Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass ( Halophila stipulacea) sediments

Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (inline-formulaCO₂) and methane (inline-formulaCH₄). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on inline-formulaCO₂ and inline-formulaCH₄ fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of inline-formulaCO₂ and inline-formulaCH₄ in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized inline-formulaCO₂ and inline-formulaCH₄ fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of inline-formulaCO₂ and inline-formulaCH₄, to gradual warming from 25 inline-formula^∘C (winter seawater temperature) to 37 inline-formula^∘C, 2 inline-formula^∘C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting inline-formulaCO₂ and inline-formulaCH₄ fluxes. We detected 6-fold-higher inline-formulaCO₂ fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher inline-formulaCH₄ fluxes. Warming led to an increase in net inline-formulaCO₂ and inline-formulaCH₄ fluxes, reaching average fluxes of 10 422.18 inline-formula± 2570.12 inline-formulaµmol inline-formulaCO₂ minline-formula⁻² dinline-formula⁻¹ and inline-formula88.11±15.19 inline-formulaµmol inline-formulaCH₄ minline-formula⁻² dinline-formula⁻¹, while inline-formulaCO₂ and inline-formulaCH₄ fluxes decreased over time in sediments maintained at 25 inline-formula^∘C. Prolonged darkness led to an increase in inline-formulaCO₂ fluxes but a decrease in inline-formulaCH₄ fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of inline-formulaCH₄, while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhancing global warming.