Effects of light intensity and ammonium stress on photosynthesis in Sargassum fusiforme seedlings

During the growth period of economically cultured Sargassum fusiforme, offshore eutrophication and ammonium release from sediments, both induced by high temperatures, significantly affect the photosynthesis and growth of this seaweed. In the present study, S. fusiforme seedlings were treated with various ammonium-nitrogen (NH₄⁺-N) gradients and light intensities to evaluate the effects of these treatments on algal photosynthesis and NH₄⁺-N accumulation. The results showed that a certain concentration of NH₄⁺-N (300 μmol L^-1) increased the photosynthetic electron transport rate (rETR) and maximum photosynthetic rates (Pm) of the S. fusiforme seedlings, but a high NH₄⁺-N (900 μmol L^-1) reduced the light-saturated rETR, the maximum quantum yield, and Pm. Although the seedlings maintained high NH₄⁺-N uptake rates under high ammonium concentration, the glutamate synthase and glutamine synthetase activities were adversely affected, indicating that the absorption and assimilation of NH₄⁺-N were not synchronous. Under low light conditions, high ammonium concentrations significantly inhibited the electron transfer and photosynthetic rates of the seedlings and negatively impacted the photosynthetic pigment synthesis and photosynthetic product accumulation. Sargassum fusiforme seedlings displayed a degree of photosynthetic tolerance to the increased ammonium concentrations. However, excessively high ammonium concentrations caused stress to the S. fusiforme seedlings, and this could inhibit algal growth and even cause death by inhibition of the photosynthetic process. These results indicated the concentration effects of ammonium eutrophication on the marine cultivation of S. fusiforme. It also showed that ammonium tolerance of S. fusiforme could be enhanced by improving the light conditions under high ammonium concentrations.