Toxicity of microplastics and nanoplastics to benthic Sargassum horneri: The role of nitrogen availability in modulating stress responses

Over the past few decades, the accumulation of micro- and nanoplastics (MNPs) have identified as enduring contaminants, posing significant risks to aquatic organisms. However, the interplay of MNPs and environmental stressors (e.g. nutrient etc.) is not well understood. In this study, Sargassum horneri, a typical benthic macroalgae, was cultured with two sizes of plastic particles (MPs (5 μm), NPs (0.05 μm) and nitrogen concentrations (LN (30 μM), HN (120 μM)) for 20 days to investigate the interactive effects between MNPs and nitrogen levels by measuring different physiological and biochemical parameters. The results demonstrated that both MPs and NPs decrease growth rate, non-photochemical quenching (NPQ), and catalase (CAT) activity, but increased the chlorophyll a and c, carotenoid, and soluble protein contents at low nitrogen level. Notably, the inhibitory effect on growth rate was more pronounced in the NPs conditions. Compared to low nitrogen groups, high nitrogen concentration increased the growth rate, NPQ, the ratio of carotenoids to chlorophyll a, the energy absorbed by each reaction center (ABS/RC), the energy dissipated by each reaction center (DI₀/RC), superoxide dismutase (SOD), and CAT levels at same MPs or NPs treatment, respectively. Meanwhile, there was no significant difference among different sizes of plastic particle treatment groups in high nitrogen conditions. These results imply that NPs may exhibit potentially greater detrimental effects than MPs, when the algae were cultured under low nitrogen conditions. However, increased nitrogen availability appears to alleviate the toxic effects of MNPs by enhancing the algal photoprotective and antioxidant capacities. These findings highlight the potential for nutrient enrichment to mitigate the toxic impacts of micro- and nanoplastics on benthic macroalgae, providing valuable insights into future ecosystem response to increasing MNPs pollution in nutrient-variable coastal environments.