Screening, condition optimization and preliminary application of nitrite degrading Bacillus velezensis GXMZU-B1

To address the problem of elevated nitrite level in high-density intensive aquaculture process, and obtain probiotics that can effectively degrade nitrite, we sampled shrimps from the shrimp pond in Fangchenggang, Guangxi. First, the strains with the best nitrite degradation effect were selected by preliminary screening on methylene blue plate and calcium carbonate medium plate, and secondary screening on nitrite and nitrate medium. Then the strains were identified by morphology, physiological and biochemical characteristics as well as 16S rRNA phylogenetic tree. Furthermore, the fermentation temperature, initial pH and inorganic salts of the strains were optimized. Finally, the safety of the strain and the nitrite degradation capacity in high-concentration nitrite aquaculture tailwater by the strain were studied. The results show that the strain GXMZU-B1 with a degradation efficiency of 98% was obtained from the 53 strains which were from the preliminary screening, and was identified as Bacillus velezensis GXMZU-B1. The optimal nitrite degradation conditions of the strain GXMZU-B1 were 30 ℃, pH 6−7, and no addition of inorganic salt ions. Under these conditions, the degradation rate of nitrite peaked after 12 h, up to 98% in 0.5 g·L⁻¹ NaNO₂. The strain was used in the aquaculture tailwater with nitrite concentration (28.73±1.08) mg·L⁻¹, and the tailwater nitrite could be completely degraded after 24 h. The safety test of the strain shows that the strain did not produce hemolytic circle on the blood AGAR plate. The drug sensitivity test shows that the strain had awesome antibiotic sensitivity. In summary, B. velezensis GXMZU-B1 can be used in aquaculture to keep the content of nitrite low and maintain the normal growth of aquatic animals. It provides a new way to solve the problem of nitrite accumulation in the process of high-density aquaculture, which has significant economic and ecological values.