Аннотация:Summary Seagrass is constantly challenged with transporting sufficient O 2 from above‐ to belowground tissue via aerenchyma in order to maintain aerobic metabolism and provide protection against phytotoxins. Electrochemical microsensors were used in combination with a custom‐made experimental chamber to analyse the belowground biogeochemical microenvironment of Z ostera muelleri under changing environmental conditions. Measurements revealed high radial O 2 release of up to 500 nmol O 2 cm −2 h −1 from the base of the leaf sheath, maintaining a c . 300‐μm‐wide plant‐mediated oxic microzone and thus protecting the vital meristematic regions of the rhizome from reduced phytotoxic metabolites such as hydrogen sulphide (H 2 S). H 2 S intrusion was prevented through passive diffusion of O 2 to belowground tissue from leaf photosynthesis in light, as well as from the surrounding water column into the flow‐exposed plant parts during darkness. Under water column hypoxia, high belowground H 2 S concentrations at the tissue surface correlated with the inability to sustain the protecting oxic microshield around the meristematic regions of the rhizome. We also found increased p H levels in the immediate rhizosphere of Z. muelleri , which may contribute to further detoxification of H 2 S through shifts in the chemical speciation of sulphide. Zostera muelleri can modify the geochemical conditions in its immediate rhizosphere, thereby reducing its exposure to H 2 S.
