Strong leaf surface basification and CO2 limitation of seagrass induced by epiphytic biofilm microenvironments

Anoxic seagrass leaf environments as potential hotspots for toxin production and N2O emission

Epiphytes on seagrass leaves can render parts of the leaf phyllosphere anoxic in darkness owing to leaf/epiphyte respiration and O2 diffusion constraints. In such anoxic microenvironments, anaerobic microbes can potentially produce phytotoxins and greenhouse gases, but the actual occurrence of such processes in seagrass epiphytic biofilms remain uncertain. We used microsensors to measure O2, NO, N2O and H2S concentration gradients, as well as NO and O2 dynamics within epiphytic biofilms on seagrass (Zostera marina) leaves under changing environmental conditions. The bacterial community composition of epiphytic biofilms was analyzed with 16S rRNA gene amplicon sequencing. Flavobacteriaceae and Rhodobacteraceae were dominant bacterial community members accounting for ˃50 % of the relative abundance, and sulfate-reducing bacteria (Desulfobacterota) were omnipresent in the epiphytic biofilms. We found pronounced production of NO, N2O and H2S in anoxic parts of the seagrass phyllosphere, with NO and H2S reaching maximal concentrations of 1.0 and 4.4 μmol L-1, respectively, under slow flow and hypoxic seawater conditions, while the highest N2O concentration in the epiphytic biofilms reached 5.9 μmol L-1 in hypoxic, nitrate-rich seawater. Part of the phytotoxic NO and H2S diffused into the seagrass leaves, while no NO escaped the biofilm. In contrast, N2O emission from the biofilm in hypoxic and eutrophic seawater reached 9.6 μmol N2O m-2 day-1. Such release of the potent greenhouse gas N2O from seagrass leaves with epiphytic biofilms under eutrophic conditions could potentially offset the carbon burial capacity of seagrass meadows. Ocean eutrophication can thus stimulate denitrification and sulfate reduction within anoxic leaf microenvironments, negatively impacting seagrass fitness and ecological function.

A matter of choice: Understanding the interactions between epiphytic foraminifera and their seagrass host Halophila stipulacea

In sub/tropical waters, benthic foraminifera are among the most abundant epiphytic organisms inhabiting seagrass meadows. This study explored the nature of the association between foraminifera and the tropical seagrass species H. stipulacea, aiming to determine whether these interactions are facilitative or random. For this, we performed a "choice" experiment, where foraminifera could colonize H. stipulacea plants or plastic "seagrasses" plants. At the end of the experiment, a microbiome analysis was performed to identify possible variances in the microbial community and diversity of the substrates. Results show that foraminifera prefer to colonize H. stipulacea, which had a higher abundance and diversity of foraminifera than plastic seagrass plants, which increased over time and with shoot age. Moreover, H. stipulacea leaves have higher epiphytic microbial community abundance and diversity. These results demonstrate that seagrass meadows are important hosts of the foraminifera community and suggest the potential facilitative effect of H. stipulacea on epiphytic foraminifera, which might be attributed to a greater diversity of the microbial community inhabiting H. stipulacea.

Two-dimensional, high-resolution imaging of pH dynamics in the phyllosphere of submerged macrophyte using a new Nano-optode

The phyllosphere pH helps shape the plant microbiome and strongly influences aboveground interactions in plant canopies. Yet little is known about the distribution of pH at a microscale within the macrophyte phyllosphere and the factors promoting them because achieving high-resolution quantitative imaging of phyllosphere pH is a great challenge. Here, new ratiometric pH nano-optodes were prepared by firstly encapsulating the self-synthesized lipophilic dyes (8-acetoxypyrene-N1, N3, N6-trioctadecyl-1, 3, 6-tri-trisulfonamide) to poly(1-vinylpyrrolidone-co-styrene) nanoparticles, and then immobilizing the resulting nanoparticles in polyurethane hydrogel on transparent foils. The nano-optodes presented reversible and fast response (t95 < 80 s) to the pH range from 7.0 to 11.0, with merits of good spatial resolution, photobleaching/leaching resistance and negligible cross-sensitives toward temperature, O2 and ionic strength (< 100 mM). The nano-optodes together with a self-designed phyllosphere chamber were further applied to directly measure the pH distributions at a microscale around single leaves of V. spiralis grown in natural sediment. The pronounced pH microheterogeneity and leaf basification within the V. spiralis phyllosphere were quantitatively visualized. We also provided direct empirical evidence that the dynamic of the phyllosphere pH at high resolution was significantly controlled by the shifting light intensity and temperature. Implementation of the nano-optodes holds great potential for various laboratory applications, which will provide an in-depth insight into phyllosphere activities on the microscale.

