Nanoparticle-based measurements of pH and O-2 dynamics in the rhizosphere of Zostera marina L.: effects of temperature elevation and light-dark transitions

摘要

Seagrasses can modulate the geochemical conditions in their immediate rhizosphere through the release of chemical compounds from their below-ground tissue. This is a vital chemical defence mechanism, whereby the plants detoxify the surrounding sediment. Using novel nanoparticle-based optical O-2 and pH sensors incorporated in reduced and transparent artificial sediment, we investigated the spatio-temporal dynamics of pH and O-2 within the entire rhizosphere of Zostera marina L. during experimental manipulations of light and temperature. We combined such measurements with O-2 microsensor measurements of the photosynthetic productivity and respiration of seagrass leaves. We found pronounced pH and O-2 microheterogeneity within the immediate rhizosphere of Z. marina, with higher below ground tissue oxidation capability and rhizoplane pH_ levels during both light exposure of the leaf canopy and elevated temperature, where the temperature -mediated stimuli of biogeochemical processes seemed to predominate. Low rhizosphere pH microenvironments appeared to correlate with plant derived oxic microzones stimulating local sulphide oxidation and thus driving local proton generation, although the rhizoplane pH levels generally where much higher than the bulk sediment pH. Our data show that Z. marina can actively alter its rhizosphere pH microenvironment alleviating the local H2S toxicity and enhancing nutrient availability in the adjacent sediment via geochemical speciation shift.