Multifactor meta-analysis reveals context-dependent inhibitory efficiency and mechanisms of allelochemicals against Microcystis aeruginosa and concurrent microcystin-pollution risk.

Microcystis aeruginosa as prevalent bloom-forming cyanobacterium deteriorates water quality and excretes microcystins (MCs) to pose notable eco-risk. Allelochemicals are promising algicides, yet inconsistent research settings (allelochemical concentration, type, action time, action mode) lead to debates on their impacts on M. aeruginosa growth and MC-pollution. By integrating 2124 data points from 121 articles, this study proposed multifactor meta-analysis approach integrating univariate subgroup with multifactor interaction analyses, which fundamentally differs from conventional approaches and enables simultaneous quantification of multiple contextual outcomes to quantitatively assess context-dependent allelochemical effects on M. aeruginosa growth, physiology and MC-production/release, and confer mechanistic insights. Results revealed that allelochemicals inhibit M. aeruginosa growth with overall effect size of -1.24 (inhibitory effect), and high-level heterogeneity among articles reflected context-dependence of allelochemical effect. Ultrahigh concentration (>100 mg/L) and long action time (10-15 days) were verified to most strongly inhibit M. aeruginosa growth and MC-release. Higher concentrations and longer time further disrupted photosynthesis, antioxidant activation and MC-production and -release, thus intensifying growth inhibition. Compared to single-time addition, sustained continuous addition/release more effectively inhibited M. aeruginosa growth by more strongly suppressing antioxidant activation, photosynthesis and MC-production and release. Among allelochemical types, terpenoids compounds exert the strongest growth-inhibition without decreased MC-release, and phenolic compounds decreased MC-production/release when inhibiting growth, acting as safe algicides. Yet, nitrogen-containing and aliphatic compounds moderately restrained growth but slightly boosted MC-release, raising secondary pollution risks. Allelochemical concentration and action time jointly induced distinct nonlinear growth response across allelochemical types, with significant concentration-time nonlinearity observed for phenolic, aliphatic and terpenoids compounds, and exerted divergent nonlinear effects on MC-pollution dynamics: MC-production responded nonlinearly to phenolic and aliphatic compounds, while terpenoids caused no significant nonlinear variation in either MC-production or MC-release. This study provided novel insights for selectively coordinating allelochemical parameters for efficient M. aeruginosa blooms and MC-pollution control, which offered solid application support.