Population thermal regime modulates the response of the agarophyte Gelidium corneum to marine heatwaves

Marine heatwaves (MHWs) are intensifying with climate change, driving significant alterations in marine ecosystems, especially those dominated by macroalgae. This study assesses the stress response and recovery capacity of two geographically and environmentally distinct populations of the agarophyte Gelidium corneum under simulated MHWs. One population originated from the inner Southern Bay of Biscay (Cantabrian Sea, non-upwelling area) and the other from northern Portugal (Atlantic Ocean, upwelling-influenced). MHWs were simulated using three intensities, control (20 °C), low (22 °C) and high (24 °C), and two durations, short (7 days) and long (21 days), each followed by a 7-day recovery period. Stress responses were evaluated through growth rates, tissue damage, and photophysiological parameters (F_v/F_m, photosynthesis and respiration). Results revealed population-specific thermal responses. Short, intense MHWs induced reversible photophysiological stress in both populations, indicating a general sensitivity of the photosynthetic apparatus to abrupt temperature increases. In contrast, prolonged MHWs triggered divergent responses: warm-adapted individuals from the Cantabrian Sea displayed photosynthetic plasticity and resilience, while cold-adapted individuals from the Atlantic population suffered a sustained reduced performance, particularly under high-intensity events. These findings highlight the role of local thermal adaptation in shaping macroalgal responses to future ocean warming scenarios.