The effects of irradiance, temperature, desiccation, and salinity on the photosynthesis of the freshwater red alga Nemalionopsis shawii (Thoreales) from Japan

Abstract

The effects of irradiance (including irradiance spectral quality), temperature, desiccation, and salinity on a Japanese population of freshwater red alga Nemalionopsis shawii were investigated using dissolved oxygen sensors and pulse-amplitude-modulated chlorophyll fluorometry to improve understanding of its conservation requirements. Oxygenic photosynthesis–irradiance curves determined at 20 °C under blue, green, and red light-emitting diodes (LEDs) showed that the maximum net photosynthetic rate was highest under green light, nearly equivalent to that observed under white light (metal halide lamp), and followed by red light. Temperature responses of the maximum (F_v/F_m) and effective (ΔF/F_m') quantum yields after three days of culture across 4–40 °C under irradiances of 0 and 50 μmol photons m⁻² s⁻¹ (12 L:12D) peaked at 21.9 °C and 19.9 °C, respectively, and declined above 28 °C; F_v/F_m was less sensitive to low temperatures. Combined irradiance (200, 500, and 1000 μmol photons m⁻² s⁻¹) and temperature (28, 16, and 4 °C) treatments revealed that ΔF/F_m' decreased during light exposure across all temperatures but largely recovered after 12-h of dim light acclimation at 28 °C and 16 °C; no recovery occurred at 4 °C, indicating cold-induced photoinhibition. In the desiccation experiment (up to 8 h of aerial exposure, 20 °C, 45–50% humidity, and 20 μmol photons m⁻² s⁻¹), ΔF/F_m' remained stable for 1-h but gradually decreased as relative water content fell below 50%, approaching zero after 8 h. In the one-week salinity experiment (0–10 psu, 20 °C, 20 μmol photons m⁻² s⁻¹, 12 L:12D), ΔF/F_m' declined progressively above 6 psu, indicating low tolerance to brackish conditions. These findings provide important insights into the conservation and habitat management of N. shawii, emphasizing optimal light and temperature conditions and the avoidance of desiccation and salinity stress.