Imaging of CO2 and Dissolved Inorganic Carbon via Electrochemical Acidification-Optode Tandem

Dissolved inorganic carbon (DIC) is a key component of the global carbon cycle and plays a critical role in ocean acidification and proliferation of phototrophs. Its quantification at a high spatial resolution is essential for understanding various biogeochemical processes. We present an analytical method for 2D chemical imaging of DIC by combining a conventional CO2 optode with localized electrochemical acidification from a polyaniline (PANI)-coated stainless-steel mesh electrode. Initially, the optode response is governed by local concentrations of free CO2 in the sample, corresponding to the established carbonate equilibrium at the (unmodified) sample pH. Upon applying a mild potential-based polarization to the PANI mesh, protons are released into the sample, shifting the carbonate equilibrium toward CO2 conversion (>99%), which corresponds to the sample DIC. It is herein demonstrated that the CO2 optode-PANI tandem enables the mapping of free CO2 (before PANI activation) and DIC (after PANI activation) in complex samples, providing high 2D spatial resolution (approx. 400 μm). The significance of this method was proven by inspecting the carbonate chemistry of complex environmental systems, including the freshwater plant Vallisneria spiralis and lime-amended waterlogged soil. This work is expected to pave the way for new analytical strategies that combine chemical imaging with electrochemical actuators, aiming to enhance classical sensing approaches via in situ (and reagentless) sample treatment. Such tools may provide a better understanding of environmentally relevant pH-dependent analytes related to the carbon, nitrogen, and sulfur cycles.

A review of microplastic impacts on seagrasses, epiphytes, and associated sediment communities

Microplastics have been discovered ubiquitously in marine environments. While their accumulation is noted in seagrass ecosystems, little attention has yet been given to microplastic impacts on seagrass plants and their associated epiphytic and sediment communities. We initiate this discussion by synthesizing the potential impacts microplastics have on relevant seagrass plant, epiphyte, and sediment processes and functions. We suggest that microplastics may harm epiphytes and seagrasses via impalement and light/gas blockage, and increase local concentrations of toxins, causing a disruption in metabolic processes. Further, microplastics may alter nutrient cycling by inhibiting dinitrogen fixation by diazotrophs, preventing microbial processes, and reducing root nutrient uptake. They may also harm seagrass sediment communities via sediment characteristic alteration and organism complications associated with ingestion. All impacts will be exacerbated by the high trapping efficiency of seagrasses. As microplastics become a permanent and increasing member of seagrass ecosystems it will be pertinent to direct future research towards understanding the extent microplastics impact seagrass ecosystems.

