Chromium deposition enhances the tolerance of Chlorella vulgaris to microplastics

Microplastics (MPs) and heavy metals (HMs), well-known environmental pollutants, have attracted widespread attention owing to their increasing threats. However, the interactions of MPs and chromium (Cr) at the microscale remain poorly understood, and the effects of environmental transformation on their toxicity remain controversial. The influences of light irradiation on their conversion were investigated using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results indicate that light irradiation enhances the formation of the MP-Cr-MP complex, in which the deposition of Cr is mainly distributed in MP-MP gaps and 15.5 %-16.0 % of Cr was oxidized to the highly toxic hexavalent state. Interestingly, polystyrene (PS) MPs determined the response of Chlorella vulgaris (C. vulgaris), but the MP-Cr-MP complex mitigated the phytotoxicity of both MPs and Cr in terms of the mitochondrial membrane potential (MMP), Fv/Fm, and ultrastructure. The steric hindrance of the MP-Cr-MP complex and the increased production of extracellular polymeric substances explain the decreased toxicity. The upregulated metabolites (e.g., glycine, L-serine, and L-threonine) and downregulated genes (e.g., CHMP6 and AP2S1) in the endocytosis pathway indicated the alleviated toxicity of PS MPs to C. vulgaris in the comparison of the PS+Cr and PS groups. This study highlights the formation of the deposited MP-Cr-MP complex during environmental transformation and the possible overestimation of MP or Cr toxicity when the MP-Cr-MP complex is ignored.