Photosynthetic and metabolic analyses reveal a higher resistance to salinity in the Pyropia yezoensis (Rhodophyta) blades than in the conchocelis

Abstract The life cycle of Pyropia yezoensis contains two obviously different stages: the conchocelis and the blade, which have different ploidy and morphology. The conchocelis grows in a calcareous matrix within subtidal zone and cannot live without seawater. However, the blades live on intertidal rocks and experience severe stress conditions at low tide. When exposed to air, external salt concentration increases around the algal tissues. It has been reported that the blade has high tolerance to various stresses, including high salinity stress. However, it is unknown whether carbon fixation still occurs when the blade is exposed to high salinity stress. Furthermore, there has been very little research on the conchocelis response to salt stress. Therefore, it is important to comprehensively compare the responses of the conchocelis and the blades to salinity. In this study, photosynthesis performance, carbon metabolism, and metabolite production were measured. The Y(II) for the conchocelis was almost zero (0.006) after 1 h at 120 practical salinity units (psu), whereas Y(I) was high (0.175). The Y(II) of the blade was only 0.008 whereas its Y(I) was 0.304 after 72 h at 120 psu, which suggests that PSII is more sensitive to salinity than PSI in both the conchocelis and the blade. Higher relative abundance was detected in the blade over the whole experimental period, which showed that the blade had higher carbon fixation efficiency than the conchocelis. The proline and trehalose contents were also higher in the blade, which might explain why the blade can tolerate higher salinities than the conchocelis.