Abstract:There are many studies that focused on the effects of changing marine environmental conditions on the adult stage of seaweeds. However, little work has been done to investigate the response of the juvenile stage of them to increasing solar ultraviolet (UV) radiation and coastal eutrophication. Hizikia fusiformis (Harvey) Okamura (Sargassaceae, Phaeophyta) is an important economic brown seaweed, and the culture of young sporophytes through sexual reproduction is a practically feasible approach to the artificial seedlings production of this species. Detailed information derived from physiological experiments is essential for successful controlling and management of growing seedlings in land-based system. In the present work, the effects of solar radiation on the photochemcial properties of photosynthesis were investigated in young sporophytes of Hizikia fusiformis grown at two different N availabilities. The results showed that during culture period, the young sporophytes exhibited a considerable increment of length. However, the relative growth rates were not statistically different between the young sporophytes grown under high N level and the ones grown under normal N level. The contents of Chl a, carotenoids and UV-absorbing compounds (UVAC) were significantly higher in high N-grown algae than the normal N-grown algae. The effective photosynthetic quantum yield and the relative electronic transfer rate (rETR) were drastically reduced in the young sporophytes when exposed to high solar radiation, with the decrease fluctuation being greater in treatment with all spectrum solar radiation (PAB) than the one with solar photosynthetically active radiation (PAR). The decrease trend of the effective photosynthetic quantum yield was similar between the algae grown under normal N level and high N level when the young sporophytes were exposed to PAR or PAB. However, when exposed to PAB, rETR of the thalli grown under high N level was higher than those grown under normal N level. This indicated that the young sporophytes cultured under high N level exhibited higher capacity of defending UV radiation, which might be related with the higher contents of UVAC and carotenoids in the young sporophytes. Our result showed that the young sporophytes of Hizikia fusiformis possessed the higher ability against UV damage than the adult sporophytes, which could be reflected from the recovery of the thalli which was damaged by UV radiation. It is proposed that in artificial seedlings production of Hizikia fusiformis, an appropriate addition of N nutrient in the seawater media would enhance the ability of young sporophytes to resistance the environmental stresses such as solar UV radiation, although the N fertilization had no obvious effect on the growth (length of the young sporophytes).