Seasonal response of respiration and thermal tolerance of PSII to sea surface temperature variation in two foundation seaweed species

Amidst escalating sea surface temperatures, comprehending how marine organisms acclimate to temperature fluctuations is paramount. This understanding is not only needed to anticipate the ecological ramifications of future climatic conditions, but also forms the cornerstone of our efforts to effectively restore and safeguard these invaluable marine habitats. The mechanisms of acclimation in seaweed physiology in response to seasonal shifts in temperature have not yet been sufficiently explored. Here, we examined the physiological responses of two foundational seaweeds, Ecklonia radiata and Phyllospora comosa, to seasonal sea surface temperature (SST) variation. Our approach combined a fluorophore O₂-sensor system to measure respiration and chlorophyll fluorescence to assess critical heat thresholds (T_crit) with PSII. By measuring T_crit and respiration across a full year, we explored the metabolic costs and some of the trade-offs in seaweed acclimation and examined whether a higher T_crit entails a greater respiratory carbon cost, or, whether respiration acclimates as SST increases, reducing metabolic expenses. Our findings reveal significant seasonal shifts in T_crit, for both species. For metabolic activity, Phyllospora exhibited temporally stable respiration, indicating homeostatic regulation, while Ecklonia demonstrated potential energy deficits during warmer months. These findings are likely linked to the species' habitat differences, with Phyllospora occupying variable and extreme intertidal environments, and Ecklonia residing in more stable subtidal habitats. The relationship between the predicted temperature at maximum respiration (T_max) and T_crit indicated a coupling between thermal stability of respiration and photosynthesis within seaweeds. Our approach has uncovered important connections between thermal tolerance and respiration, providing new insights into seaweed physiology which offer a foundation for future research and conservation strategies under global warming.