Impacts of nighttime hypoxia on the physiological performance of Red Sea macroalgae under peak summer temperature

Eutrophication-induced hypoxic sites are increasingly reported in coastal regions. At the same time, ocean warming, water column stratification, and changing circulation lead to open-ocean deoxygenation. In areas reefs with dense vegetation, aquatic organisms can be exposed oxygen limitation stress where concentration reaches extremely low levels, particularly during nighttime once photosynthetic O 2 production has ceased. Despite scientists being aware of this for decades, little is known about impact deoxygenation on physiology marine primary producers, such as macroalgae. Red Sea, particular, physiological adaptations macroalgae under future climate scenarios nonexistent. Here, we investigate different levels (6.5, 2.5, 1.3 mg L -1 ) at night three conspicuous Sea species Halimeda opuntia Padina boryana (calcareous) brown algae Sargassum latifolium (noncalcifying). We monitored algal responses a 12-hour (dark) period 32°C by measuring photochemical efficiency ( F v /F m ), respiration rates, cellular viability. No lethal thresholds were detected. However, both treatments decreased rates induced changes activity, only severe hypoxia was decrease observed all species. calculated sublethal SLC (50) 1.2 ± 0.1, 1.5 1.7 0.1 H. , P. S. respectively. Therefore, effects evident over short timescales may ecosystems via reduced production. Future consequences persistent subsequent performance multifaceted stressor exposures will provide fundamental understanding hypoxia’s threat biodiversity ecosystems.