Ecotoxicological impact of virgin and environmental microplastics leachate on Chlorella vulgaris: Synergistic microbial-pollutant drivers cripple photosynthesis.

Microplastics (MPs, < 5 mm) leachate poses significant threats to aquatic ecosystems; however, their toxicity across different sources remains poorly understood. This study examined the toxicological effects of leachates derived from virgin MPs (VMPs) and environmentally weathered MPs (EMPs) on Chlorella vulgaris in seawater. EMPs leachate exhibited 2.5-3.4 times higher toxicity than VMPs leachate, with growth inhibition rates reaching 77.66 ± 10.25 % and 32.64 ± 6.99 %, respectively. EMPs leachate exposure induced more pronounced disruptions to algal photosynthesis than VMPs leachate, including a 35.3 % reduction in chlorophyll a content and significant downregulation of photosynthesis-related genes (e.g., PsbS, PsbY). EMPs leachate also altered carbon metabolism (59 differentially expressed genes (DEGs)) and elevated oxidative stress markers, evidenced by a 22.2 % increase in malondialdehyde (MDA) compared to VMPs leachate. In contrast, VMPs leachate primarily affected amino acid biosynthesis (44 DEGs). Crucially, EMPs leachate introduced 580 unique bacterial taxa and anthropogenic pollutants (e.g., ciprofloxacin), which synergistically exacerbated algal toxicity through nutrient competition and oxidative damage. These findings highlight the heightened ecological risks posed by EMPs due to synergistic interactions among pollutants and microorganisms, emphasizing the urgent need for targeted regulatory strategies to mitigate MPs pollution.