Planar Optode Research Articles

Localized Intensification of Arsenic Release within the Emergent Rice Rhizosphere.

Behavior of trace elements in flooded/lowland rice soils is controlled by root-zone iron oxidation. Insoluble iron species bind/capture toxic elements, i.e., arsenic. However, it was recently observed that within this territory of arsenic immobilization lies a zone of prolific iron release, accompanied by a significant flux of arsenic in close proximity to rice root apices. Questions still remain on how common this phenomenon is and whether the chemical imaging approaches or soils/cultivars used influence this event. Here, three types of ultrathin/high-resolution diffusive gradient in thin films (DGT) substrates were integrated with oxygen planar optodes in a multilayer system, providing two-dimensional mapping of solute fluxes. The three DGT approaches revealed a consistent/overlapping spatial distribution with localized flux maxima for arsenic, which occurred in all experiments, concomitant with iron mobilization. Soil/porewater microsampling within the rhizosphere revealed no significant elevation in the solid phase's total iron and arsenic concentrations between aerobic and anaerobic zones. Contrary to arsenic, phosphorus bioavailability was shown to decrease in the arsenic/iron flux maxima. Rice roots, in addition to their role in nutrient acquisition, also perform a key sensory function. Flux maxima represent a significant departure from the chemical conditions of the bulk/field environment, but our observations of a complete rhizosphere reveal a mixed mode of root-soil interactions.

Impact of Bed Form Celerity on Oxygen Dynamics in the Hyporheic Zone

Oxygen distribution and uptake in the hyporheic zone regulate various redox-sensitive reactions and influence habitat conditions. Despite the fact that fine-grain sediments in streams and rivers are commonly in motion, most studies on biogeochemistry have focused on stagnant sediments. In order to evaluate the effect of bed form celerity on oxygen dynamics and uptake in sandy beds, we conducted experiments in a recirculating indoor flume. Oxygen distribution in the bed was measured under various celerities using 2D planar optodes. Bed morphodynamics were measured by a surface elevation sensor and time-lapse photography. Oxygenated zones in stationary beds had a conchoidal shape due to influx through the stoss side of the bed form, and upwelling anoxic water at the lee side. Increasing bed celerity resulted in the gradual disappearance of the upwelling anoxic zone and flattening of the interface between the oxic (moving fraction of the bed) and the anoxic zone (stationary fraction of the bed), as well as in a reduction of the volumetric oxygen uptake rates due shortened residence times in the hyporheic zone. These results suggest that including processes related to bed form migration are important for understanding the biogeochemistry of hyporheic zones.

Luminescence Lifetime Imaging of O2 with a Frequency-Domain-Based Camera System.

We describe a method to image dissolved oxygen (O2), in 2D at high spatial (< 50-100 µm) and temporal (< 10 s) resolution. The method employs O2 sensitive luminescent sensor foils (planar optodes) in combination with a specialized camera system for imaging luminescence lifetime in the frequency-domain. Planar optodes are prepared by dissolving the O2-sensitive indicator dye in a polymer and spreading the mixture on a solid support in a defined thickness via knife coating. After evaporation of the solvent, the planar optode is placed in close contact with the sample of interest - here demonstrated with the roots of the aquatic plant Littorella uniflora. The O2 concentration-dependent change in the luminescence lifetime of the indicator dye within the planar optode is imaged via the backside of the transparent carrier foil and aquarium wall using a special camera. This camera measures the luminescence lifetime (µs) via a shift in phase angle between a modulated excitation signal and emission signal. This method is superior to luminescence intensity imaging methods, as the signal is independent of the dye concentration or intensity of the excitation source, and solely relies on the luminescence decay time, which is an intrinsically referenced parameter. Consequently, an additional reference dye or other means of referencing are not needed. We demonstrate the use of the system for macroscopic O2 imaging of plant rhizospheres, but the camera system can also easily be coupled to a microscope.

