Effects of polystyrene microplastics on Sargassum fusiforme: concentration- and size-dependent responses

Microplastics (MPs) pose a growing threat to global marine ecosystems. Sargassum fusiforme, a commercial brown seaweed, remains understudied with respect to its physiological responses to polystyrene MPs (PS-MPs). This study investigated the effects of PS-MPs concentrations and particle sizes on key physiological parameters of S. fusiforme. Results demonstrated that, elevated PS-MPs concentrations and reduced particle size significantly inhibited their growth rate, which declined to 2.295% under 100 mg L^-1, 1 μm PS-MPs and to 2.468% under 50 mg L^-1, 1 μm PS-MPs. Concomitantly, photosynthetic performance deteriorated that the maximum electron transport rate, light-use efficiency, and maximal photochemical efficiency all decreased with most markedly in the MPS1 group. In contrast, nitrate uptake and the activities of nitrogen assimilation enzymes including nitrate reductase, glutamine synthetase, and glutamate synthase, were significantly enhanced, with peak activity observed in both MPC100 and MPS1 treatments. Soluble carbohydrate and soluble protein contents increased progressively with PS-MPs stress intensity, reaching maxima of 8.485 mg g^-1 and 4.826 mg g^-1, respectively. Moreover, malondialdehyde content and antioxidant enzyme activities, including superoxide dismutase and catalase, rose markedly with increasing PS-MPs concentration and decreasing particle size, indicating escalating oxidative stress. Collectively, these findings indicated that PS-MPs suppress S. fusiforme growth primarily by impairing photosynthetic function and inducing oxidative damage. Concurrently, the alga mounted a compensatory response via accumulation of osmoprotective metabolites and upregulation of nitrogen metabolism. The results of this study provided a fundamental mechanism for ecological risk assessment and offer a reference for sustainable management strategies for seaweed farming in plastic-polluted marine areas.