No abstract

We determined the response of photochemical efficiency to desiccation and salinity in an edible brown alga, Saccharina japonica (var. japonica), using a pulse amplitude modulation chlorophyll fluorometer. Natural populations of this species in Muroran, Hokkaido, Japan are most of the time immersed in the subtidal zone; however, those at the uppermost part of the sublittoral are sometimes emersed during low tide. In the laboratory experiment, the alga under aerial exposure up to 8 h under dim light at 20 C and 50% humidity showed that the effective quantum yields (ΔF/F m 0 ) of Photosystem II quickly declined after more than 45 min of emersion; ΔF/F m 0 also failed to recover to the initial level even after 1 day of rehydration in seawater. Under emersion, the ΔF/F m 0 values were relatively stable and above 0.5 when the absolute water content (AWC, %) was greater than 50%; however, it declined as the AWC decreased. Once the algae were dehydrated to an AWC of less than 50%, their ΔF/F m 0 did not return to the initial level despite subsequent reimmersion in seawater, suggesting the low ability of photosynthetic recovery from dehydration. Results of in situ measurements in Muroran showed that the ΔF/F m 0 of S. japonica declined during the tidal exposure; however, it recovered after they were again immersed in seawater due to the rising tide. Furthermore, S. japonica showed a stenohaline photosynthetic response between 20 and 40 psu, as their ΔF/F m 0 values were found to be unchanged in these salinities after 5 days of laboratory culture; this suggests the seaweed's photosynthetic tolerance and/or acclimation within such a relatively narrow range, which might be one of the traits that preclude its occurrence in the intertidal and brackish waters. Indeed, desiccation risk and fluctuating salinity are important limiting factors that influence the stability of kelp beds for sustainable utilization in the regional fishery including mariculture of this species.

Ecklonia radicosa (Laminariales), a canopy-forming brown alga, was reported to have disappeared since 2016 at a site surveyed under a nationwide long-term monitoring survey for the natural ecosystem (Monitoring Site 1000, Japanese Ministry of Environment) in Nagashima Island, Kagoshima Prefecture, Japan. The distribution of seaweed assemblages along the coast of the island was surveyed in 2019 to compare with that in 2005. In 2005, E. radicosa and the temperate species of Sargassum (Fucales) were widely distributed, and comprised the large seaweed assemblages along the coast facing both the East China Sea and Yatsushiro Bay. However, in 2019, these species, including an edible alga, Sargassum fusiforme, were not found oŠ the coast of the former, suggesting the degradation of kelp and fucoid assemblages. These algae were still found at the sites in Yatsushiro Bay as large seaweed assemblages; however, some tropical species of Sargassum that were uncommon in 2005 were also observed in 2019. The reason for the disappearance of these canopy-forming algae has not been elucidated yet;