In-situ, high-resolution evidence from water-sediment interface for significant role of iron bound phosphorus in eutrophic lake

Potential release of phosphorus (P) bound to iron (Fe) is critical because of the aggravating effects on P load in aquatic ecosystems. However, the process is largely unknown due to the absence of in-situ high-resolution evidence. Dissolved oxygen (DO), ferrous ion (Fe2+), and dissolved reactive phosphate (DRP) in interstitial water of sediment columns from a eutrophic shallow lake were measured using the novel colorimetric planar optode imaging method and ZrO-Chelex DGT technology during controlled experimental episodes. The solid Fe and P fractions in sediments were also simultaneously evaluated by employing sequential extraction procedure and spectra scanning analysis including SEM-EDS and 57Fe-Mössbauer spectroscopy. The results demonstrated that the DO penetration depths were accordingly regulated with time, the depths depended on the oxygen supply patterns, and oxygen depletion occurred at anaerobic intervals. Considerable increases of concentrations and diffusion of Fe2+ and DRP in interstitial water upward from the deep layer into the overlying water were mirrored by decreased concentrations of solid Fe bound P and mineral phase Fe(II) during an anaerobic episode. This confirmed that the re-dissolution of solid Fe bound P pools is the most important source of labile P, and aggravates the P budget in lake water via anaerobic intervals. The reduction-precipitation mechanism of Fe bound P during different oxidation scenarios indicated that the Fe bound P in sediments can act as intermediates between Po and Ca bound P, and result in the permanent burying of authigenic Ca bound P. Significantly positive correlations (R2 ≥ 0.7783, n = 74) between labile Fe2+ and DRP on both redox conditions also provided explicit evidence for the critical role of redox controlling Fe in labile P cycling at the lacustrine sediment-water interface. These findings provide improved insight for potential controlling effort of Fe coupled P to labile P depending on the oxygen supply in shallow-water hypereutrophic lakes.

The effect of unsteady streamflow and stream-groundwater interactions on oxygen consumption in a sandy streambed

Streamflow dynamics are often ignored when studying biogeochemical processes in the hyporheic zone. We explored the interactive effects of unsteady streamflow and groundwater fluxes on the delivery and consumption of oxygen within the hyporheic zone using a recirculating flume packed with natural sandy sediments. The flume was equipped with a programmable streamflow control and drainage system that was used to impose losing and gaining fluxes. Tracer tests were used to measure hyporheic exchange flux and a planar optode was used to measure subsurface oxygen concentration patterns. It was found that the volume of the oxic zone decreased when the losing flux declined, and was drastically decreased when gaining conditions were applied. It was also found that unsteady streamflow led to a slight increase in the average volume of the oxic zone, compared to the average volume of the oxic zone under steady streamflow. However, the average oxygen consumption rates were significantly higher under unsteady streamflow compared to steady streamflow under all groundwater conditions with the exception of the highest losing flux. The present study provides the first insight into the interactions between streamflow unsteadiness and losing/gaining fluxes and improve understanding of their impact on microbial metabolism in the hyporheic zone.

Constraints on CaCO3 precipitation in superabsorbent polymer by aerobic bacteria.

