Lack of root oxygen loss from the dominant tropical Atlantic-Caribbean seagrass, Thalassia testudinum, in a highly sulfidic carbonate lagoon

Abstract

Although seagrasses transport leaf and water column oxygen (O₂) to belowground rhizome and root tissues, mortality events from hypoxia-driven H₂S intrusion occur, resulting in large-scale seagrass mortality events worldwide and in Florida Bay (40 km²), a large (2200 km²) seagrass-dominated estuary at the terminus of the Florida (USA) Peninsula. We examined the dominant tropical Atlantic-Caribbean seagrass Thalassia testudinum's ability to transport O₂ to belowground tissues and diffuse O₂ into root rhizospheres to constrain H₂S diffusion into roots. We examined O₂ in the rhizosphere in the light and dark by deploying 2-D planar O₂ optode sensors and measured pO₂ and H₂S dynamics in rhizospheres using microsensors. Our planar optode experiments revealed no detectable root O₂ loss (ROL) to rhizospheres which remained anoxic in >9 h of light (500 μmol photons m⁻² s⁻¹). Sediment O₂ and H₂S microsensor profiles of the rhizosphere and light/dark gas dynamic studies revealed that rhizospheres remained anoxic and H₂S concentrations reached ∼200 μM in the light and dark. At the same time, belowground tissue internal pO₂ was above air saturation (>21 kPa) in the light (3−12h) and remained oxic in the dark (12h) with no detectable H₂S intrusion. The lack of oxic regions in rhizospheres under high internal tissue pO₂ indicate a restriction of ROL in this slow-growing species. A limitation of root-sediment gas exchange in T. testudinum may be an evolutionary strategy to prevent O₂ loss along deep roots and minimize H₂S intrusion in sulfidic carbonate sediments.