Seagrasses are marine angiosperms (flowering plants). They create habitats for various organisms, maintain water quality, and prevent sediment erosion. Seagrasses are responsible for
sequestering about 10% of the total oceanic organic carbon while covering only 0.2% of the ocean floor, thus playing a crucial role in the coastal and global carbon cycles. It is generally accepted that ocean acidification due to rising atmospheric carbon dioxide levels will benefit seagrasses. However, decreasing ocean pH increases sulfide toxicity by shifting sulfide speciation toward more harmful hydrogen sulfide (H2S). In this study, we explore how changes in pH influence sulfide toxicity on a tropical seagrass, Halophila stipulacea. We exposed seagrass plants to sulfide (0.2 and 2 mM of total sulfide) under different pH levels (8.1, 7.9, 7.6, and 7.2). We measured the plant physiological responses using chlorophyll fluorescence (Fv/Fm) and photosynthesis rates and found that at the same total sulfide level, a lower pH negatively correlates with seagrass health. A dynamic population model demonstrates that while ocean acidification may boost seagrass growth, the simultaneous increase in sulfide toxicity due to decreasing pH offsets these gains. Our findings suggest that ocean acidification may introduce complex, competing feedbacks in coastal ecosystems, complicating our understanding on the role of seagrass in the future global carbon cycle.