From macro-to microplastics: marine microalgae drive polystyrene degradation via EOM-mediated photochemical aging

The process of degradation and fragmentation of plastic into microplastics (MPs) by algae has been rarely addressed. Here we reported two marine algae capable of degrading polystyrene (PS) characterized as Pseudochloris wilhelmii K1 and Jaaginema sp. S1. Their PS degradation activity was confirmed by changes in plastic weight, chemical groups, and physical properties. K1 and S1 generated peak microplastic concentrations of 1.06 × 10⁷ and 2.23 × 10⁷ particles/L in light incubators, versus 7.33 × 10⁶ and 1.07 × 10⁷ particles/L under natural light. The results show that light exerted obvious influences on microplastic generation. The algal extracellular organic matter (EOM), particularly the humic acid-like fraction, significantly enhanced the photochemical aging of PS-MPs under natural sunlight irradiation. This aging process of MPs was predominantly mediated by reactive oxygen species (ROS) produced by marine algal EOM, leading to 3.3–3.8 % weight loss, MPs changes including increased oxygen-containing functional groups, and a significant reduction in particle size. Therefore, the widespread marine algae unprecedently participate in plastic degradation, fragmentation and aggregation, thereby interfering with the fate of plastic debris in marine environments.