Sargassum golden tides have bloomed frequently in many sea areas throughout the world, and negatively impacted on the local marine ecology. Sargassum muticum commonly inhabits rocky shores. It is now distributed worldwide due to its invasiveness, and recently drifting individuals have been observed on the coasts of Canary Islands. However, as a potential golden tide alga, physiological, and ecological studies of this species have not been frequently explored. To investigate the responses of S. muticum to light and nitrogen, two key environmental factors in golden tide formation, we established three light levels (LL, low light, 10 μmol photons m–2 s–1; ML, medium light, 60 μmol photons m–2 s–1, and HL, high light, 300 μmol photons m–2 s–1) and two nitrogen levels (LN, low nitrogen, 25.0 μM of natural seawater; HN, high nitrogen, 125.0 μM), and cultivated the thalli under different conditions for 12 days before measuring the physiological properties of alga. The results showed that higher light and/or nitrogen levels enhanced the relative algal growth rate. The maximum net photosynthesis rate of alga increased with the light, while it remained unaffected by the nitrogen. The HN treatment had no effect on the apparent photosynthetic efficiency of algae in the LL culture, while increased it in the ML and HL cultures. The irradiance saturation point of photosynthesis was approximately 300 μmol photons m–2 s–1 with no significant difference among the six treatments, except for a slight increase under HLHN in contrast to the LLHN and MLLN treatments. HL treatment decreased the maximum quantum yield of photosynthesis (Fv/Fm) in both nitrogen levels. In the HN culture, ML and HL led to lower values of photoinhibition, indicating higher survivability in the alga. The HN culture led to higher nitrogen uptake but had no effects on Fv/Fm and the contents of pigments and soluble protein, regardless of culture light level. Based on these results, we speculate that drifting individuals of S. muticum would be possible to form a golden tide owing to its rapid growth rate at light level of 300 μmol photons m–2 s–1, when they encountered the sustained lower light level on the sea surface (≤300 μmol photons m–2 s–1). A high nitrogen supply caused by eutrophication of seawater might facilitate this process. Our results provide an important reference for the prediction of golden tides formed by S. muticum.

In recent years, massive Sargassum drifting on the sea surface, known as “golden tides,” negatively impacted on the local marine ecology. However, the physiological mechanisms of its formation remain unclear. To investigate the photosynthetic responses of golden tide algae to UVR, one key factor for drifting Sargassum population, we cultivated thalli of S. horneri, a golden tide alga, under three light treatments: P (photosynthetically active radiation, PAR), PA (PAR+UVA) and PAB (PAR+UVA+UVB) for 120 mins, followed by low light recovery for 240 mins. The photosynthetic characteristics of alga were determined. The results showed that UVR exposure decreased photosynthetic activity, reflected by depressed maximum photochemical quantum yield (Fv/Fm) and contents of Chla and Chlc in the PA and PAB treatments. Higher content of malondialdehyde (MDA) was found in thalli exposed to UVR, which verified the damage of UVR. Electron transfer rate (ETR) was slowed down by UVR, accompanied by the increments of net closing rate of the reaction center and the energy absorbed and dissipated by unit reaction center in PSII. In these effects on photosynthesis of UVR, the PAB treatment expressed more significant inhibition, indicating a remarkable role of UVB. However, based on our results, S. horneri also took some strategies to protect itself from photodamage of UVR. UVR exposure enhanced the contents of UV-absorbing compounds (UVACs) and carotenoid, and simultaneously expedited heat consumption of excess light energy, indicated by the increased non-photochemical quenching coefficient (NPQ) in the PA and PAB treatments. Increased activities of superoxide dismutase (SOD) and peroxidase (POD), and higher content of PsbA (D1) protein were found in the treatments with UVR, which suggested that antioxidant system and the turnover of D1 protein played important roles in protection from UV-induced damages. Due to the above protection pathways, Fv/Fm and ETR gradually recovered when thalli were transferred to low light recovery. Therefore, we suggest that various protection and restoration pathways in S. horneri work together to effectively protect against UVR damage, which may be the reason why drifting populations can adapt to UVR on the seawater surface and form golden tide in case of suitable temperature and nutrients.