Ca(II) alleviates microplastic toxicity to Microcystis aeruginosa via cyanobacteria induced carbonate precipitation

Microplastics (MPs), as a globally emerging contaminant, present significant and increasing threats to aquatic ecosystem health and adversely impact cyanobacterial physiology. Ca(II) are ubiquitous essential ions in natural waters, yet their influence on MPs toxicity to cyanobacteria remains insufficiently understood. This study systematically investigated the mitigating effects and mechanisms of Ca(II) on MPs-induced toxicity in Microcystis aeruginosa through an integrated approach involving toxicological assays, microscopic characterization, and microbiome analysis. Both low (2 mg L⁻¹) and high (10 mg L⁻¹) concentrations of MPs induce oxidative stress and inhibit the growth of M. aeruginosa. However, the addition of Ca(II) significantly mitigated these adverse effects. Mechanistically, Ca(II) promoted cyanobacteria induced carbonate precipitation (CICP), leading to the immobilization of approximately 32.3-34.7 % of MPs, thereby reducing cellular exposure to MPs. Consequently, this attenuated MPs-induced stress on glucosiolate biosynthesis and 2-oxocarboxylic acid metabolism pathways, while mitigating damage to photosynthetic components, including photosystem II (PSII), photosystem I (PSI), and the photosynthetic electron transport chain (PETC). This study provides evidence that Ca(II) protects M. aeruginosa from MPs toxicity by activating the CICP pathway to establish a calcite-based defense system. These findings enhance our understanding of cyanobacteria-MPs interactions under environmentally relevant ionic conditions.