Microbially induced CaCO3 precipitation (MICP) can give concrete self-healing properties. MICP agents are typically bacterial endospores which are coated into shelled granules, infused into expanded clay, or embedded into superabsorbent polymer (SAP). When small cracks appear in the cured concrete, the encapsulation is broken and the metabolic CO2 production from the germinated bacteria causes healing of the cracks by precipitation of CaCO3. Such systems are being tested empirically at large scales, but survival of endospores through preparation and application, as well as germination and growth kinetics of the germinated vegetative cells, remains poorly resolved. We encapsulated endospores of Bacillus subtilis and Bacillus alkalinitrilicus in crosslinked acrylamide-based SAP and quantified their germination, growth, and, in the case of B. alkalinitrilicus, CaCO3 precipitation potential. The endospores survived crosslinking and desiccation inside the polymer matrix. Microcalorimetry and microscopy showed that ~ 80% of the encapsulated endospores of both strains readily germinated after rehydration of freeze-dried SAP. Germinated cells grew into dense colonies of cells inside the SAP, and those of B. alkalinitrilicus calcified with up to 0.3g CaCO3 produced per g desiccated SAP when incubated aerobically. Measurements by planar optodes indicated that the precipitation rates were inherently oxygen limited due to diffusional constraints, rather than limited by electron donor or Ca2+ availability. Such oxygen limitation will limit MICP in all water-saturated and oxygen-dependent systems, and MICP agents based on anaerobic bacteria, e.g., nitrate reducers, should be developed to broaden the applicability of bioactive self-healing concretes to wet and waterlogged environments.

Evaluation of Ebselen-azadioxatriangulenium as redox-sensitive fluorescent intracellular probe and as indicator within a planar redox optode

Redox homeostasis is essential for cellular survival and is therefore tightly regulated. Visualizing changes in the local redox state is essential for detecting oxidative stress, especially with the help of red, long emission lifetime probes. We synthesized a new reversible, redox-sensitive fluorescent probe, ebselen-azadioxatriangulenium (Ebselen-ADOTA), and characterized its potential use for intra- and extracellular redox sensing. The probe consists of a photostable triangulenium-based fluorophore (emission maximum at 564 nm) modulated by a redox-active ebselen motif. The reduced form only shows around 1/10 of the emission intensity of the oxidized form. The Ebselen-ADOTA probe exhibited reversible oxidation and reduction in solution under extracellular conditions. In addition, Ebselen-ADOTA can visualize an increase in oxidative stress within anaerobic bacteria when exposing them to air, the probe did not readily respond to changes in the redox environment inside mammalian cells, probably due to unfavorable localization, aggregation or binding to cysteine containing proteins. When incorporated inside a polymer matrix (hydrogel D4 mixed with poly(acrylic acid)), the probe showed a rapid and fully reversible response to redox changes, thus enabling the preparation of a redox-sensitive optode.

Source of organic detritus and bivalve biomass influences nitrogen cycling and extracellular enzyme activity in estuary sediments

In aquatic ecosystems, natural processes that remove nitrogen from the biologically available pool (e.g. denitrification) have been intensively studied as an ecosystem function that reduces eutrophication. The quantity of sediment organic matter is a key driver of denitrification with percent organic content positively related to rates of nitrogen removal; however, few studies have investigated the influence of the quality of organic matter on nitrogen cycling in estuarine sediments despite shifts in primary producers with eutrophication. This laboratory study using intact benthic communities investigates the influence of various organic detritus sources, which vary in their C:N ratio, on nitrogen gas (N2) and solute fluxes and extracellular enzyme activity in estuarine sediments. A custom-built tank with a removable front plate was used with a planar optode film to image sediment oxygenation. Mangrove leaf detritus significantly increased the net N2 production in sediments, while the deposition of other detrital sources and control sediments produced net N2 consumption. Sulfatase activity was significantly reduced in the mangrove leaves and seagrass treatments, suggesting alteration of heterotrophic microbial activity with reducing oxygen conditions. Leucine aminopeptidase activity, indicating nitrogen cycling, was reduced in all treatments, suggesting the organic detritus provided a nitrogen supplement or reduced the activity of extracellular enzymes producing microbes. Bivalve biomass increased net nitrogen gas fluxes in some treatments. Our results indicate different detrital sources may have varying impacts on the removal of bioavailable nitrogen through denitrification and show that feedbacks in biogeochemical cycles may occur with changes in organic detrital source pools.

Elevated micro-topography boosts growth rates in Salicornia procumbens by amplifying a tidally driven oxygen pump: implications for natural recruitment and restoration.

