Understanding microplastic aging driven by photosensitization of algal extracellular polymeric substances

The aging of microplastics (MPs) is extremely influenced by photochemically-produced reactive intermediates (PPRIs), which are mediated by natural photosensitive substances. Algal extracellular polymeric substances (EPS) can produce PPRIs when exposed to sunlight. Nonetheless, the specific role of EPS in the aging process of MPs remains unclear. This work systematically explored the aging process of polystyrene (PS) MPs in the EPS secreted by Chlorella vulgaris under simulated sunlight irradiation. The results revealed that the existence of EPS accelerated the degradation of PS MPs into particles with sizes less than 1 μm, while also facilitating the formation of hydroxy groups on the surface. The release rate of dissolved organic matter (DOM) from PS MPs was elevated from 0.120 mg∙L-1∙day-1 to 0.577 mg∙L-1∙day-1. The primary factor contributing to the elevated levels of DOM was humic acid-like compounds generated through the breakdown of PS. EPS accelerated the aging process of PS MPs by primarily mediating the formation of triplet excited states (3EPS⁎), singlet oxygen (1O2), and superoxide radicals (O2·−), resulting in indirect degradation. 3EPS⁎ was found to have the most substantial impact. This study makes a significant contribution to advance understanding of the environmental fate of MPs in aquatic environments impacted by algal blooms. In aquatic environments, microplastics (MPs) and microalgae often coexist, as the former is frequently enriched by the latter to form aggregates. The toxicity of MPs to microalgae is influenced by changes in their physicochemical properties. This study demonstrates that extracellular polymeric substances exposed to sunlight can generate photochemically-produced reactive intermediates, which play a significant role in changing the physicochemical properties of MPs. The leachates and dynamic changes in the physicochemical properties of MPs may cause unpredictable toxic impacts on microalgae. This study provides a theoretical foundation for further research on the potential toxicity of microplastics to aquatic organisms.