We determined the effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from its native distributional range in Japan by using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–irradiance curves at three temperatures (8, 20, and 28°C) showed that the maximum net photosynthetic rates (NPmax) and saturation irradiance were highest at 28°C. Gross photosynthesis determined at 8–36°C (every four increments) and 300 μmol photons m−2 s−1 showed that the maximum gross photosynthetic rate (GPmax) occurred at 19.5°C (ToptGP\documentclass[12pt]{minimal}
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\begin{document}$$ {T}_{\mathrm{opt}}^{\mathrm{GP}} $$\end{document}), which is consistent with the seawater temperature at its peaked abundance in Japan. The maximum quantum yields (Fv/Fm) of photosystem II (PSII) during the 72-h temperature exposures were above 0.60 at 8–28°C but dropped at higher temperatures. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m−2 s−1 at three temperatures showed remarkable decline in the effective quantum yields (ΔF/Fm') of PSII under high irradiance at 8°C only; the Fv/Fm measured after 12-h dark acclimation also did not recover to initial values, signifying its sensitivity to photoinhibition at 8°C. Furthermore, the alga exhibited tolerance to 2 h of desiccation with 80% of water loss from the thallus, and ΔF/Fm' recovered after 24 h of rehydration in seawater, suggesting potential of photosynthetic recovery of this alga at such low hydration threshold. In conclusion, the adaptation of S. muticum to relatively high irradiance, to broad range of temperature (8–28°C), and to desiccation explains its potentially high invasive capacity.
