Ecotype-specific light intensity responses in Sargassum horneri: bloom-forming vs. benthic forms

This study compared light-driven physiological adaptations between floating and benthic Sargassum horneri ecotypes using five light intensities (50, 100, 150, 300, and 600 μmol photons m⁻² s⁻¹). Both ecotypes showed bell-shaped relative growth rate (RGR) curves, with floating S. horneri peaking at 6.74 % day⁻¹ at 150 μmol photons m⁻² s⁻¹ and benthic ecotype reached its highest RGR of 6.90 % day⁻¹ at 300 μmol photons m⁻² s⁻¹. At 600 μmol photons m⁻² s⁻¹, both the maximum photochemical efficiency (F_v/F_m) and the maximum relative electron transfer rate (rETR_max) of the two ecotypes were observed to be inhibited. Notably, the inhibition was more pronounced in the floating S. horneri. Simultaneously, both ecotypes exhibited resilience to elevated light stress by curtailing pigments synthesis, enhancing the light saturation point (I_k) and tissue carbon storage, and elevating catalase (CAT) and glutathione reductase (GR). The concentrations of soluble proteins and UV-absorbing compounds in benthic S. horneri were significantly greater than those found in floating S. horneri at light intensities of ≥300 μmol photons m⁻² s⁻¹. The results highlight ecotypes-specific physiological adaptations of S. horneri to varying light intensities. In comparison to the floating S. horneri, the benthic S. horneri collected during the same period exhibited a greater resilience and photosynthetic performance under higher light conditions. The study suggests that benthic S. horneri, if dislodged to surface waters, could proliferate rapidly under transient high-light exposure due to its robust photosynthetic plasticity.