The role of epiphytes in seagrass productivity under ocean acidification

Ocean Acidification (OA), due to rising atmospheric CO₂, can affect the seagrass holobiont by changing the plant's ecophysiology and the composition and functioning of its epiphytic community. However, our knowledge of the role of epiphytes in the productivity of the seagrass holobiont in response to environmental changes is still very limited. CO₂ vents off Ischia Island (Italy) naturally reduce seawater pH, allowing to investigate the adaptation of the seagrass Posidonia oceanica L. (Delile) to OA. Here, we analyzed the percent cover of different epiphytic groups and the epiphytic biomass of P. oceanica leaves, collected inside (pH 6.9–7.9) and outside (pH 8.1–8.2) the CO₂ vents. We estimated the contribution of epiphytes to net primary production (NPP) and respiration (R) of leaf sections collected from the vent and ambient pH sites in laboratory incubations. Additionally, we quantified net community production (NCP) and community respiration (CR) of seagrass communities in situ at vent and ambient pH sites using benthic chambers. Leaves at ambient pH sites had a 25% higher total epiphytic cover with encrusting red algae (32%) dominating the community, while leaves at vent pH sites were dominated by hydrozoans (21%). Leaf sections with and without epiphytes from the vent pH site produced and respired significantly more oxygen than leaf sections from the ambient pH site, showing an average increase of 47 ± 21% (mean ± SE) in NPP and 50 ± 4% in R, respectively. Epiphytes contributed little to the increase in R; however, their contribution to NPP was important (56 ± 6% of the total flux). The increase in productivity of seagrass leaves adapted to OA was only marginally reflected by the results from the in situ benthic chambers, underlining the complexity of the seagrass community response to naturally occurring OA conditions.

The Second Skin of Seagrass Leaves: A Comparison of Microalgae Epiphytic Communities Between Two Different Species Across Two Seagrass Meadows in Lesser Sunda Islands

Epiphytes as the important features in the seagrass ecosystems have been studied widely, and their functions as a primary producer, influence rates of herbivory grazer, and prevent seagrass leaf from desiccation is well known. However, patterns and distribution among seagrasses especially in Indonesia, which was known as hotspot marine biodiversity is not well understood. Therefore, this study aimed to examined epiphytic assemblages on two seagrass species with different morphological and longevity, Enhalus acoroides and Cymodocea rotundata, in two different meadows (conservation area and non-conservation area) in Lesser Sunda Islands (Bali and Lombok). A total of 22 taxa of microalgae epiphytes species were identified from eight sites and 2 different species of seagrass. The highest number of collected species between class was from Bacillariophyceae (18), followed by Cyanophyceae (3) and Fragilariophyceae (1). Analysis of similarity (ANOSIM) revealed a significant difference of microalgae epiphytes assemblages between sites and seagrasses. Epiphytes assemblages in conservation area were more abundant than non-conservation area, both in Bali and Lombok. On seagrass comparison, Enhalus acoroides showed higher abundance of epiphytes assemblages than those on Cymodocea rotundata. Based on principal component analysis (PCA), this study highlights the microalgae epiphytic communities strongly influenced by seawater temperature, phosphate's concentration, and pH in sediment. This study also demonstrated that the assemblages of microalgae epiphytic communities affected by differences of seagrass morphological and longevity.

Total Dissolved Inorganic Carbon Sensor Based on Amperometric CO2 Microsensor and Local Acidification

We present a dipping probe total dissolved inorganic carbon (DIC) microsensor based on a localized acidic microenvironment in front of an amperometric CO₂ microsensor. The acidic milieu facilitates conversion of bicarbonate and carbonate to CO₂, which in turn is reduced at a silver cathode. Interfering oxygen is removed by an acidic CrCl₂ oxygen trap. Theoretical simulations of microsensor functioning were performed to find a suitable compromise between response time and near-complete conversion of bicarbonate to CO₂. The sensor exhibited a linear response over a wide range of 0-8 mM DIC, with a calculated LOD of 5 μM and a 90% response time of 150 s. The sensor was successfully tested in measuring DIC in bottled mineral water and seawater. This DIC microsensor holds the potential to become an important tool in environmental sensing and beyond for measurements of DIC at high spatial and temporal resolution.

