Crossing thermal limits: functional collapse of the surfgrass Phyllospadix scouleri under simulated extreme marine heatwave.

Phyllospadix scouleri experiences a critical physiological breakdown at 26.5 °C, just 5–6 °C above current summer maxima, indicating a potential thermal tipping point.

• Why it matters: Understanding the thermal limits of seagrasses is essential to predict their resilience under intensifying marine heatwaves caused by climate change, yet these limits remain poorly defined.
• What they did: Researchers conducted a 27-day mesocosm experiment exposing P. scouleri to severe (23.5 ± 1.5 °C) and extreme (26.5 ± 1.5 °C) heatwaves, monitoring photosynthesis, respiration, nitrogen metabolism, oxidative stress, and growth.
• The result: The study found that P. scouleri maintained function under severe warming but showed persistent physiological disruption at extreme temperatures, reducing growth and recovery, which suggests a rapid erosion of thermal safety margins in temperate seagrasses.