The seagrass phyllosphere can become anoxic in darkness owing to epiphyte respiration. Within anoxic microenvironments, anaerobic microbial processes can potentially produce phytotoxins and greenhouse gases, but the actual occurrence of such processes in seagrass epiphytic biofilms remains uncertain.
Here, we used microsensors to measure O₂, NO, N₂O and H₂S concentration gradients, as well as NO and O₂ dynamics within epiphytic biofilms on seagrass (Zostera marina) leaves under varying environmental conditions. We found pronounced production of NO, N₂O and H₂S in anoxic parts of the seagrass phyllosphere. Under hypoxic seawater conditions, the anoxic zone expanded within the seagrass leaf microenvironment, with NO and H₂S reaching maximal concentrations of 1.0 and 4.4 μmol L^-1, respectively. Furthermore, increasing seawater temperatures (from 14 to 24 °C) stimulateed H₂S production and accumulation within anoxic seagrass phyllospheres. The denitrification process was enhanced in eutrophic seawater, where the highest N₂O concentration in the epiphytic biofilms reached 5.9 μmol L^-1. Part of the phytotoxic NO and H₂S diffused into the seagrass leaves, while no NO escaped the biofilm. In contrast, N₂O emission from the biofilm in hypoxic and eutrophic seawater reached 9.6 μmol N₂O m^-2 day^-1. Such release of the potent greenhouse gas N₂O from seagrass leaves with epiphytic biofilms could potentially offset the carbon burial capability of seagrass meadows.
Multiple environmental stress conditions involving high temperatures, eutrophication and reduced O₂ availability in the water-column, can thus stimulate denitrification and sulfate reduction within anoxic leaf microenvironments, negatively impacting seagrass fitness and ecological function.

seagrass,anaerobic microbial activity,microenvironment,deoxygenation,water eutrophication