Imaging O₂ dynamics and microenvironments in the seagrass leaf phyllosphere with magnetic optical sensor nanoparticles

Abstract

Eutrophication leads to epiphyte blooms on seagrass leaves that strongly affect plant health, yet the actual mechanisms of such epiphyte-induced plant stress remain poorly understood. We used magnetic optical sensor nanoparticles in combination with luminescence lifetime imaging to map the O 2 concentration and dynamics in the heterogeneous seagrass phyllosphere under changing light conditions. By incorporating magnetite into the sensor nanoparticles, it was possible to image the spatial O 2 distribution under flow over seagrass leaf segments in the presence of a strong magnetic field. Local microniches with low leaf surface O 2 concentrations were found under thick epiphytic biofilms, often leading to anoxic microhabitats in darkness. High irradiance led to O 2 supersaturation across most of the seagrass phyllosphere, whereas leaf microenvironments with reduced O 2 conditions were found under epiphytic biofilms at low irradiance, probably driven by self-shading. Horizontal micro-profiles extracted from the O 2 images revealed pronounced heterogeneities in local O 2 concentration over the base of the epiphytic biofilm, with up to 52% reduction in O 2 concentrations in areas with relatively thick (>2 mm), compared with thin (≤1 mm), epiphyte layers in darkness. We also present evidence of enhanced relative internal O 2 transport within leaves with epiphyte overgrowth, compared with bare seagrass leaves, in light as a result of limited mass transfer across thick outward diffusion pathways. The local availability of O 2 was still markedly reduced in the epiphyte-covered leaves, however. The leaf phyllosphere is thus characterized by a complex microlandscape of O 2 availability that strongly affects microbial processes occurring within the epiphytic biofilm, which may have implications for seagrass health, as anoxic microhabitats have been shown to promote the microbiological production of reduced toxic compounds, such as nitric oxide.

How plants recruit their microbiome? New insights into beneficial interactions

“…In an interesting study, the availability of O 2 in the marine plant phyllosphere was considered an important factor in the conformation of the associated microbiome, which modulates the microbial metabolic processes that occur within epiphytic biofilms [135] . Although O 2 is available in terrestrial environments, other gases such as CO 2 can affect the diversity of the phyllosphere (and endosphere) of rice plants [136] .…”

Luminescence lifetime imaging of three-dimensional biological objects

“…The fields of aquatic and plant biology research, among others, also benefit from 3D FLIM (Moßhammer et al, 2019). For instance, O 2 imaging using various nanoparticles is useful for the study of corals and plantbased inter-species systems such as those that exist in the rhizosphere (Fabricius-Dyg et al, 2012;Brodersen et al, 2020). Similar to NAD(P)H sensing in biomedical imaging, chlorophyll autofluorescence FLIM can be used to analyse the metabolic function of coral symbionts.…”

Editors’ Choice—Luminescent Oxygen Sensors: Valuable Tools for Spatiotemporal Exploration of Metabolism in In Vitro Systems

“…This allows complex sample geometries to be analyzed. The user can, for example, grow biofilms around the particles to monitor gradients, add a dimension to rhizosphere analysis, 53,54 disperse them throughout a batch culture for continuous monitoring of O 2 metabolism, 48 or spray paint the particles on to complex surfaces to characterize respiration and photosynthetic activity in 3D. 55 Like some dendrimeric porphyrins, 26 nanoparticles have the potential to push the realm of O 2 sensing into intracellular settings, albeit with various difficulties.…”