Saccharina japonica kelps grow naturally in the subtidal zone up to more than 20 m, where blue light (400–500 nm) is predominant; they possess the xanthophyll pigment fucoxanthin, in addition to chlorophyll, that aids in broadening the spectrum of light absorption. However, the knowledge of photosynthetic performance under different light wavelengths (e.g., blue, green, red light) remains insufficient for this alga. Moreover, the interactive effects of light and temperature on the photosynthetic performance of S. japonica is important for gaining a more wholistic understanding of its ecophysiology and distribution. Hence, this study focused on determining the thermal–light and the spectral light availability responses in the photosynthesis of S. japonica (= S. japonica var. japonica) using the PAM-chlorophyll fluorometer and DO sensors. Results of the study highlighted that 24°C was the photosynthetic temperature threshold for this seaweed, given that the PSII efficiency (effective quantum yields) gradually declined at low temperatures up to 4°C. Accelerated photoinhibition at low temperature was likewise observed. Moreover, S. japonica can well photosynthesize under blue light, with maximum photosynthetic output when exposed to such light wavelength. The photoinhibitory sensitivity to chilling-light stress and the enhanced photosynthesis under blue light of S. japonica presented additional evidence of their physiological adaptation to cold-temperate subtidal waters.

Due to its economic importance as an aquaculture crop, understanding the adaptation of S. japonica to the subtidal environment where both irradiance and spectral light availability are limiting is essential. Such information could provide significant contributions toward the sustainable utilization including mariculture of this species, in addition to providing baselines for potential shifts on seaweed distribution amid climate change.

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