaculture.