Physiological and molecular responses of tropical Seagrass Enhalus acoroides exposed to simultaneous high temperature and hypoxia stress

Abstract

High temperature and hypoxia pose significant threats to coastal ecosystems, often co-occurring and intensifying damage to seagrass meadows. While the independent effects of these stresses have been extensively documented, their combined impact on seagrasses remains underexplored. This study investigates the effects of high temperature and hypoxia on Enhalus acoroides, a dominant tropical seagrass. Results indicated that E. acoroides could tolerate high temperature (35 °C) and hypoxia (2.65 mg L⁻¹) individually for 24 h. However, exposure to both stresses simultaneously led to severe, irreversible physiological damage, highlighting a synergistic effect that surpassed the additive impact of each stressor alone. Combined stresses markedly impaired PSII, reduced photosynthetic rate and chlorophyll content, alongside elevated oxidative stress. Transcriptomic analysis indicated that high temperature intensified metabolic stress, while oxygen deficiency forced a shift from aerobic to anaerobic respiration, resulting in energy deficits. Furthermore, the lack of oxygen caused the accumulation of electrons, which triggered excessive production of reactive oxygen species. Despite the antioxidant enzyme system's active response, it was unable to mitigate the overwhelming oxidative stress, leading to irreversible oxidative damage. The above results suggested that in the context of global warming and eutrophication, the combined effects of high temperature and hypoxia may accelerate the degradation of seagrass meadows at a far greater rate than previously anticipated.