From inhibition to recovery: metabolic rebound of Microcystis aeruginosa following pulse exposure to polystyrene nanoplastics.

Nanoplastics (NPs) are an emerging global environmental concern. While extensive research has focused on the toxicological effects of NPs on microalgae, the post-exposure consequences remain largely unexplored. In this study, Microcystis aeruginosa was exposed to 5 and 50 mg/L of polystyrene (PS) for 15 days, followed by a 15-day post-exposure period in PS-free medium. Exposure to 5 mg/L PS did not significantly affect growth, phycobiliprotein content, or oxidative stress responses. In contrast, 50 mg/L PS significantly inhibited algal growth and impaired photosynthetic capacity. Transcriptomic analysis revealed that this toxicity was underpinned by a broad downregulation of central energy metabolism, including photosynthesis, glycolysis, the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation. Notably, following the removal of PS, M. aeruginosa exhibited a remarkable recovery capacity driven by metabolic rebound. This was manifested as a robust upregulation of previously suppressed pathways, particularly the pentose phosphate pathway and oxidative phosphorylation, facilitating a substantial, though potentially incomplete, restoration of physiological function. These findings highlight the metabolic resilience of M. aeruginosa and underscore the importance of considering post-exposure phases in ecological risk assessments of NPs.