Green under siege: how the polyhexamethylene guanidine disinfectant attacks microalgae in aquatic ecosystems

In aquatic ecosystems, microalgae experience the adverse effects of the widely used disinfectant component, polyhexamethylene guanidine (PHMG). Our research focused on the model alga, <i>Chlorella vulgaris</i>, examining how PHMG exposure influences its growth, photosynthesis, metabolic byproducts, and antioxidant defenses. We observed a range of effects from slight disturbances at low PHMG concentrations to significant growth inhibition at higher levels, suggesting a hormesis-like response. Specifically, PHMG exposure led to reduced chlorophyll content, impaired Photosystem II efficiency, and decreased photosynthetic activity. Interestingly, the algae responded to stress by increasing antioxidant enzyme activities and stress biomarkers. Structural equation modeling (SEM) revealed that PHMG primarily disrupts the photosynthetic apparatus, which in turn affects metabolic and antioxidant responses, culminating in reduced algal biomass. Our results contribute to the understanding of the threats posed to aquatic life by the ubiquitous and ever-increasing pollution from chemical disinfectants, and highlight the urgency of mitigation measures.