Effects of Different Carbon Sources on Water Quality, Physiology, Growth, and Intestinal Microbiota in Cultured Sea Cucumber (Apostichopus japonicus) With Biofloc Technology

The biofloc technology (BFT) in sea cucumber (Apostichopus japonicus) farming utilizes nutrient cycling, driven by heterotrophic microorganisms, to improve water quality and promote sustainable aquaculture. This study explored the potential of sodium bicarbonate and sodium acetate as novel carbon sources, which may offer advantages in maintaining water stability and enhancing microbial diversity compared to traditional carbon sources. Juvenile sea cucumbers were cultured in a 64-day trial and assigned to five treatment groups: S (sucrose), SSa (S + sodium acetate), SSb (S + sodium bicarbonate), Nc (no carbon source), and Cg (control group), each treatment group was repeated three times (n = 3). The inclusion of sodium bicarbonate and sodium acetate significantly improved water quality parameters. The group SSb exhibited a higher pH (8.28), and significantly lower ammonia nitrogen (0.501 mg L⁻¹), and nitrite levels (0.031 mg L⁻¹) compared to other groups. Additionally, the biofloc volume (BFV; 17 mL L⁻¹), total suspended solids (TSSs; 238 mg L⁻¹), and protein content (13.9%) in the group SSb were all within the tolerance range for sea cucumbers. Group SSb also showed the significantly highest weight gain rate (WGR; 78.5%) and specific growth rate (SGR; 0.91%), alongside increased enzyme activities, including superoxide dismutase (SOD; 179 U mL⁻¹) and peroxidase (POD; 1.59 U mL⁻¹), which were significantly higher than those in other groups. Furthermore, the group SSb demonstrated enhanced gut microbiota diversity, characterized by a higher abundance of beneficial Bacillus and lower levels of harmful Vibrio and Pseudomonas. The addition of SSb significantly improved water quality by maintaining optimal pH and reducing harmful nitrogen compounds, which in turn supported enhanced physiological growth and increased gut microbiota diversity, indicating its potential as a superior carbon source in biofloc systems.