The photosynthetic response of the freshwater red alga Thorea okadae to environmental gradients of temperature, irradiance, desiccation, and salinity: Adaptations to its stream habitat
摘要
The response of photosynthesis in a freshwater red alga Thorea okadae to environmental gradients of temperature, irradiance, desiccation, and salinity, was examined using dissolved oxygen sensors and a pulse amplitude modulation (PAM) fluorometer. The net photosynthesis–irradiance (P–E) curve, determined at 20 °C under blue, green, red (light-emitting diode, LED), and white lights (metal-halide lamp), revealed that this alga exhibited highest maximum photosynthetic output under green light, followed by blue and white light. The temperature response of maximum (Fv/Fm) and effective (ΔF/Fm') quantum yields after a three-day culture period at temperatures ranging from 4–40 °C under 0 and 50 µmol photons m⁻2 s⁻1, showed a peak at 21.9 and 22.9 °C, respectively, and sharply declined above 28 °C. The combined effects of irradiance (200 [low], 500 [medium], 1000 [high] µmol photons m⁻2 s⁻1) and temperature (28, 16, 4 °C) revealed that ΔF/Fm' declined during exposure to all irradiance treatments. However, it mostly recovered after a subsequent 12-h period of dim-light acclimation at 28 °C and 16 °C, as well as under low and medium irradiance at 4 °C. In contrast, samples exposed to high irradiance at 4 °C failed to recover, indicating the occurrence of low-temperature light stress. Under desiccation stress, ΔF/Fm' quickly decreased as the desiccation period extended beyond one hour, dropping to zero after four hours. This decline was closely related to the loss of internal water in the thallus, as ΔF/Fm' decreased rapidly when relative water content fell below 50%. Under salinity stress, after a seven-day culture at 0–10 psu, ΔF/Fm' decreased rapidly with increasing salinity, reaching zero at over 8 psu. This alga appears well-adapted to the freshwater stream environment at the study site. However, winter temperatures combined with strong light, dehydration during droughts, and exposure to brackish water when the alga drifts downstream may negatively impact the survival of this species.