Background and AimsThe growth rate of pioneer species is known to be a critical component determining recruitment success of marsh seedlings on tidal flats. By accelerating growth, recruits can reach a larger size at an earlier date, which reduces the length of the disturbance-free window required for successful establishment. Therefore, the pursuit of natural mechanisms that accelerate growth rates at a local scale may lead to a better understanding of the circumstances under which new establishment occurs, and may suggest new insights with which to perform restoration. This study explores how and why changes in local sediment elevation modify the growth rate of recruiting salt marsh pioneers.MethodsA mesocosm experiment was designed in which the annual salt marsh pioneer Salicornia procumbens was grown over a series of raised, flat and lowered sediment surfaces, under a variety of tidal inundation regimes and in vertically draining or un-draining sediment. Additional physical tests quantified the effects of these treatments on sediment water-logging and oxygen dynamics, including the use of a planar optode experiment.Key ResultsIn this study, the elevation of sediment micro-topography by 2 cm was the overwhelming driver of plant growth rates. Seedlings grew on average 25 % faster on raised surfaces, which represented a significant increase when compared to other groups. Changes in growth aligned well with the amplifying effect of raised sediment beds on a tidally episodic oxygenation process wherein sediment pore spaces were refreshed by oxygen-rich water at the onset of high tide.ConclusionsOverall, the present study suggests this tidally driven oxygen pump as an explanation for commonly observed natural patterns in salt marsh recruitment near drainage channels and atop raised sediment mounds and reveals a promising way forward to promote the establishment of pioneers in the field.

High-resolution imaging of rhizosphere oxygen (O2) dynamics in Potamogeton crispus: effects of light, temperature and O2 content in overlying water

Radial oxygen loss (ROL) for macrophytes is intimately involved in their survival and growth, thus detailed characterizations of ROL and its implication for geochemical processes are of particular interest. We experimentally investigated ROL patterns from the submerged macrophyte species Potamogeton crispus and determined how light, temperature and O2 content in overlying water regulate O2 micro-distributions in the rhizosphere. Planar optodes were firstly used for non-destructive imaging of the O2 micro-distributions around single roots of P. crispus planted in rhizotrons containing sediment. The dynamic changes in below-ground O2 concentrations and oxygenation expansion at different light intensities (0–216 μmol photons m−2), temperatures (14 and 25 °C) and O2 content in overlying waters (0–256 μmoL−1) were quantified. P. crispus-mediated ROL is predominantly localized to the root apices with an average rate of 168.1 ± 21.4 nmol m−2 s−1 under nearly natural conditions (light, saturated overlying water at 14 °C), maintaining a visible oxygenated rhizosphere zone with a radius of 1.33 ± 0.21 mm. ROL is closely correlated with light intensity, suggesting photosynthetic O2 evolution. In darkness, the rhizosphere O2 availability is significantly reduced and strongly affected by the O2 content in overlying water, which passively diffuses into the plant leaves. Elevated temperatures lead to diminished ROL as a result of increased O2 demand of the surrounding sediment. A high O2 microheterogeneity around P. crispus root apices is firstly demonstrated, and we provide direct empirical evidences that ROL are regulated dynamically by the shifting light, temperature and O2 content in overlying water.

Coupling zymography with pH mapping reveals a shift in lupine phosphorus acquisition strategy driven by cluster roots

Phosphorus (P) availability affects the spatial and temporal distribution of phosphatase activity and acidification in the rhizosphere: two strongly interactive key strategies of nutrient acquisition by roots. Zymography was coupled with pH planar optode mapping to reveal the effects of P-availability (P deficiency, or with phytate or Ca(H2PO4)2 amendment) on the spatial distribution of phosphatase activity, pH and rhizosphere extent around the taproot of lupine before and after cluster root formation. Before cluster root formation, phosphorus deficiency increased acid phosphatase activities by 20%, decreased pH by 0.8 units and broadened the rhizosphere extent by 0.4 mm across the taproot. Phytate (80 mg kg−1) addition dampened these changes before cluster root formations. In contrast, the rhizosphere extent of phosphatase activity after cluster root formation was 0.2 mm narrower under P-deficiency than with Ca(H2PO4)2 amendment. Due to cluster root formation, the hotspot areas of alkaline phosphatase activity were 40% larger for lupine grown under P-deficiency than amended with Ca(H2PO4)2. Lupine rhizosphere strategies shifted during growth: increasing phosphatase activity, acidifying soil and broadening the rhizosphere around the taproot are dominant mechanisms before cluster root formation. After cluster root development, the main mechanism is increasing the area of phosphatase activity hotspots around cluster roots to enlarge the exploited soil volume.

