Effects of desiccation time on performance of a cultured (Neopyropia yezonensis) and fouling alga (Ulva prolifera)

Neopyropia yezoensis, a kind of red alga (Rhodophyta) that is a source of nori, is a highly valuable macroalga widely cultivated in coastal regions of East Asia. However, in recent years, Neopyropia cultivation areas have been frequently disturbed by green tide algae, particularly Ulva prolifera. To address this issue, we conducted a seven-day experiment subjecting both N. yezoensis and raft-attached U. prolifera to desiccation treatment twice daily, with exposure times ranging from 0 to 6 hours per treatment. The physiological responses were evaluated through microscopic structure analysis, maximum quantum yield of photosystem II (Fv/Fm), biomass measurements, and pigment content determination. Results showed that increasing desiccation time significantly affected the physiological status of U. prolifera. With prolonged desiccation time, both U. prolifera and N. yezoensis exhibited decreased Fv/Fm values, reduced biomass growth rate, and significant changes in pigment content. Desiccation periods of 5–6 hours reduced the photosynthetic efficiency of photosystem II (PSII) in U. prolifera. Although a single desiccation treatment temporarily inhibited the physiological status of N. yezoensis, after undergoing desiccation–rehydration cycles, the alga not only recovered its physiological function but also exhibited higher Fv/Fm values than the continuously submerged control group. However, for treatments with desiccation times exceeding 5 hours, productivity was somewhat affected, with the growth rate being approximately 24 % lower than that of the continuously submerged group. This indicates that desiccation-rehydration cycles promote the physiological status and biomass of N. yezoensis, demonstrating its superior adaptation to desiccation conditions. This study identified 5 hours as the optimal desiccation duration for N. yezoensis cultivation. Under these conditions, N. yezoensis maintained high growth rates and favorable physiological status while effectively inhibiting U. prolifera growth, providing a viable method for controlling U. prolifera in cultivation areas.