Cyanostatic Potential of an Oleaginous Chlorella vulgaris Strain Against the Bloom-Forming Cyanobacterium Microcystis aeruginosa

Interactions between cyanobacteria and eukaryotic algae are traditionally viewed as favoring cyanobacteria, which are often the dominant competitors in freshwater ecosystems. However, increasing evidence suggests that eukaryotic algae also possess effective chemical defenses capable of suppressing cyanobacterial proliferation. In this study, the anti-cyanobacterial potential of a methanolic biomass extract of the green microalga Chlorella vulgaris against the bloom-forming cyanobacterium Microcystis aeruginosa was investigated. Optical density measurements revealed the inhibition of cyanobacterial growth, with extract concentrations of 0.4–1.6 mg mL−1 significantly reducing biomass accumulation. The lowest concentration (0.2 mg mL−1) stimulated the accumulation of phycobiliproteins, whereas concentrations of 0.8–1.6 mg mL−1 significantly decreased phycobiliprotein content, indicating progressive physiological impairment. The results suggest that the highest extract concentration applied may also cause alteration in phosphate acquisition. Lipid profiling of the Chlorella biomass showed that 97% of the extractable lipids consisted of methanol-soluble fatty acids, with C18 fatty acids predominating among the 20 identified compounds. These metabolites are reported to possess allelopathic activity and therefore represent plausible contributors to the observed inhibition. These findings support that anti-cyanobacterial activity may represent a widespread ecological trait of Chlorella and potentially other green microalgae, providing a promising foundation for environmentally friendly cyanobacterial bloom management.