The effect of temperature, light‐spectrum, desiccation and salinity gradients on the photosynthetic performance of a subtidal brown alga, Sargassum macrocarpum, from Japan

The effect of temperature, light‐spectrum, desiccation and salinity gradients on the photosynthesis of a Japanese subtidal brown alga, Sargassum macrocarpum (Fucales), was determined using a pulse amplitude modulation‐chlorophyll fluorometer and dissolved oxygen sensors. Temperature responses of the maximum (Fv/Fm in darkness) and effective (ΔF/Fm′ at 50 μmol photons m−2 s−1; = ΦPSII) quantum yields during 6‐day culture (4–36°C) remained high at 12–28°C, but decreased at higher temperatures. Nevertheless, ΔF/Fm′ also dropped at temperatures below 8°C, suggesting light sensitivity under chilling temperatures because Fv/Fm remained high. Photosynthesis–irradiance responses at 24°C under red (660 nm), green (525 nm), blue (450 nm) and white light (metal halide lamp) showed that maximum net photosynthesis under blue and white light was greater than under red and green light, indicating the sensitivity and photosynthetic availability of blue light in the subtidal light environment. In the desiccation experiment, samples under aerial exposure of up to 8 h under dim‐light at 24°C and 50% humidity showed that ΔF/Fm′ quickly declined after more than 45 min of emersion; furthermore, ΔF/Fm′ also failed to recover to initial levels even after 1 day of rehydration in seawater. Under the emersion state, the ΔF/Fm′ remained high when the relative water content (RWC) was greater than 50%; in contrast, it quickly dropped when the RWC was less than 50%. When the RWC was reduced below 50%, ΔF/Fm′ did not return to initial levels, regardless of subsequent re‐hydration, suggesting a low capacity of photosynthesis to recover from desiccation. The stenohaline response of photosynthesis under 3‐day culture is evident, given that ΔF/Fm′ declined when salinity was beyond 20–40 psu. Adaptation to subtidal environments in temperate waters of Japan can be linked to these traits.