The rhizosphere of aquatic plants is a habitat for cable bacteria.

ABSTRACTCable bacteria belonging to the family Desulfobulbaceae couple sulfide oxidation and oxygen reduction by long-distance electron transfer over centimeter distances in marine and freshwater sediments. In such habitats, aquatic plants can release oxygen into the rhizosphere. Hence, the rhizosphere constitutes an ideal habitat for cable bacteria, which have been reported on seagrass roots recently. Here, we employ experimental approaches to investigate activity, abundance, and spatial orientation of cable bacteria next to the roots of the freshwater plant Littorella uniflora. Fluorescence in situ hybridization (FISH), in combination with oxygen-sensitive planar optodes, demonstrated that cable bacteria densities are enriched at the oxic–anoxic transition zone next to roots compared to the bulk sediment in the same depth. Scanning electron microscopy showed cable bacteria along root hairs. Electric potential measurements showed a lateral electric field over centimeters from the roots, indicating cable bacteria activity. In addition, FISH revealed that cable bacteria were present in the rhizosphere of Oryza sativa (rice), Lobelia cardinalis and Salicornia europaea. Hence, the interaction of cable bacteria with aquatic plants of different growth forms and habitats indicates that the plant root–cable bacteria interaction might be a common property of aquatic plant rhizospheres.

Successful control of phosphorus release from sediments using oxygen nano-bubble-modified minerals

Due to the limited aeration capacity of current aeration techniques at the sediment-water interface (SWI), we developed a specialized aeration material aimed at the SWI, known as oxygen nano-bubble-modified minerals (ONBMMs). Furthermore, we simulated its aeration efficiency at the SWI and the control effects of internal phosphorous (P) release under anaerobic conditions during 20 days. High resolution diffusive gradients in thin films (DGT) and Planar luminescent optode (PO) technologies were used to measure the temporal variation of reactive P, reactive Fe (II) and dissolved oxygen (DO) of the SWI. These results show that ONBMMs can effectively increase the content of DO at the SWI and decrease the release flux of internal P from sediments. The use of ONBMMs reduced 97.9% of the soluble reactive P concentration of the overlaying water and reduced the release flux of DGT-P from sediments by 78.9%. Inhibition of reductive dissolution of FeP from sediments was the primary principle that effectively inhibited the input of internal P by ONBMMs. Therefore, ONBMMs are potentially promising technology for the treatment of internal P pollution in eutrophic lakes.

Root-induced soil acidification and cadmium mobilization in the rhizosphere of Sedum plumbizincicola: evidence from a high-resolution imaging study

