LAUSR

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a temperate Japanese edible brown alga, Cladosiphon umezakii (Chordariaceae, Ectocarpales), from Honshu Island, Japan, was determined in the heteromorphic life history stages (macroscopic and microscopic) by using pulse-amplitude modulation (PAM) fluorometry and optical dissolved oxygen sensors. The microscopic stage is well-adapted to relatively low PAR environment, as revealed by its lower maximum net photosynthesis (NPmax), saturation (Ek), and compensation PAR (Ec), and higher value of the initial slope (α) compared to the macroscopic stage. Both stages share similar temperature optima (ToptGP$$ {T}_{opt}^{GP} $$ 15.9–25.8 °C for macroscopic stage, 12.6–27.4 °C for microscopic stage), suggesting the possible occurrence of both generations in their habitats throughout the year. While these optimum temperatures are within the range of seawater temperature in the distribution of this species, they are close to the physiological limit of thermal inhibition. Continuous exposures (6 h) to 200 (low) and 1000 (high) μmol photons m−2 s−1 at 8, 16, and 28 °C revealed greater declines in effective quantum yields of photosystem II (ΦPSII) in the microscopic stage, confirming its low PAR adaptation. Low temperature-induced photoinhibition was likewise observed in both stages. Existing farming methods for Japanese Cladosiphon (especially for subtropical Cladosiphon okamuranus from Ryukyu Islands, Japan) may need to be modified to meet the optimum requirements of C. umezakii in the temperate region of Japan.