O2 and pH microdynamics around the rhizome and roots of seagrasses determined via novel optical nanoparticle-based sensors

Seagrasses provide important eco-engineering services in coastal environments but have over the past century been declining with alarming rates mainly due to anthropogenic activity. Seagrasses are constantly challenged to... [ view full abstract ]

Seagrasses provide important eco-engineering services in coastal environments but have over the past century been declining with alarming rates mainly due to anthropogenic activity. Seagrasses are constantly challenged to aerate their belowground tissue and the surrounding sediment to prevent intrusion of reduced phytotoxic compounds such as hydrogen sulfide and to ensure aerobic metabolism. In present study, we developed a novel bioimaging approach to determine O2 and pH microdynamics and distributions around the belowground tissue of Zostera marina L. and Zostera muelleri spp. capricorni by means of optical nanoparticle-based O2 and pH sensors incorporated into transparent, artificial sediments consisting of a deoxygenated, pH-buffered, sulphidic seawater/agar matrix. Seagrass growth and photosynthetic activity did not seem affected by the experimental setup based on root growth rates (~5 mm d-1) and photosynthetic quantum yields (~0.7), which both were comparable to healthy seagrasses growing in their natural habitat. Oxygen release and the pH heterogeneity were visualised and analysed on a whole rhizosphere level, which is a substantial improvement to existing methods such as via microsensors and/or planar optodes. Our images determined higher oxygen release from the belowground tissue in light as compared to darkness and that water column hypoxia leads to reduced oxygen levels around the rhizome and roots. We found pronounced spatial pH microheterogeneity within the immediate rhizosphere of Z. marina L. Light exposure of the leaf canopy and elevated temperature resulted in higher rhizoplane pH levels (rhizome/roots surface ∆pH of up to 0.9 pH units). Low rhizosphere pH microenvironments (pH levels down to ~4) appeared to correlate with the plant-mediated oxic microniches, although the rhizoplane/tissue surface pH levels overall were much higher than the pH of the surrounding sediment (~0.4 pH units higher; based on averaged measurements in selected regions of interest (ROIs)). Seagrass plants thus alters the pH levels of their immediate rhizosphere, an important chemical defence mechanism that further alleviates the H2S toxicity in the rhizoplane through geochemical speciation shift towards non-tissue-permeable HS- ions, as well as the plant-derived low pH microenvironments leads to nutrient mobilization, which then becomes available for plant assimilation.

Keywords: optical nanosensors, O2 distribution, pH microheterogeneity, seagrasses