The effects of four stressors, irradiance, temperature, desiccation, and salinity on the photosynthesis of a red alga, Agarophyton vermiculophyllum (Gracilariales) from a native distributional range in Japan

We examined the effects of four stressors, irradiance, temperature, desiccation, and salinity on the photosynthesis of a red alga, Agarophyton vermiculophyllum (= Gracilaria vermiculophylla, Gracilariales, Rhodophyta) from its native distributional range in Hokkaido and Kagoshima, Japan. Photosynthesis–irradiance (P–E) curves at 8, 16, and 28°C showed that the maximum net-photosynthetic rates (NPmax) and saturation irradiance (Ek) were highest at 28°C for both strains. Gross-photosynthesis determined at 8–40°C at 200 μmol photons m-2 s-1 showed that the maximum gross-photosynthetic rate (GPmax) occurred at 25.3°C for Hokkaido and 28.0°C for Kagoshima (ToptGP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {T}_{opt}^{GP} $$\end{document}), which is almost consistent with the summer-time seawater temperature at each habitat. The temperature responses (4–40°C) of effective quantum yields (ΔF/Fm') of photosystem II during 7-day exposures were similar to that of oxygenic photosynthesis and the optimum temperature (ToptΔF/Fm′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {T}_{opt}^{\varDelta F/ Fm\prime } $$\end{document}) was 20.0°C for Hokkaido and 30.1°C for Kagoshima. In the desiccation experiment, the ΔF/Fm' decreased with decreasing absolute water content (AWC); nevertheless, for samples with an AWC above 20%, ΔF/Fm' retuned to initial levels after subsequent 1-day rehydration in seawater, suggesting relatively strong tolerance to desiccation. This alga also showed a broad range of tolerance to salinity ranging from 20 to 60 psu in 7-day exposures, and the ΔF/Fm' tolerated 0 psu at 3-day exposure. The adaptations of A. vermiculophyllum from its native range in Japan to relatively high irradiance, a broad range of temperature, and strong osmotic (desiccation and salinity) tolerance explain its potentially high invasive capacity.