Size-dependent mitigation by humic acid: rescuing metabolic homeostasis from nano-plastics but not micro-plastics in Microcystis aeruginosa

This is the authors' abstract. We don't add key points for this paper.

Microplastics (MPs) are ubiquitous freshwater contaminants, yet their combined effects with humic acid (HA) on phytoplankton remain poorly understood. This study investigated the physiological and metabolic responses of Microcystis aeruginosa exposed to polystyrene microplastics (PS) (0.1 µm and 1.0 µm; 0.5–50 mg/L) with or without HA (5 mg/L). PS exposure promoted chlorophyll a (Chl a) synthesis but induced oxidative stress and enhanced extracellular polymeric substance (EPS) secretion. HA modulated these effects in a size-dependent manner: it further enhanced Chl a synthesis under 1.0 μm PS stress but suppressed it under 0.1 μm PS stress. Furthermore, HA effectively mitigated oxidative stress at high-concentration PS exposures. Loosely bound EPS (LB-EPS) was the most sensitive fraction, exhibiting increased humic- and protein-like fluorescence under co-exposure to micro-sized PS and HA. Metabolomics revealed that nano-sized PS disrupted primary metabolism, whereas micro-sized PS triggered defensive secondary metabolism. HA restored metabolic homeostasis specifically under nano-sized PS stress. Ultimately, HA plays a dual, particle-size-dependent role in regulating PS toxicity, clarifying the ecological implications of MP-organic matter interactions.