Abstract: In recent years, the frequent occurrence of harmful algal blooms (HABs) has severely damaged freshwater ecosystems worldwide, while efficient in situ bloom control methods are still lacking. Microalgae-inhibiting bacteria are critical in modulating the life cycle of cyanobacterial proliferation, and their enhanced inhibitory capabilities may accelerate bloom decay. This study aims to elucidate the mechanisms by which the photosynthetic bacterium Rhodopseudomonas faecalis JS enhances the suppressive effects of these algae-restraining microorganisms. In this study, the enhancement effect of photosynthetic bacterium Rhodopseudomonas faecalis JS on the algal inhibition ability of in situ bacteria was first investigated through the algal inhibition experiments with indigenous bacteria in eutrophic water bodies. The promotion effect of R. faecalis JS on the algal inhibition ability of different bacterial species was further investigated through the isolation and identification of the pure bacteria from mixed colonies. Bioactive compounds responsible for these enhancements in R. faecalis JS were isolated and identified using ethanol extraction, HPLC and liquid-liquid chromatography. The results showed that the fermentation broth of photosynthetic bacterium R. faecalis JS enhanced the algal inhibition effect of in situ bacterial flora in eutrophic water bodies. Among the 38 isolated pure bacterial strains, six (designated AB-3, AB-6, AB-13, AB-26, AB-31 and AB-38) displayed an increased ability to suppress algal growth when synergistically combined with R. faecalis JS. Notably, strain AB-31 exhibited the most pronounced enhancement, with its algal inhibition efficacy rising from a modest 13.59% to an impressive 81.70%. The phylogenetic identification of these bacteria showed that all bacteria whose algal inhibition was enhanced by the R. faecalis JS belonged to the Pseudomonadota phylum. Given that R. faecalis JS also belongs to the Pseudomonadaceae phylum, this suggests that the enhancement of the bacterial algal inhibition ability by R. faecalis JS is specific at the phylum level. The fermentation broth of R. faecalis JS was extracted with ethanol, separated by HPLC and analyzed by liquid mass spectrometry to obtain 22 potentially active metabolites. Final validation revealed that among the metabolites, N-hexanoyl-DL-homoserine lactone and N-butanoyl-DL-homoserine lactone significantly enhanced the algal inhibitory effect of AB-31 bacterium. This suggests that the photosynthetic bacterium R. faecalis JS may enhance the algal inhibition of bacteria by releasing quorum sensing signaling molecules. The findings of this research reveal that Rhodopseudomonas faecalis JS possesses a remarkable capability to amplify the algae-suppressing properties of indigenous bacterial populations. This discovery introduces novel strategies and conceptual frameworks to address the pressing issue of cyanobacterial bloom management.