Particle-associated exposure of microcystin-LR in Daphnia magna

Ecological risk assessments of microcystin-LR (MC-LR) predominantly rely on freely dissolved exposure, despite growing evidence that ingestion-based, particle-associated pathways can substantially alter toxin bioavailability and toxicity. This study investigated how the toxicity of MC-LR changes when exposure occurs via particulate carriers, using polystyrene (PS) particles as a well-defined model particulate system. Four exposure scenarios–control, PS alone, freely dissolved MC-LR, and MC-LR–adsorbed PS particles–were established to disentangle intrinsic particle effects from particle-associated toxin delivery in Daphnia magna. Freely dissolved MC-LR induced mild oxidative responses, whereas MC-LR associated with PS elicited the most pronounced and integrated biological responses, including reduced survival and fecundity, altered energy metabolism, and broad transcriptional activation of antioxidant, detoxification, transport, and oxygen-transport pathways. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses of gut-recovered particles revealed nitrogen- and oxygen-containing functionalization, consistent with surface-associated transformations during gut passage. Multivariate analysis further demonstrated that particle-associated exposure produced a distinct biological response profile compared with freely dissolved MC-LR. These results demonstrate that MC-LR toxicity is strongly dependent on exposure form, and that particle-associated delivery–represented here by PS particles as a model carrier–can modify apparent MC-LR potency by promoting ingestion-based exposure and prolonged gut residence. These findings highlight the need to incorporate particle-associated cyanotoxin exposure into ecological risk assessment frameworks.