Simultaneous inhibition of Microcystis growth, toxin production, and release by a high-efficacy algicidal bacterium Aeromonas sp. N3

AIMS: This study aimed to isolate an algicidal bacterium able to strongly inhibit Microcystis growth and microcystins (MCs) production/release simultaneously, and to consolidate bacterial efficacy for potential practice. METHODS AND RESULTS: Algicidal bacterium Aeromonas sp. N3 effectively inhibited growth of 105 [non-quorum sensing (QS)-activated] and 108 cells/mL (QS-activated) Microcystis by 98.95% and 90.22% by day 26, respectively, and sharply decreased extracellular and intracellular MCs by > 90% for both densities, as detected by enzyme-linked immunosorbent assay. Quantitative polymerase chain reaction revealed algicidal effects involved stably strong downregulation of algal energy acquisition, nutrient assimilation, DNA repair, and heat-shock response. N3 exerted algicidal effect via excreted algicidal compounds, as its primary filtrates also stably suppressed Microcystis growth and MC-production/release with inferior efficacy than free-living N3, while its secondary filtrate with pre-exposure to Microcystis showed weaker efficacy. To consolidate N3's application potential, its immobilized agent was optimally constructed by dropping procedures with orthogonal tests using various polyvinyl alcohol-sodium alginate ratios. Alike to free-living N3, immobilized agent with equivalent cell loading posed equally strong inhibitory effects on Microcystis growth (> 90% inhibition) and extracellular/intracellular MCs content (> 95% inhibition), thus diminishing MC-pollution by blocking MC-synthesis/release, which is superior to by degrading aqueous MCs after release. CONCLUSIONS: Aeromonas sp. N3 had functional robustness and stability against Microcystis growth and MC-pollution, regardless of Microcystis density, exhibiting robust potential for mitigating both low- and high-density toxic Microcystis-dominated cyanobacterial blooms (corresponding to early- and mature-stage MCBs). Optimal construction of immobilized agent further consolidated its applicability in effective MCBs and MC-pollution mitigation.