Optimal nitrite degradation by isolated Bacillus subtilis sp. N4 and applied for intensive aquaculture water quality management with immobilized strains.

The toxicity of nitrite is an issue that cannot be overlooked in nitrogen pollution within aquaculture. A highly efficient bacterium capable of simultaneous nitrification and denitrification was screened from natto, and its 16S rRNA gene sequence was compared to existing records, confirming its identification as Bacillus subtilis sp. N4. The optimal conditions for nitrite degradation by B. subtilis sp. N4 were identified using response surface methodology as 167 rpm, pH 6.4, 1 g/8 mL feed, 0.6 OD, and 30 °C, with a predicted 99 % nitrite removal efficiency. The B. subtilis sp. N4 demonstrated a maximum nitrite concentration tolerance of 60 mg/L, with μ and Ks values calculated using a Monod model analysis of 1.67 mg/L/h and 0.29 mg/L, respectively. Immobilized B. subtilis sp. N4 could be reused for ten cycles while maintaining a nitrite degradation efficiency of >99 %, and retained a high nitrite-degrading ability after being refrigerated at 4 °C for three months. Immobilized B. subtilis sp. N4 effectively reduced ammonia nitrogen, nitrite, and nitrate concentrations in Nile tilapia aquaculture, maintaining them at consistently low levels. Therefore, free or immobilized B. subtilis sp. N4, with both nitrification and denitrification capabilities, has considerable potential for application in the aquaculture industry in the future.