Study on the purification effect of immobilized Nannochloropsis oculata on nutrients in greenhouse aquaculture effluent of Penaeus vannamei

Nutrient accumulation and pathogenic Vibrio proliferation in greenhouse aquaculture effluent of Penaeus vannamei pose significant threats to sustainable shrimp production. This study evaluated the remediation efficacy of sodium alginate-immobilized Nannochloropsis oculata on real P. vannamei greenhouse aquaculture effluent under ambient greenhouse conditions. Microalgae harvested during the exponential growth phase were encapsulated via sodium alginate gelation, and the effects of algal bead dosages (0, 1, 2, and 3 g/L) on the removal of inorganic nitrogen species, phosphorus, and total Vibrio were systematically assessed. Results demonstrated dose-dependent enhancement in nutrient removal. NH₄⁺-N was removed most rapidly, with the 3 g/L group achieving 98.08 ± 1.58% removal by day 3, significantly exceeding the control (31.73 ± 2.8%; P < 0.05). PO₄³ ⁻-P removal in all treatment groups surpassed 96% by day 3. NO₃⁻-N and NO₂⁻-N removal exhibited a temporal lag consistent with the sequential nitrogen assimilation hierarchy (NH₄⁺-N > NO₃⁻-N > NO₂⁻-N), reaching 95.44 ± 0.91% and 85.71 ± 1.24% in the 3 g/L group by day 5 ( P < 0.05). TIN and TIP removal rates in the 3 g/L group reached 88.70 ± 1.37% and 83.49 ± 2.25%, representing 3.48- and 4.57-fold improvements over the control, respectively. Additionally, total Vibrio counts in the 3 g/L group decreased to 11.42 ± 0.68 CFU/mL by day 5, compared with 122.20 ± 5.36 CFU/mL in the control ( P < 0.05). Reusability assessments indicated that the system retained high efficiency across two treatment cycles, though bead structural degradation in Cycle 3 compromised performance, suggesting a two-cycle reuse protocol. These findings demonstrate that immobilized N. oculata at 3 g/L achieves optimal effluent purification with dual functionality in nutrient bioremediation and pathogenic bacterial suppression, providing a scientific basis for scalable implementation in greenhouse aquaculture wastewater management.