Warm Acclimation Modulates the Physiological Responses of Young Saccharina japonica to Marine Heatwaves

Frequent marine heatwaves (MHWs) pose significant threats to and have a profound impact on the structure and functioning of marine ecosystems. The response of marine organisms to MHWs may depend on the background temperature of their habitats. Saccharina japonica is a cold-temperate species that is vulnerable to thermal stress. However, it remains unclear how habitat-related temperatures impact responses of S. japonica to MHWs. In this study, the algae were exposed to MHWs (+∆4 °C) under two background temperatures (control level, 8 °C; warmer level, 12 °C), and the growth, photosynthetic performance, and biochemical composition were measured at the end of the MHW and the recovery periods. The results showed that a warmer background temperature increased the growth and pigment contents (Chlorophyll c and fucoxanthin) of S. japonica, but decreased chlorophyll fluorescence parameters, including the effective quantum yield, relative electron transport rate, non-photochemical quenching, and photochemical quenching. The MHWs had no significant effect on growth rate, photosynthetic performance, or biochemical compositions at the background temperature of 8 °C. In contrast, MHWs reduced growth and biochemical composition contents (Chlorophyll a, Chlorophyll c, fucoxanthin and soluble protein) at a warmer level, but the negative influence was alleviated after the recovery period. Furthermore, there was a considerable increase in the total antioxidative capacity caused by MHWs under warmer conditions. Overall, the stimulation in growth demonstrated that warmer conditions (12 °C) may provide a better growth temperature for young S. japonica but also increase the risks of biomass losses and physiological damage triggered by MHWs. These findings deepen our understandings of the tolerance and resistance of S. japonica to heatwaves and provide useful information for managing and modulating the cultivation of this important commercial seaweed.