Humidity-dependent variation in photochemical efficiency during aerial exposure in sporophytes of the cultivated red alga Pyropia yezoensis f. narawaensis (=Neopyropia yezoensis f. narawaensis; Bangiaceae) from Saga, Japan

The response of photochemical efficiency to aerial exposure was examined in cultivated sporophytes of the red alga, Pyropia yezoensis f. narawaensis (= Neopyropia yezoensis f. narawaensis ; Bangiaceae), from Saga, Japan under different humidity conditions (40–99%). Continuous real-time measurements of the photochemical efficiency (effective quantum yield of photosystem II, ΔF/F m ' ) of sporophytes attached to dead oyster shells were obtained every 20 s using a pulse-amplitude-modulated (PAM) chlorophyll fluorometer (Mini Imaging-PAM) during aerial exposure under humidity conditions of 40, 50, 60, 70, 80, and 90% for up to 40 min at 24 °C. Under saturated humidity (99%), measurements were conducted once daily over a 50-day exposure period at 24 °C under a 12 L:12 D photoperiod. At 40–60% humidity, ΔF/F m ' declined rapidly and approached zero within ten min of exposure. At 70% humidity, the decline was slower, reaching near-zero values after approximately 20 min. At 80% and 90% humidity, ΔF/F m ' decreased even more gradually, taking over 30 min to approach zero, indicating that high humidity reduced internal water loss. Under saturated humidity (99%), sporophytes remained healthy and maintained ΔF/F m ' for at least seven days, followed by a gradual decline that did not reach zero over the 50-day exposure. In Japanese Nori cultivation, sporophytes attached to dead oyster shells are highly sensitive to desiccation and typically cannot withstand more than ten minutes of dehydration. This was confirmed under low-humidity conditions; however, the rapid decline in ΔF/F m ' was clearly inhibited under high-humidity conditions. These results suggest that careful humidity management during the handling of sporophytes in the Nori cultivation process may improve their survival by reducing water loss. • The sporophyte of Pyropia yezoensis f. narawaensis is less tolerant to desiccation. • Photosynthetic activity drops within ten minutes of dehydration at 50% humidity. • However, it is maintained for a longer duration under high and saturated humidity. • Careful humidity management may improve Nori cultivation.