Evaluating the potential of Rhodopseudomonas faecalis PA2 as a probiotic and biofloc supplement for Nile tilapia (Oreochromis niloticus)

Supplementing biofloc with probiotics is a promising approach in modern aquaculture, potentially enhancing overall fish health. Identifying new microorganisms with high nutritional value is key to improving aquaculture productivity. In this study, we investigated the effects of the purple phototrophic bacterium Rhodopseudomonas faecalis PA2, as a probiotic alone and as a biofloc supplement, on the growth, proximate composition, blood chemistry, and immune response in Nile tilapia ( Oreochromis niloticus ), as well as on water quality. Its effects were compared with those of Bacillus subtilis (used alone and as a biofloc supplement), conventional biofloc, and clear water treatment (control). Fish treated with biofloc alone exhibited superior final weight, weight gain, and survival, underscoring its superior efficacy. Fish treated with R. faecalis PA2 alone exhibited significantly reduced aspartate aminotransferase, alanine aminotransferase, and total ammonia levels, along with substantial improvements in lipid, carotenoid, antioxidant enzyme, and non-specific immune responses compared with those supplemented with B. subtilis alone. Supplementing biofloc with R. faecalis PA2 increased the protein and carotenoid content, as well as globulin and total protein, compared with their contents in the B. subtilis -supplemented biofloc treatment. Although the growth performance of R. faecalis PA2, whether administered alone or in combination with biofloc, was lower than that of biofloc alone, survival rates still exceeded 80 %, indicating acceptable viability under these conditions. These findings confirm that R. faecalis PA2 can be integrated into Nile tilapia cultures to improve nutritional quality, immune function, and water quality without negatively affecting kidney and liver health. This strain represents a new probiotic that can enhance the sustainability and productivity of aquaculture systems, whether used alone or as a biofloc supplement. • R. faecalis PA2 was investigated as a probiotic for Nile tilapia. • R. faecalis PA2 enhances lipid, carotenoid, and immune responses in fish. • R. faecalis PA2 as a biofloc supplement increases protein and carotenoids in fish. • R. faecalis PA2 may enhance aquaculture productivity and sustainability.