Heavy metal–microplastic co-exposure modulates toxicity in microalgae: A meta-analysis
Abstract
Background: Microplastics and heavy metals frequently co-occur in aquatic environments, but their joint effects on microalgae remain inconsistent across experimental studies. This uncertainty limits ecological risk assessment for combined pollution because microalgae form the base of many aquatic food webs and contribute substantially to primary production. Methods: We conducted a meta-analysis of combined exposure to heavy metals and microplastics in microalgae. Six databases (PubMed, Web of Science, ScienceDirect, CNKI, CQVIP, and Wanfang) were searched through December 2025. After screening 10,862 records, 28 studies and 1,151 observations were included. Seven physiological response categories were analysed: growth, oxidative response, photosynthesis, pigments, cellular metabolism, membrane damage, and toxin production. Interaction effect sizes were calculated as Hedges' d and pooled using multilevel mixed-effects models with study and response indicator as random effects. Moderator analyses examined heavy metal type, heavy metal concentration and speciation, microplastic polymer type, size and concentration, exposure time, algal phylum, and growth environment. Results: Combined exposure produced significant antagonistic effects on growth (d = -1.73, 95% CI: -3.23 to -0.22, P = 0.024) and oxidative response (d = -1.29, 95% CI: -2.47 to -0.11, P = 0.032). Effects on photosynthesis, pigments, cellular metabolism, membrane damage, and toxin production were not statistically significant. Moderator analyses showed that the direction of interaction depended strongly on exposure context. At low heavy metal concentrations, combined exposure shifted to synergism for growth (d = 3.45, 95% CI: 2.37 to 4.54, P < 0.001) and oxidative response (d = 1.67, 95% CI: 0.68 to 2.66, P = 0.001). Aged microplastics and several polymer types showed strong antagonistic effects for oxidative response, whereas short-term exposure (≤96 h) was associated with antagonism in growth. Random forest models identified exposure time and heavy metal concentration as the strongest predictors for growth, and heavy metal type and microplastic concentration as important predictors for oxidative response. Conclusions: The combined toxicity of heavy metals and microplastics to microalgae is concentration- and time-dependent. High-concentration acute studies tend to capture antagonism, whereas environmentally relevant low-concentration conditions may produce synergistic toxicity. Risk assessment should therefore incorporate concentration-dependent shifts in interaction direction and give greater attention to long-term exposure at environmentally realistic concentrations.
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