Nitrogen removal characteristics of Lactobacillus paracasei HHWX6 and construction of rice husk biochar-based microbial microcapsules for enhanced nitrogen removal in aquaculture waters

Nitrogen accumulation is a major cause of water quality deterioration and ecological risks in aquaculture systems, highlighting the need for efficient and sustainable nitrogen removal technologies. In this study, a strain designated HHWX6, capable of heterotrophic nitrification and aerobic denitrification (HNAD), was isolated from a shrimp farming pond and identified as Lactobacillus paracasei based on 16S rRNA gene analysis. The strain exhibited strong nitrogen removal capability, achieving over 97.01% removal of both NH₄ + -N and NO₂ - -N under single nitrogen-source conditions. To enhance bacterial survival and treatment performance, NaOH–Mg 2+ co-modified rice husk biochar-based material was incorporated into alginate–chitosan microcapsules to construct rice husk biochar-based microbial microcapsules (RHBMM). The optimized microcapsules maintained nitrogen removal efficiencies above 80% across a broad range of temperature (22–40 °C), pH (7–9), C/N ratio (5–20), and salinity (20–50‰), while retaining high activity after 60 d of storage or five reuse cycles. Structural characterization demonstrated that the porous biochar matrix promoted bacterial colonization and proliferation. In practical aquaculture water validation, both the 4% RHBMM and 8% RHBMM demonstrated significantly better TN removal performance than the free bacterial strain HHWX6. Within 24 h, 4% RHBMM and 8% RHBMM achieved 100% TN (3.50 ± 0.03 mg/L) removal in aquaculture pond water; for TN in aquaculture wastewater (21 ± 0.06 mg/L), denitrification rates of 93.20% and 100%, respectively, were achieved within 72 h. These results indicate that RHBMM has the potential to enhance nitrogen removal performance in aquaculture waters.