Abstract: The frequent occurrence of golden tides caused by Sargassum horneri in China’s coastal areas has brought negative impacts on the nearshore ecosystems and coastal economy. Eutrophication of seawater is considered an important inducement for the formation of golden tides. Focusing on two environmental factors (light intensity and inorganic phosphorus concentration) during golden tide outbreaks, we set three light intensities (LL, 10; ML, 60; and HL, 150 μmol photons·m^-2·s^-1) and two inorganic phosphorus concentrations (LP, 0.5 and HP, 50 μmol·L^-1), to explore their combined effects on photosynthetic physiology of S. horneri. The results showed that under low and medium light conditions, high phosphorus concentration did not affect the relative growth rate (RGR) and photosynthesis of S. horneri. Under high light conditions, the RGR of S. horneri in high phosphorus level reached a maximum of 3.51%·d^-1, which was 53.87% higher than that in low phosphorus level. High phosphorus level significantly increased the maximum net photosynthetic rate (P_max), dark respiration rate (R_d), and photosynthetic utilization efficiency (α) at high light level, with the P_max reaching 57.42±6.21 μmol O₂·h^-1·g^-1 FW. Compared with LL+HP and HL+LP treatments, LL+LP treatment promoted the accumulation of chlorophyll c content in S. horneri by 72.12% and 63.87% , respectively. High light intensity promoted the synthesis of soluble protein but reduced the content of soluble carbohydrate, which was not affected by phosphorus concentration. High phosphorus level significantly enhanced phosphorus absorption rate, and reached the highest value (0.254 μmol·d^-1·g^-1 FW) under low light conditions. Photosynthesis and growth rate of S. horneri significantly increased with increasing light intensity, but high phosphorus level significantly promoted photosynthesis and growth of S. horneri under high light conditions. It is speculated that the combination of eutrophic, highphosphorus seawater and high surface light environments may lead to rapid growth of S. horneri, thereby promoting the occurrence of golden tides and accelerating their development.

Key words: golden tide, Sargassum horneri, light, phosphorus, photosynthesis