Synergistic and Antagonistic Effects of Chemical Pollutants and Parasitic Fungi on Cyanobacterial Metabolism.

Freshwater ecosystems are increasingly impacted by anthropogenic pollutants, including the widely used herbicide metolachlor (MET) and cigarette butt (CB) litter. Parasites represent an additional biotic stressor that can modulate pollutant effects on their hosts. Cyanobacteria are key primary producers in aquatic ecosystems, and alterations in their metabolism can cascade through trophic webs, affecting ecosystem functioning. However, the metabolic responses of cyanobacteria to simultaneous exposure to pollutants and parasites remain poorly understood. In this study, we investigated the metabolic response of the toxigenic cyanobacterium Planktothrix agardhii using a multi-biomarker approach under simultaneous exposure to abiotic (MET and CB leachate) and biotic (the chytrid parasite Rhizophydium megarrhizum) stressors. Co-exposure to MET and chytrids led to a synergistic effect, inducing oxidative stress, primarily countered by non-enzymatic mechanisms, whereas MET alone had no measurable effect. In contrast, CB leachate alone induced oxidative stress, but this effect was mitigated when cyanobacteria were also infected by chytrids, indicating an antagonistic interaction. This study demonstrates the complexity of cyanobacterial metabolic responses to interacting biotic and abiotic stressors and highlights the importance of incorporating host-parasite interactions in ecotoxicological assessments. Metabolic alterations induced in toxigenic cyanobacteria by multiple stressors may influence bloom formation and trophic energy transfer, with broader ecological implications in freshwater ecosystems.