Aged microplastics decrease the bioavailability of coexisting heavy metals to microalga Chlorella vulgaris

Environmental aging of ubiquitous microplastics (MP) occurs through the action of biotic and abiotic factors, and aged MP exhibit different physicochemical properties and environmental behavior from virgin MP. This study aimed to investigate the aged micro-sized polystyrene (mPS) and polyvinyl chloride (mPVC), and the heavy metals copper (Cu) and cadmium (Cd), and examine the effects of their combined toxicities on microalga Chlorella vulgaris. Results showed that the presence of MP inhibited cell growth as compared with the control, the inhibition rate (I) decreased as concentrations of MP rose and aged MP exhibited stronger inhibition of cells than did virgin MP. The largest I was achieved in each culture with the MP concentration of 0.01 g/L, in which aged mPS with the maximal of 36.84% (I_{aged mPS}) followed by aged mPVC (I_{aged mPVC} = 30.03%), virgin mPS (I_{virgin mPS} = 29.10%) and virgin mPVC (I_{virgin mPVC} = 16.72%). Addition of the heavy metals Cu²⁺ and Cd²⁺ significantly inhibited cell growth, and toxicity increased with concentrations in a range of 0.5-2.0 mg/L; the maximum I values were 19.50% (I_Cu) and 85.14% (I_Cd), respectively. The combined toxicity of aged MP + Cu or aged MP + Cd was less than that of individual heavy metals. In particular, as compared with the maximal I_Cd of 85.14% achieved by single Cd²⁺, the toxicity of Cd²⁺ was greatly reduced when combined with aged mPS and mPVC, with the I value decreased to 27.55% (I_{aged mPS}) and 32.51% (I_{aged mPVC}), respectively. Both single and combined treatments caused cell damage to the microalga, accompanied by increased superoxide dismutase (SOD) and intracellular malonaldehyde (MDA) content.