Low pH aggravates the toxicity of polystyrene microplastics in crab Eriocheir sinensis: Evidence from metabolome and intestinal microflora

Microplastics (MPs) pollution and freshwater acidification represent emerging threats to aquatic ecosystems. The combined toxic effects of these stressors on crustaceans and their underlying immune-metabolic mechanisms remain poorly understood. This study investigated the individual and combined toxic effects of low pH and polystyrene MPs on the Chinese mitten crab ( Eriocheir sinensis ). Through a 21-day exposure experiment combined with enzyme activity analysis, gut microbiota profiling, and hepatopancreas metabolomics, the findings revealed: (1) Combined exposure to low pH (pH 6.5) and MPs exacerbated oxidative damage and immune suppression, demonstrating synergistic toxicity; (2) MPs alone primarily affected pyrimidine metabolism, whereas combined exposure significantly disrupted the tricarboxylic acid (TCA) cycle and arginine biosynthesis while activating serotonin metabolism; (3) Although the α-diversity of gut microbiota remained stable, its COG functional profile underwent significant alterations. The results indicate that freshwater acidification amplifies the toxic effects of MPs on crustaceans through immune-metabolic interactions, highlighting the urgency of assessing multiple environmental stressors in the context of climate change. The study provides novel mechanistic insights for aquatic ecological risk assessment. • Compared to individual stressors, the combined exposure to low pH and MPs exhibits stronger toxic effects on E. sinensis . • Low pH may exacerbate the toxicity of MPs and produce a synergistic effect. • Single and combined exposures have different impacts on the metabolism of E. sinensis . • Low pH and MPs jointly showed no significant effect on E. sinensis gut microbiota.