Effects of increasing temperature on NO, N2O and H2S production and dynamics within epiphytic seagrass leaf biofilms

Leaf epiphytes have potential to reduce seagrasses nighttime O₂ supply leading to microbial production of greenhouse gases and phytotoxins that may be harmful to the plant. However, it remains unknown how global warming affects anaerobic processes like denitrification and sulfate reduction in the seagrass phyllopshere during darkness. We used electrochemical microsensors to measure gradients of O₂, NO, N₂O and H₂S in the leaf microenvironment of heavily epiphyte-covered seagrass (Zostera marina L.) during darkness as a function of temperature at 14 and 24 °C. Increasing seawater temperature stimulated phytotoxic H₂S production and accumulation within anoxic seagrass phyllospheres in darkness, which can be harmful to the plant as H₂S readily diffuses through the thin cuticle of seagrass leaves. The seagrass phyllosphere also exhibited production of NO, which was diffusing into the plant as measured in the basal leaf meristem, as well as the potent greenhouse gas N₂O during darkness. Increased temperature and reduced O₂ availability in the water-column, e.g., during marine heatwaves in eutrofied waters, can thus induce hostile chemical conditions in the seagrass phyllosphere that potentially can be detrimental for the plant’s performance and ecological function. The phyllosphere dynamics of NO and N₂O displayed limited response to increasing seawater temperatures as compared to H₂S, but the N₂O production in epiphyte-covered seagrass leaves and subsequent emission to the surrounding seawater could have implications for the role of seagrass meadows in greenhouse gas mitigation.