Effects of temperature, irradiance and varying light spectral qualities on photochemical efficiency of the edible red alga, Gloiopeltis frutex (Endocladiaceae) from Kagoshima Prefecture, Japan

The effects of temperature and irradiance, including light spectral quality, on the photochemical efficiency of the edible red alga Gloiopeltis frutex (Endocladiaceae) were determined using a pulse-amplitude-modulated (PAM) chlorophyll fluorometer. The temperature response (4–36 °C) during a 3-day culture period (50 µmol photons m⁻ 2 s⁻ 1 ; 12L:12D photoperiod) revealed that the effective quantum yield ( ΔF/F m ' ) was maintained at a high level below 24 °C, with a maximum value of 0.605 (95% credible interval [CI]: 0.591–0.619) occurring at 17.2 °C (95% CI: 14.7–19.9 °C). Non-photochemical quenching (NPQ) showed a similar response and remained stable below 24 °C. In contrast, both parameters declined sharply above 24 °C and decreased to nearly zero at 32 °C. The combined effects of irradiance (1000 µmol photons m⁻ 2 s⁻ 1 ) and temperature (28, 20, 12, and 4 °C) revealed that ΔF/F m ' declined during a 6-h period of light exposure. However, it fully recovered after a subsequent 12-h period of dim-light acclimation at 20 and 12 °C, suggesting a strong tolerance to high irradiance. In contrast, ΔF/F m ' was not fully restored at 28 and 4 °C, suggesting that both low and high temperatures enhanced photoinhibitory stress. The effects of high irradiance (1000 µmol photons m⁻ 2 s⁻ 1 ) under blue (450 nm), green (525 nm), red (660 nm), and white light (metal halide lamp) differed among spectral qualities. ΔF/F m ' decreased significantly under green light, suggesting that photoinhibitory stress may have been enhanced under green wavelengths, which are absorbed by the light-harvesting phycobiliprotein phycoerythrin. Overall, these findings suggest that G. frutex is well adapted to the temperature and irradiance conditions that prevail during its occurrence period from mid-winter to early spring in the upper intertidal zone at the study site. The results provide important physiological insights into the ecology, cultivation potential, and stress tolerance of this species under varying environmental conditions.