Comparison of the photo-acclimation potential of floating and benthic thalli of Sargassum horneri (Phaeophyta) during autumn and winter

Sargassum horneri is a foundational species and an important contributor to the floating seaweed stock along the northeastern coast of Asia. In this study, benthic and floating thalli of S. horneri were collected from Changdao Island (37°54′N, 120°43′E), Bohai Bay, China. We conducted an in-situ and an indoor experiment to study the acclimation potential in S. horneri to abiotic conditions at sea surface in autumn and winter. Both benthic and floating thalli were cultured in situ for two months (from October to December) at different depths: 0 m above sea level (masl) and 3 m below sea level (mbsl), and their growth rate, biochemical content, and photosynthetic performance were compared. During the first month of culture, the relative growth rate of floating thalli was 2-fold greater than that of benthic thalli at 0 masl. The photosynthetic rate of most thalli was significantly higher at 0 masl than at 3 mbsl. In the indoor experiments, floating and benthic thalli were exposed to high light intensity (400 µmol photons/(m2·s) photosynthetically active radiation (PAR)) for 21 d, and their photo-acclimation capacities were compared. Under high light intensity, the two types of thalli showed low maximum quantum yield (Fv/Fm) and light utilisation efficiency (α) but high light saturation point (Ek). Floating thalli showed higher photosynthetic rate and photoprotective ability than benthic thalli at high light intensity. The effective quantum yield of photosystem II [Y(II)] of both types of thalli recovered after a 6-day treatment with low light intensity (40 µmol photons/(m2·s)). These findings suggest that S. horneri is highly acclimated to the sea surface environment, which possibly contributes to its rapid accumulation and long free-floating periods at the sea surface.