Synergistic and antagonistic effects of chemical pollutants and parasitic fungi on cyanobacterial metabolism

Freshwater communities are increasingly impacted by anthropogenic pollutants, such as the widely used herbicide metolachlor (MET) and chemicals leached from cigarette butts (CBs), one of the most common types of litter worldwide. Cyanobacteria often interact with their parasitic chytrid fungi, which play a role in controlling their growth. Parasites represent an additional biotic stressor that can alter the effects of pollutants on their hosts. However, the metabolic responses of cyanobacteria to simultaneous exposure to pollutants and parasites remain poorly understood. In this study, we investigated the combined effects of abiotic (MET and CB leachate) and biotic (chytrid parasite Rhizophydium megarrhizum ) stressors on the metabolic response of the toxigenic cyanobacterium Planktothrix agardhii . Co-exposure to MET and chytrids led to a synergistic effect, inducing oxidative stress, countered primarily 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 responses to interacting biotic and abiotic stressors and highlights the importance of considering host-parasite interactions in ecotoxicological assessments of cyanobacteria. A deeper understanding of how pollutants such as MET and CB leachate affect cyanobacteria-chytrid interactions at the metabolic level is crucial for evaluating their broader ecological impacts.