Abstract: The effects of light intensity on the growth and photosynthetic characteristics of Sargassum horneri, a golden tide causing species, were investigated under different nitrate (NO₃^--N) levels to reveal the physiological mechanism underlying the outbreak of golden tide. There were two NO₃^--N levels and three light intensities: i.e. high N (100 μmol·L^-1), low N (50 μmol·L^-1, the normal NO₃^- concentration in natural seawater), high-light (150 μmol photons·m^-2·s^-1), medium-light (60 μmol photons·m^-2·s^-1) and low-light (10 μmol photons·m^-2·s^-1). The growth, pigment concentration, soluble sugar concentration, soluble protein concentration, nitrogen absorption rate and the maximum photosynthetic oxygen evolution rate (P_max) of S. horneri thalli were measured after they had acclimated to different NO₃^--N and light levels for seven days. The results showed that the growth rate, soluble sugar concentration, soluble protein concentration, nitrogen absorption rate, and P_max ofS. horneri thalli all increased with increased light intensities, while the pigment concentration decreased with the increasing light intensities. For the lowN acclimated S. horneri thalli, except for carotenoids, all the above mentioned indicators under the high light treatment were significantly higher than those under the low light treatment (P<0.05). For the highN acclimatedones, except for the soluble sugar, all the indicators under high light treatment were significantly higher than those under the low light treatment (P<0.05). For the S. horneri thalli acclimated under same light intensities, pigment concentration, soluble protein concentration, nitrogen absorption rate andP_max increased with the increasing NO₃^--N level, while the soluble sugar concentration decreased with the increasing NO₃^--N level. For the S. horneri thalli acclimated to the three light conditions, the P_max under the highN treatment was significantly higher than that under the lowN treatment. In conclusion, after seven days acclimation to different light intensities and NO₃^--N levels, S. horneri thalli showed higher potentials in growth and photosynthesis under the condition of high light intensity and high nitrogen level. Therefore, eutrophication in seawater would aggravate the occurrence of golden tide in that S. horneri thalli can obtain higher light intensities when they break from the stems and drift on the sea surface, which would stimulate the potentials of growth and photosynthesis.

Key words: Vicia faba, Soil bulk density, Soil compaction, Seed production, Root rot