Plant roots can significantly alter soil pH and the chemical concentration and distribution of different elements in the rhizosphere environment. Here we ask whether cadmium (Cd) bioavailability in the rhizosphere of Cd-hyperaccumulator Sedum plumbizincicola can be influenced by root-induced effects on soil pH. The Cd-hyperaccumulator S. plumbizincicola and the Cd non-hyperaccumulator ecotype Sedum alfredii were both grown in four different Cd-contaminated soils. We used the planar optode imaging technique to produce two-dimensional and high-resolution measurements of soil pH. Shoot excess cation concentration, root architecture and Cd concentrations ([Cd]) in soil pore water were also measured. Spatial analyses based on kernel density estimate of roots (KDE) and a Moran’s I correlogram were performed to assess spatial patterns and potential relationships among root distribution, soil pH and [Cd]. Both Sedum species showed root-induced increases in soil acidification (i.e. soil pH decreases of 0.1 to 0.62 units), which were clearly associated with greater root density of these plants. Remarkable excess cation uptakes by both Sedum species were detected and likely a driving factor for the root-induced acidification. The presence of the roots of S. plumbizincicola were then related to higher [Cd] in the rhizosphere than in bulk soil in Orthic Acrisol (+342%) and in Hydragric Antrosol soils (+296%). The hyperaccumulator S. plumbizincicola had larger root systems, higher acidification ability, and was associated with greater soil [Cd] than S. alfredii. Spatial patterns of root distribution and soil pH were similar between Sedum plants, however, spatial patterns of [Cd] differed across polluted soils. Rhizosphere acidification induced by S. plumbizincicola plants can play an important role on soil Cd mobilization, but overall effects on soil Cd bioavailability will depend on intrinsic soil biogeochemical properties.

Experimental study on the oxygenation efficiency of nano-bubble modified mineral particles at the sediment-water interface in lakes

季节性缺氧导致夏季沉积物内源磷强烈释放，加剧水体富营养化，是我国西南地区深水湖泊（水库）面临的重要挑战.有效增加夏季缺氧期深水沉积物-水界面的含氧量，是减少内源磷释放的关键.现有的深水增氧技术由于缺乏对沉积物-水界面增氧的针对性，因此治理效果有限.近年来，纳米气泡已被证实具有的稳定性好、氧传质速率高和环境风险低等优点，为新型深水增氧技术研发提供了巨大潜力.本文以天然矿物材料白云母、绢云母、硅藻土和沸石为基底，负载纳米气泡，研发纳米气泡改性矿物颗粒技术，开展湖泊沉积物-水界面增氧模拟实验研究，运用平面光电极技术评估其界面增氧效果.结果表明，纳米气泡改性矿物颗粒对沉积物-水界面具有比较明显的增氧效果.其中，改性白云母、绢云母和沸石的界面持续增氧时间可达7天以上，增氧后的界面最大溶解氧（DO）浓度达4.40 mg/L，而改性硅藻土不具有增氧能力.其次，矿物粒度对改性颗粒的增氧效果有一定影响：粒度越细，界面的最大增氧浓度越高，且持续增氧时间越长.纳米气泡改性矿物颗粒技术有望成为夏季缺氧期深水沉积物-水界面精准增氧和内源污染控制的有效技术手段.;Seasonal hypoxia that enhancing the release of phosphorus from sediment in summer and exacerbating the water eutrophication is a big challenge for deep-water lakes (reservoirs) in southwest China. Increasing the oxygen level of the sediment-water interface (SWI) during summer anoxic periods is the key to reduce the internal phosphorus release. The existing deep water oxygenation technology has limited efficiency due to lacking pertinence to the SWI. In recent years, the nano-bubbles provide great potential for the development of a new oxygenation technology, which have advantages of good stability, high oxygen mass transfer rate and low environmental risk. In this study, the natural minerals, i.e., white mica, sericite, diatomite and zeolite, were modified by nano-bubbles to develop a new oxygenation technology. Simulation experiments were conducted to assess the efficiency of the newly established oxygenation technology by using planar luminescent optode technology. Our results showed that the nano-bubbles modified mineral particles obviously increased the oxygen concentrations of the SWI. The duration of continuous oxygenation for white mica, sericite and zeolite can be over 7 days, while maximum dissolved oxygen concentration at the SWI was more than 4.40 mg/L. The modified diatomite had no ability to increase oxygen level of the SWI. Minerals with fine grain size have the stronger oxygenation efficiency and the longer oxygenation duration. The nano-bubbles modified mineral technology provides new technical support for the precise oxygenation of SWI and effective control of sediment pollution in lakes.

