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We present a new experimental set-up enabling fine-scale examination of how changing environmental conditions affect the below-ground biogeochemical microenvironment of aquatic macrophytes. By means of micro-sensor and planar optode technology, the influence of plant-mediated radial [O.sub.2] release on the below-ground chemical microenvironment of Zostera muelleri and Halophila ovalis was determined in high spatio-temporal resolution. The seagrass specimens were cultured in a new split flow chamber with artificial sediment made of a deoxygenated seawater-agar solution with added sulphide. Microelectrode measurements revealed radial [O.sub.2] release from the root-shoot junction of both Z. muelleri and H. ovalis during both light stimulation and darkness, resulting in a rapid decrease in [H.sub.2]S concentration, and a significant drop in pH was observed within the plant-derived oxic microzone of Z. muelleri. No radial [O.sub.2] release was detectable from the below-ground tissue of Z. muelleri during conditions of combined water-column hypoxia and darkness, leaving the plants more susceptible to sulphide invasion. The spatial [O.sub.2] heterogeneity within the immediate rhizosphere of Z. muelleri was furthermore determined in two dimensions by means of planar optodes. [O.sub.2] images revealed a decrease in the spatial extent of the plant-derived oxic microzone surrounding the below-ground tissue during darkness, supporting the microelectrode measurements. This new experimental approach can be applied to all rooted aquatic plants, as it allows for direct visual assessment of the below-ground tissue surface during microprofiling, while enabling modification of the above-ground environmental conditions., Introduction Seagrass meadows provide a diverse range of key ecosystem services such as coastal protection from erosion and enhancement of marine biodiversity (Larkum et al. 2006; Orth et al. 2006; [...]

Seagrass is constantly challenged with transporting sufficient O2 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 Zostera muelleri under changing environmental conditions., Measurements revealed high radial O2 release of up to 500 nmol O2 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 (H2S). H2S intrusion was prevented through passive diffusion of O2 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 H2S 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 H2S through shifts in the chemical speciation of sulphide., Zostera muelleri can modify the geochemical conditions in its immediate rhizosphere, thereby reducing its exposure to H2S. [ABSTRACT FROM AUTHOR]

Mapping of O 2 with luminescent sensors within intact animals is challenging due to attenuation of excitation and emission light caused by tissue absorption and scattering as well as interfering background fluorescence. Here we show the application of luminescent O 2 sensor nanoparticles (∼50-70 nm) composed of the O 2 indicator platinum(II) tetra(4-fluoro)phenyltetrabenzoporphyrin (PtTPTBPF) immobilized in poly(methyl methacrylate- co -methacrylic acid) (PMMA-MA). We injected the sensor nanoparticles into the gastrovascular system of intact colony fractions of reef-building tropical corals that harbor photosynthetic microalgae in their tissues. The sensor nanoparticles are excited by red LED light (617 nm) and emit in the near-infrared (780 nm), which enhances the transmission of excitation and emission light through biological materials. This enabled us to map the internal O 2 concentration via time-domain luminescence lifetime imaging through the outer tissue layers across several coral polyps in flowing seawater. After injection, nanoparticles dispersed within the coral tissue for several hours. While luminescence intensity imaging showed some local aggregation of sensor particles, lifetime imaging showed a more homogeneous O 2 distribution across a larger area of the coral colony. Local stimulation of symbiont photosynthesis in corals induced oxygenation of illuminated tissue areas and formation of lateral O 2 gradients toward surrounding respiring tissues, which were dissipated rapidly after the onset of darkness. Such measurements are key to improving our understanding of how corals regulate their internal chemical microenvironment and metabolic activity, and how they are affected by environmental stress such as ocean warming, acidification, and deoxygenation. Our experimental approach can also be adapted for in vivo O 2 imaging in other natural systems such as biofilms, plant and animal tissues, as well as in organoids and other cell constructs, where imaging internal O 2 conditions are relevant and challenging due to high optical density and background fluorescence.