The effects of desiccation and salinity gradients in the photochemical efficiency of a mesophotic green alga, Ryuguphycus kuaweuweu (Ulvaceae) from Kagoshima, Japan

SUMMARY In mesophotic coral ecosystems (MCEs) at depths of 30–40 m, the light environment is characterized by low intensity and limited spectral quality. However, due to minimal freshwater input, salinity remains relatively stable, and there are no tidal aerial exposures. We examined the effects of desiccation and salinity gradients on the photochemical efficiency of the mesophotic green alga, Ryuguphycus kuaweuweu (Ulvaceae), collected from a depth of 35 m off Mageshima Island in the northern end of the Ryukyu Islands, Japan, to evaluate its tolerance to desiccation and salinity. A pulse‐amplitude modulation (PAM) chlorophyll fluorometer was used for the measurements. Desiccation experiments, conducted for up to 4 h of aerial exposure under 50% humidity, at 20°C, and with an irradiance of 20 μmol photons m⁻ 2 s⁻ 1 , revealed that the effective quantum yields ( ΔF/F m ' ) of photosystem II dropped rapidly, approaching nearly zero after 45 min of aerial exposure. Since the decline in Δ F/F m ' was closely related to the loss of internal water from the thallus, the Δ F/F m ' decreased once the absolute water content (AWC) fell below 50% and never recovered to initial levels, suggesting a low tolerance to desiccation. Salinity experiments, conducted over 7 days under a broad range of salinities (0–100 psu), at 20°C, and with an irradiance of 20 μmol photons m⁻ 2 s⁻ 1 under a 12‐h light/12‐h dark photoperiod, demonstrated euryhaline responses, as the Δ F/F m ' of the thallus remained stable between 10 and 60 psu. Given that this alga thrives at mesophotic depths, where it is never exposed to air, it is unlikely to flourish at intertidal depths. The euryhaline characteristics of this alga might not provide an advantage in habitats without freshwater influence; however, species of Ulvaceae, which are widely found in shallow coastal areas worldwide, including estuaries, may commonly possess this euryhaline tolerance.