Allelopathic inhibition of Oocystis borgei on Microcystis aeruginosa: Mechanisms and identification of the predominant allelochemicals

The increasing frequency of blue-green algae blooms has exerted significant detrimental impacts on aquatic ecosystem health. This study investigated the allelopathic effects of Oocystis borgei filtrate (at concentrations of 0 %, 25 %, 50 %, 75 %, and 100 %) on Microcystis aeruginosa and identified key inhibitory compounds. Our results demonstrate that O. borgei filtrate significantly inhibited M. aeruginosa growth, as indicated by reduced cell density, Chl-a content, and photosynthetic efficiency metrics (F_v/F_m and Y(II)). Mechanistic analyses demonstrated that the filtrate disrupted cell membrane integrity, thereby inducing intracellular toxin release and altering cellular composition through a concomitant decrease in protein-like substances and increase in humic-like substances. GC–MS analysis identified multiple bioactive allelochemicals in the O. borgei filtrate, including phenolic compounds (3,5-di-tert-butylphenol, 2,4-di-tert-butylphenol), phenol derivatives (2,2′-methylenebis(4-methyl-6-tert-butylphenol)), and esters/phthalates (erucamide, tributyl citrate, dibutyl phthalate, di(2-ethylhexyl) phthalate, and diisooctyl phthalate). Notably, 3,5-di-tert-butylphenol exhibited the strongest inhibitory effect (7-day EC₅₀ = 3.61 mg/L, IQ =1.83). These findings underscore the capacity of O. borgei-derived allelochemicals to suppress harmful M. aeruginosa growth, offering a species-specific allelopathic strategy to precision-target harmful Microcystis blooms.