This study explored the impact of combining biofloc technology with microalgae on the growth and immune gene expression of red tilapia (Oreochromis sp.). Four experimental treatments were set up: C (Biofloc only), T1 (Chlorella vulgaris and Nannochloropsis sp. in a 1:1 ratio), T2 (Biofloc plus Chlorella vulgaris and Nannochloropsis sp., 1:1), and T3 (Biofloc plus Chlorella vulgaris and Nannochloropsis sp., 2:1), all in 500L fiberglass-reinforced plastic (FRP) tanks over 60 days, with each treatment in triplicate. The incorporation of microalgae significantly enhanced water quality, particularly in T2 and T3, which showed the lowest ammonia and nitrite levels. The control treatment (C) had the highest volumes of floc and sludge. T3 recorded the highest chlorophyll a and b concentrations, while T2 had the highest carotenoid content. Bacterial counts were highest in T3, and algal counts were highest in T1.
Microalgae also contributed to better fish growth. T2 showed the best results, with the highest weight gain (142 ± 0.7g), specific growth rate (SGR: 1.61 ± 0.02), and the lowest feed conversion ratio (FCR: 1.79 ± 0.009). In terms of immune gene expression, T2 had the highest upregulation of cathepsin L (CTSL, 7.8-fold), toll-like receptor 7 (TLR7, 3-fold), interleukin-1 β (IL-1β, 6.7-fold), tumor necrosis factor-alpha (TNF-α, 4.7-fold), and metallothionein (MT, 2.8-fold) in the intestine. In the head kidney, MT expression was most elevated in T3 (7.2-fold), while TNF-α and TLR7 were most upregulated in T2 (5.9-fold and 5-fold, respectively). The liver gene expressions were highest in T3, with 6.4-fold upregulation of MT, 5-fold for CTSL, 2.7-fold for TLR7, 3-fold for IL-1β, and 5.4-fold for TNF-α.
The findings demonstrate a synergistic effect between algae and bacteria, leading to enhanced immune and antioxidative responses in red tilapia. Incorporating microalgae into biofloc systems proves to be a beneficial approach to improving fish health and performance, presenting a viable strategy for sustainable aquaculture using biofloc technology.