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

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 O₂ 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 O₂ distribution under flow over seagrass leaf segments in the presence of a strong magnetic field. Local microniches with low leaf surface O₂ concentrations were found under thick epiphytic biofilms, often leading to anoxic microhabitats in darkness. High irradiance led to O₂ supersaturation across most of the seagrass phyllosphere, whereas leaf microenvironments with reduced O₂ conditions were found under epiphytic biofilms at low irradiance, probably driven by self-shading. Horizontal micro-profiles extracted from the O₂ images revealed pronounced heterogeneities in local O₂ concentration over the base of the epiphytic biofilm, with up to 52% reduction in O₂ 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 O₂ 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 O₂ was still markedly reduced in the epiphyte-covered leaves, however. The leaf phyllosphere is thus characterized by a complex microlandscape of O₂ 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.

Flow and epiphyte growth effects on the thermal, optical and chemical microenvironment in the leaf phyllosphere of seagrass (Zostera marina)

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.

Epiphytes provide micro-scale refuge from ocean acidification

Coralline algae, a major calcifying component of coastal shallow water communities, have been shown to be one of the more vulnerable taxonomic groups to ocean acidification (OA). Under OA, the interaction between corallines and epiphytes was previously described as both positive and negative. We hypothesized that the photosynthetic activity and the complex structure of non-calcifying epiphytic algae that grow on corallines ameliorate the chemical microenvironmental conditions around them, providing protection from OA. Using mesocosm and microsensor experiments, we showed that the widespread coralline Ellisolandia elongata is less susceptible to the detrimental effects of OA when covered with non-calcifying epiphytic algae, and its diffusive boundary layer is thicker than when not covered by epiphytes. By modifying the microenvironmental carbonate chemistry, epiphytes, facilitated by OA, create micro-scale shield (and refuge) with more basic conditions that may allow the persistence of corallines associated with them during acidified conditions. Such ecological refugia could also assist corallines under near-future anthropogenic OA conditions.

Fast Responding Amperometric CO2 Microsensor with Ionic Liquid-Aprotic Solvent Electrolytes

Knowledge about the microscale distribution of CO₂ is essential in many environmental and technical settings, and electrochemical CO₂ sensing may be optimized to yield such information. The performance of a Clark-type CO₂ sensor was greatly improved by adding 20% dimethylformamide (DMF) to the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA) previously used as an electrolyte. The addition of DMF resulted in a much faster response to increasing (95% response of about 100 s) or decreasing CO₂ concentration, a negligible interference from low concentrations of N₂O, and a signal temperature dependence similar to that of O₂ microsensors. The use of 80% EMIM-DCA/20% DMF as an electrolyte leads to CO₂ reduction at -0.72 V (vs standard hydrogen electrode), reducing the overpotential by 0.2 V as compared to the use of 100% EMIM-DCA. The CO₂ microsensor has a calculated limit of detection of 0.5 Pa CO₂, and sensors optimized for high sensitivity exhibited a linear response within the range of 0-4.6 kPa (0-1.7 mM) CO₂. A set of four sensors exhibited no noticeable change of zero current and CO₂ sensitivity during 4 months of continuous polarization.

Microsensors in plant biology: in vivo visualization of inorganic analytes with high spatial and/or temporal resolution

This Expert View provides an update on the recent development of new microsensors, and briefly summarizes some novel applications of existing microsensors, in plant biology research. Two major topics are covered: (i) sensors for gaseous analytes (O2, CO2, and H2S); and (ii) those for measuring concentrations and fluxes of ions (macro- and micronutrients and environmental pollutants such as heavy metals). We show that application of such microsensors may significantly advance understanding of mechanisms of plant-environmental interaction and regulation of plant developmental and adaptive responses under adverse environmental conditions via non-destructive visualization of key analytes with high spatial and/or temporal resolution. Examples included cover a broad range of environmental situations including hypoxia, salinity, and heavy metal toxicity. We highlight the power of combining microsensor technology with other advanced biophysical (patch-clamp, voltage-clamp, and single-cell pressure probe), imaging (MRI and fluorescent dyes), and genetic techniques and approaches. We conclude that future progress in the field may be achieved by applying existing microsensors for important signalling molecules such as NO and H2O2, by improving selectivity of existing microsensors for some key analytes (e.g. Na, Mg, and Zn), and by developing new microsensors for P.