Effects of compound probiotics on the growth of Conger myriaster, water quality parameters, and microbial communities in industrial recirculating aquaculture systems

The aim of this study was to determine the effects of adding compound probiotics to culture water on fish growth, water quality parameters, and microbial communities in industrial recirculating aquaculture systems (RAS) of Conger myriaster. Two systems were established: a control group system without any compound probiotics and a test group system with compound probiotics added to the culture water (1.0 × 109 CFU/mL of viable bacteria, a dosage of 8 mL/m3, and a frequency of 1 time/7 d). The addition of compound probiotics to culture water in industrial RAS significantly improved the growth performance of C. myriaster (P < 0.05). Additionally, it significantly reduced the levels of ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), nitrite nitrogen (NO2−-N), total nitrogen (TN), and chemical oxygen demand (COD) in the culture tanks and biofilters in the test group on day 70 (P < 0.05). It also increased the average removal rates of NH4+-N, NO3−-N, NO2−-N, TN, COD, total phosphorus (TP), and PO43--P in the culture tanks and biofilter water in the test group. The relative abundance of beneficial bacteria related to water purification was significantly increased in the culture water, biofilter water, and biofilter carriers in the test group at the end of the experiment (P < 0.05). Furthermore, the compound probiotics significantly reduced the influence of environmental factors on the microbial community in the water of the test group. Therefore, it played a dual role in regulating water quality and bacterial community structure. The bacterial communities in the water of the culture tanks and biofilters of the test group were mainly involved in functional roles such as chemoheterotrophy, aerobic chemoheterotrophy, and nitrite respiration. Overall, adding compound probiotics to culture water significantly improved the C. myriaster growth and water quality parameters, regulated the structure and function of the microbial community in the RAS, and promoted the degradation of nitrogenous compounds and organic matter.