Aqueous phase leachates from pure and commercial microplastics differentially affect the physiology and metabolism of Chlorella sp. MM3.

Microplastic (MP) leachates contain chemically complex mixtures of additives that may adversely affect aquatic primary producers; however, the extent to which polymer identity and additive complexity influence toxicity remains poorly understood. We hypothesised that low-molecular-weight phthalates that are readily released from MPs under controlled leaching conditions serve as indicator compounds of additive release and elicit physiological and metabolic responses in the freshwater microalga, Chlorella sp. MM3. To test this hypothesis, aqueous leachates were prepared from pure (primary) and commercial (secondary) polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC) at loading rates of 100 and 1000 mg L-1. Targeted analysis revealed distinct phthalate profiles among polymer types and between pure and commercial plastics. Exposure to the leachates induced pronounced polymer-dependent responses, with PET and PVC producing greater toxicity than PS. Chlorophyll a declined by up to 47% in pure PET and 42% in commercial PVC, while reactive oxygen species and lipid peroxidation increased by more than three-fold relative to the controls. Metabolomic analyses identified polymer-specific pathway perturbations, with PET disrupting central carbon metabolism, PS enriching glutathione metabolism, and PVC altering amino acid, nitrogen, glyoxylate, and purine metabolism. Integration of multi-level responses showed that pure PET produced the highest physiological disturbance (PII = 0.82), whereas commercial PET elicited the strongest metabolic response (MII = 1.00). These findings demonstrate that polymer identity and additive complexity jointly determine MP leachate toxicity and provide a mechanistic framework for improving ecological risk assessment of plastic-derived contaminants.