Humic acid can mitigate or magnify nanoplastic toxicity to freshwater microalgae: what are the factors driving these contrasting effects?

Abstract

Given the continuous breakdown of plastic waste in aquatic environments, understanding how nanoplastics (NPs) impact the health of aquatic organisms has drawn global attention. Despite being considered deleterious to aquatic organisms, NPs can interact with natural organic matter, such as humic acid (HA), through their surface charges, possibly altering their toxicity. Thus, using freshwater microalga Chlorella vulgaris as the study species, we assessed whether HA (1–10 mg L⁻¹) affects the toxicity of unfunctionalized polystyrene NPs (PS) with negative surface charges and amine-functionalized polystyrene NPs (NH₂PS) with positive surface charges (1–50 mg L⁻¹) by measuring the biochemical responses of microalgae after 96 h exposure. We found that NH₂PS exhibited higher toxicity to microalgae than PS primarily due to stronger physical interactions, resulting in greater membrane damage. At a high concentration (10 mg L⁻¹), HA alone also induced membrane damage in microalgae. However, when combined with either type of NPs, HA at this concentration can mitigate NP toxicity by reducing oxidative stress due to ROS production and boosting antioxidant activities (e.g., SOD, CAT or GSH). This protective effect of HA was mediated by reducing the direct contact between NPs and microalgae through lowered surface hydrophobicity and enhanced dispersion stability, rather than through settling of microalgae. However, HA at low concentrations (1–5 mg L⁻¹) was unable to entirely reduce the oxidative stress and membrane damage caused by NPs. These results demonstrate that the capacity of HA to alleviate NP toxicity is subject to its concentration and interaction with specific NPs and has a limit beyond which HA can become a stressor. Taken together, this study highlights the significance of surface functionalization and natural organic matter in determining NP toxicity to living organisms, suggesting that the biological impacts of NPs would be more complicated in natural aquatic environments than previously thought.