Using cell-free supernatant of Bacillus sp. AK3 in combination with Chlorella to remove harmful algal bloom species, TP, TN, and COD from water

The harmful algal bloom (HAB) issue is intensifying and affecting water supplies around the world. The cell-free supernatant (CFS) of algicidal bacteria has been extensively used for HAB control. However, the remaining nutrients in the water may cause the HAB species to bloom again. In this study, we used the CFS of Bacillus sp. AK3 obtained from nutrient broth medium (NB-CFS) and M9 minimal medium (M9-CFS), to eradicate HAB species. Based on extracellular metabolomics data, seven antimicrobial compounds, i.e., cyclo (leu-pro), cyclo (phe-pro), norharman, trans-3-indoleacrylic acid, hypoxanthine, kanosamine, and betaine, were detected in both NB-CFS and M9-CFS, albeit at different intensities. In the treatment of HAB water, NB-CFS and M9-CFS exhibited strong Microcystis and Pseudanabaena (harmful cyanobacteria) growth inhibition, whereas Chlorella (green algae) growth was promoted, indicating a specific inhibitory effect of CFS. The TP, TN, and COD levels in the water significantly increased after adding both CFSs. Interestingly, NB-CFS proved superior in promoting the beneficial Chlorella species, which can completely remove TP (100%) and effectively reduce TN and COD in HAB water at the end of the experiment. The combination of species-specific algicidal compounds with Chlorella may be a feasible strategy for the simultaneous elimination of HAB species and nutrients from water.