ALBERT

Description: Intensified coastal eutrophication can result in an overgrowth of seagrass leaves by epiphytes, which is a major threat to seagrass habitats worldwide, but little is known about how epiphytic biofilms affect the seagrass phyllosphere. The physico-chemical microenvironment of Zostera marina L. leaves with and without epiphytes was mapped with electrochemical, thermocouple and scalar irradiance microsensors as a function of four irradiance conditions (dark, low, saturating and high light) and two water flow velocities (approx. 0.5 and 5 cm s-1), which resemble field conditions. The presence of epiphytes led to the build up of a diffusive boundary layer and a thermal boundary layer which impeded O2 and heat transfer between the leaf surface and the surrounding water, resulting in a maximum increase of 0.8°C relative to leaves with no epiphytes. Epiphytes also reduced the quantity and quality of light reaching the leaf, decreasing plant photosynthesis. In darkness, epiphyte respiration exacerbated hypoxic conditions, which can lead to anoxia and the production of potential phytotoxic nitric oxide in the seagrass phyllosphere. Epiphytic biofilm affects the local phyllosphere physico-chemistry both because of its metabolic activity (i.e. photosynthesis/respiration) and its physical properties (i.e. thickness, roughness, density and back-scattering properties). Leaf tissue warming can lead to thermal stress in seagrasses living close to their thermal stress threshold, and thus potentially aggravate negative effects of global warming.

Description: Steep geochemical gradients surround roots and rhizomes of seagrass and protect the plants against the harsh conditions in anoxic sediment, while enabling nutrient uptake. Imbalance of these gradients, due to e.g., low plant performance and/or changing sediment biogeochemical conditions, can lead to plant stress and large-scale seagrass meadow die-off. Therefore, measuring and mapping the dynamic gradients around seagrass roots and rhizomes is needed to better understand plant responses to human impact and environmental changes. Historically, electrochemical microsensors enabled the first measurements of important chemical species like O2, pH or H2S with high sensitivity and spatial resolution giving important insights to the seagrass rhizosphere microenvironment; however, such measurements only provide information in one dimension at a time. In recent years, the use of reversible optical sensors (in the form of planar optodes or nanoparticles) and accumulative gel sampling methods like Diffusive Gradients in Thin films (DGT) have extended the array of analytes and allowed 2-D mapping of chemical gradients in the seagrass rhizosphere. Here, we review and discuss such microscale methods from a practical angle, discuss their application in seagrass research, and point toward novel experimental approaches to study the (bio)geochemistry around seagrass roots and rhizomes using a combination of available techniques, both in the lab and in situ.