Effects of UVR on Photosynthesis in Sargassum horneri (Turner) C. Agardh Adapted to Different Nitrogen Levels

In recent years, golden tides caused by drifting Sargassum horneri (Turner) C. Agardh have caused serious ecological disasters in coastal areas of China. Eutrophication is an important cause of the formation of the golden tide. Additionally, the drifting population on the surface of the ocean is exposed to more ultraviolet radiation (UVR) than the attached population on the sea floor. In this study, the thalli of S. horneri were cultivated under two levels of nitrogen (LN: natural seawater, in which the concentration of NO3−-N was 1 µmol L−1; HN: NO3−-enriched seawater, in which the concentration of NO3−-N was 200 μmol L−1) for 6 days with low photosynthetically active radiation (PAR), and then exposed to three levels of radiation (P: photosynthetically active radiation (PAR), 400–700 nm; PA: PAR + UVA, 320–700 nm; PAB: PAR + UVA + UVB, 280–700 nm) under each level of nitrogen for 2 h to investigate the effects of high UVR and nitrogen on photosynthesis. The results showed that the high level of N (HN) only enhanced the synthesis of pigments after 6 days of pre-cultivation under low PAR. After 2 h of high UVR exposure, high P, PA, and PB decreased the maximum photochemical quantum yield (Fv/Fm) and increased non-photochemical quenching (NPQ) in S. horneri regardless of the N level, and PAB significantly decreased Fv/Fm compared to PA under the LN condition alone. Under the LN condition, compared to the P group, PA and PAB significantly promoted the synthesis of carotenoids. Under the HN condition, compared to the P group, PAB increased the absorbed flux by active RCs (ABS/RC) and dissipated the energy flux by active RCs (DI0/RC) in S. horneri alone. Furthermore, HN increased Fv/Fm, ABS/RC, and DI0/RC more in S. horneri with PAB in comparison to those in the LN and PAB group. However, no significant differences in these parameters were observed between the LN and HN conditions under the same UVR treatments. These results demonstrate that drifting S. horneri on the surface of seawater could be inhibited by the high P; however, S. horneri living in eutrophic high-nitrogen seawater may have a stronger ability to resist high UVR damage, especially with regard to PAB radiation, which may be one of the reasons for the formation of golden tides in coastal seawater.