The coexistence of Bacillus haynesii and Bacillus tequilensis achieves synchronous algicidal activity and denitrification and enhances the MC-LR degradation of phycosphere microbiota.

The key challenge in current biological technologies for the control of harmful algal blooms (HABs) is that algicidal bacteria's lytic mechanisms during bloom treatment results in structural rupture of algal cells. This results in the massive release of intracellularly stored nutrients (such as nitrogen/phosphorus compounds) and secondary metabolites such as microcystins into aquatic environments, causing secondary pollution. This study isolated two Bacillus strains (Bacillus haynesii and Bacillus tequilensis) from phycosphere microbiota, applying them synergistically against Microcystis aeruginosa. Microbial diversity analysis and metatranscriptomic techniques were employed to investigate their algae-control efficacy. Experimental data revealed a 99.57 % nitrate removal efficiency and 57.49 % algicidal rate after treatment. By day 3 of treatment, algal lysis occurred with simultaneous toxin release, yet final detection confirmed complete microcystin-LR (MC-LR) degradation. Molecular ecological network analysis demonstrated that Bacillus exhibited the highest network connectivity (with a 10-fold higher degree centrality than other groups), indicating enhanced niche stability and environmental adaptability through complex interspecies interactions. On day 2, metatranscriptomics showed peak expression levels of nitrate reductase (Nap), nitrite reductase (Nir), nitric oxide reductase (Nor), and nitrous oxide reductase (Nos) genes. By day 6, significant upregulation of phenylacetic acid degradation (paa) genes was observed, revealing a phased metabolic strategy where phycosphere microbiota prioritised denitrification before MC-LR degradation. Our findings demonstrate that coexistence between Bacillus strains enables simultaneous algal lysis, nitrogen removal, and enhanced MC-LR degradation capacity within phycosphere microbial communities, providing novel solutions for developing environmentally sustainable HAB control technologies.