Physiological responses of reproductive Sargassum horneri to increased light and nitrogen levels

The Sargassum golden tide has resulted in severe ecological impacts, and the eutrophication of seawater is considered as a major trigger for it. To explore the physiological responses of reproductive S. horneri, a golden tide species, to increased light and nitrogen levels, two light intensities (LL and HL) and two nitrate concentrations (LN and HN) were set in this study. The results of two-factor interaction experiment showed that light and nitrate interactively influenced the photosynthesis of reproductive S. horneri. In LN treatment, a significant photoinhibition caused by high light was found, reflected by decreased maximum photochemical quantum yield (F_v/F_m) and photosynthetic rate, enhanced non-photochemical quenching (NPQ), and reduced Chla and Chlc contents. However, the HN culture remarkably alleviated such photoinhibition, even exhibited a higher photosynthetic rate in HL treatment, with the elevated Chla and Chlc contents. The increments of Car and ultraviolet-absorbing compounds (UVACs) contents, electron transport efficiency (α), and dark respiration rate in HN treatment may contribute to protecting and repairing the photodamage. Additionally, HL and HN treatments significantly increased the C and N contents in the branches and receptacles of alga. The HLHN treatment significantly enhanced the relative growth rate (RGR) at initial culture, and increased the number of reproductive receptacle and reproductive effort. Based on these findings, we hypothesize that under eutrophic conditions, reproductive S. horneri, after detachment and floating to the sea surface, is more likely to maintain rapid growth and reproduction and form golden tide after adapting to high light conditions.