Gradient experiment reveals physiological stress from heavy metal zinc on the economically valuable seaweed Sargassum fusiforme

Zn is a common heavy metal pollutant in water bodies and accounts for the largest proportion of heavy metal pollutants in many rivers entering the sea. This study investigated the growth and physiological response characteristics of Sargassum fusiforme under different divalent Zn ion concentration gradients. We observed that low concentration Zn²⁺ treatment (<2 mg L⁻¹) exerted no significant effect on the growth rate, photosynthesis, and nitrogen metabolism-related indicators of S. fusiforme. Treatment with medium to high Zn²⁺ concentrations (2–25 mg L⁻¹) significantly affected the growth rate, photosynthetic activity, nitrogen absorption rate, antioxidant enzyme activity, membrane lipids, and DNA peroxidation damage-related indicators of S. fusiforme. Under medium-to-high concentration treatments, the SOD activity of S. fusiforme decreased with increasing concentration, and the CAT activity increased with increasing treatment concentration. The MDA and H₂O₂ contents increased with increasing Zn²⁺ concentrations. At a Zn²⁺ concentration of 5 mg L⁻¹, the relative conductivity of S. fusiforme significantly increased. Treatment with higher Zn²⁺ concentrations significantly increased the 8-hydroxydeoxyguanosine (8-OHdG) content, poly ADP-ribose polymerase (PARP) activity, and Histone H2AX content of S. fusiforme, thus indicating that Zn²⁺ stress causes DNA damage. All Zn²⁺ concentrations induced mannitol accumulation, and soluble protein content decreased with increasing Zn²⁺ concentration. In summary, we observed that a Zn²⁺ concentration of 2–5 mg L⁻¹ may be the critical value for the response of S. fusiforme to Zn²⁺ stress. Higher concentrations of Zn in the environment can exert toxic effects on the growth, development, and biomass accumulation of S. fusiforme. This study provides a reference for the risk assessment and aquaculture management of seaweeds.