Combined Influences of Light and Nitrogen Enrichment on the Physiological Performance of a Golden Tide Alga (<i>Sargassum horneri</i>)

Sargassum golden tides (GT) are common in numerous coastal areas all over the world, and it adversely affects local marine life. Eutrophication is critical for Sargassum GT development. However, its physiological and ecological mechanism remains unclear. To investigate the responses of drifting Sargassum horneri, the species causing GT in the western Pacific, to light and enriched nitrogen, we set three light conditions (Low-light (LL), 10 μmol photons m−2 s−1; Middle-light (ML), 60 μmol photons m−2 s−1; and High-light (HL), 300 μmol photons m−2 s−1) and three nitrogen conditions (Natural seawater, the final concentration of N was 31.8 μmol L−1, including 30.5 μmol L−1 of NO3− and 1.3 μmol L−1 of NH4+; Enrichment of NO3−, final concentration of N was 200 μmol L−1; and Enrichment of NH4+, the final concentration of N was 200 μmol L−1), and grew the thalli under varying conditions for 10 days before determining the growth and utilization of carbon and nitrogen. Based on the accumulated data, the elevated light level led to a higher growth rate of alga. In the LL culture, the higher capacity for carbon utilization, which was reflected by the higher maximum photosynthetic carbon fixation rate (Vmax), resulted in the elevated growth rates of thalli in the nitrogen-enriched media as compared with the natural seawater. Furthermore, a higher growth rate was found in the enrichment of NH4+ despite a low affinity for inorganic carbon indicated by a higher value of the half-saturation constant (K0.5). In the ML treatment, an insignificant difference in growth rate was found in three nitrogen cultures, except for a slight increase in the enrichment of NH4+ than the enrichment of NO3−. In the HL treatment, compared with natural seawater culture, enrichment of NO3− or NH4+ accelerated the growth of alga, with no significant difference between the two nitrogen sources. Such enhancement in growth was related to the more photosynthetic carbon fixation, indicated by the higher value of Vmax and soluble carbohydrates content of alga cultured with NO3− and NH4+ enrichments. Additionally, the uptake and assimilation products of nitrogen, such as pigments and soluble proteins, remained unaffected by nitrogen source enrichment of NO3− or NH4+ at all three light levels. In conclusion, enrichment of NO3− and NH4+ exhibited different influences on the growth of S. horneri at different light levels, which was mainly associated with the capacity and efficiency of photosynthetic carbon utilization. At the HL level, both the enrichment of NO3− and NH4+ dramatically accelerate the growth of alga by stimulating the photosynthetic carbon fixation. Accordingly, we speculated that drifting S. horneri, exposed to HL level on the surface of the sea, were likely to develop rapidly to form GT in eutrophic oceanic areas with upwelled and river plume NO3− or NH4+ nutrients