Effect of partial denitrification of water on biofloc system quality for Litopenaeus vannamei

Shrimp aquaculture significantly contributes to greenhouse gas emissions and environmental degradation, mainly through diesel use, organic waste generation, and mangrove deforestation in major producing regions. With an estimated 6.04 ± 0.04 kg of CO₂ emitted per kilogram of shrimp delivered to market, and up to 50% of mangrove loss linked to pond construction, more sustainable farming approaches are urgently needed. Biofloc technology (BFT) offers a promising alternative, improving feed efficiency and reducing water treatment costs. However, nitrate accumulation remains a major challenge in intensive BFT systems without water exchange. This study aimed to optimize an anaerobic denitrification protocol for biofloc-based systems cultivating Litopenaeus vannamei, focusing on nitrate removal, water quality, and microbial dynamics. Phase 1 focused on understanding the dynamics of key genes involved in nitrification and anaerobic denitrification through the qPCR technique. A significant increase in narG expression from hour 32 to 48 was observed during nitrate reduction, while other denitrification genes (nirK, nirS, cNor, nosZ) remained stable. Nitrifying genes like 16S rRNA of Nitrospira sp. decreased under anaerobic conditions, suggesting inhibition of nitrification. Gene concentration throughout the experiment ranged from 10⁷ to 10¹⁰ copies/ml, higher than in previous studies. In Phase 2 of the study, weekly 20% volume exchanges through denitrification reactors using molasses under anoxic conditions led to complete nitrate removal within 96 hours relative to the initial concentrations measured at hour 0. Nitrite remained below toxicity thresholds, and physicochemical parameters (pH, dissolved oxygen, alkalinity, and total suspended solids) remained stable with the help of buffering agents. Phosphate levels were lower in the water of the treatment group, and survival rates averaged 87.6%, with no significant differences in growth or feed conversion between the control and treatment groups. These results confirm that biofloc systems can support complete anaerobic denitrification by maintaining anoxic conditions, supplying an external carbon source (molasses), and fostering microbial communities capable of nitrate reduction, as evidenced by narG gene expression and stable water quality parameters. This highlights the need for continued optimization to improve nitrogen removal efficiency and long-term system stability.