Dual Stress of Ocean Acidification and Sulfide: Physiological Adaptation Mechanisms of the Tropical Seagrass Thalassia hemprichii

ABSTRACT

Ocean acidification and sulfide intrusion are key environmental stressors affecting seagrass bed ecosystems. Although sulfide stress drives large‐scale seagrass mortality, the physiological response of seagrasses under ocean acidification remains unclear. Therefore, this study selected Thalassia hemprichii , a dominant species of seagrass beds, to investigate the effects of ocean acidification (pH 7.7), sulfide stress, and their interaction on its physiological responses and growth. Our results showed that acidification alone relieved photosynthetic carbon limitation of T. hemprichii , significantly increasing net photosynthetic oxygen release and growth rate ( p  < 0.05). Sulfide stress alone significantly decreased the maximum quantum yield ( Fv / Fm ) and net photosynthetic rate and increased tissue H ₂ S content ( p  < 0.05), while the growth rate showed a decreasing trend. Under combined acidification and sulfide stress, acidification promoted sulfide intrusion and significantly increased leaf H ₂ S content ( p  < 0.05), whereas no significant reduction in growth rate was observed. Compared with sulfide treatment alone, combined acidification and sulfide stress enhanced the electron transport rate and radial oxygen loss (ROL) in T. hemprichii , thereby significantly increasing O ₂ contents in the sediment ( p  < 0.05). Meanwhile, the antioxidant defense systems of T. hemprichii were activated. These coordinated responses jointly mitigated sulfide toxicity. Therefore, under ocean acidification, T. hemprichii alleviated sulfide toxicity by coordinating carbon acquisition, rhizosphere oxygen cycling, and antioxidant defense. These findings provide key physiological and ecological evidence for predicting the adaptive capacity of seagrass beds under future climate change and underscore the importance of interactions among multiple stressors for seagrass conservation.