Diffusive gradients in thin films: devices, materials and applications

A large number of ex situ sampling techniques have been used traditionally to investigate the impact and fate of pollutants in soil, sediment and waters. However, the distribution and form of chemical species present are often altered prior analysis, due to the alterations during sampling and transfer to the laboratory. Alternatively, a robust in situ passive sampling technique , diffusive gradients in thin films (DGT), has been developed for the measurement of labile concentrations, species and distribution of various solutes in soil, sediment and waters. Here we review the recent developments in DGT device configurations and components, e.g., binding agents, diffusive phases and filter membranes. We highlight new configurations for effectively reducing the measurement errors and the disturbance of environmental media. We discuss DGT applications for the analysis of soil, sediment and water, such as evaluation of bioavailability and toxicity, measurement of nutrients and organic substances, and assessment of relationships between multiple solutes. We also present the coupling of DGT with other in situ measurement techniques such as dialysis samplers (Peeper), diffusive equilibrium in thin films (DET) and planar optodes (PO).

Background-Free Fluorescence-Decay-Time Sensing and Imaging of pH with Highly Photostable Diazaoxotriangulenium Dyes.

Novel fluorescent diazaoxatriangulenium (DAOTA) pH indicators for lifetime-based self-referenced pH sensing are reported. The DAOTA dyes were decorated with phenolic-receptor groups inducing fluorescence quenching via a photoinduced-electron-transfer mechanism. Electron-withdrawing chlorine substituents ensure response in the most relevant pH range (apparent p Ka' values of ∼5 and 7.5 for the p, p-dichlorophenol- and p-chlorophenol-substituted dyes, respectively). The dyes feature long fluorescence lifetimes (17-20 ns), high quantum yields (∼60%), and high photostabilities. Planar optodes are prepared upon immobilization of the dyes into polyurethane hydrogel D4. Apart from the response in the fluorescence intensity, the optodes show pH-dependent lifetime behavior, which makes them suitable for studying 2D pH distributions with the help of fluorescence-lifetime-imaging techniques. The lifetime response is particularly pronounced for the sensors with high dye concentrations (0.5-1 wt % with respect to the polymer) and is attributed to the efficient homo-FRET mechanism.

In Situ Selective Measurement of SeIV in Waters and Soils: Diffusive Gradients in Thin-Films with Bi-Functionalized Silica Nanoparticles.

The speciation of selenium (Se) controls its fate and behavior, determining both its biological and environmental activities. However, in situ monitoring of SeIV presents a significant challenge due to its sensitivity to redox change. A novel diffusive gradients in thin films (DGT) technique containing mercapto-, amino-bifunctionalized SBA15 mesoporous silica nanoparticles was developed and evaluated in a series of laboratory and field deployment tests. The SBA-DGT exhibited a linear accumulation of SeIV ( r2 > 0.997) over a 72 h deployment, with negligible accumulation of SeVI(<5%). Consistent prediction of SeIV occurred within ionic strength and pH ranges of 0.1-200 mmol L-1 and 3.6-8, respectively. Limits of detection of the SBA-DGT were 0.03 μg SeIV L-1, which is suitable for natural waters. Moreover, the properties of the bifunctionalized SBA15 enable it to be fabricated within ultrathin (0.05 mm) gel layers for use in conjunction with O2 planar optode imaging. This new sandwich sensor technology with SBA-DGT was validated by mapping the two-dimensional distribution of SeIV and oxygen simultaneously in rice rhizospheres. This study shows that SBA-DGT provides a selective measurement of SeIV in situ, demonstrating its potential for both environmental monitoring and as a research tool for improving our understanding of Se biogeochemical processes.

Planar optode observation method for the effect of raindrop on dissolved oxygen and pH diffusion of air–water interface

The air–water interface is an important boundary where material exchanges, it has important influence on ecosystem and biogeochemical cycle. The rain can change the balance of interface, improve the exchange rate of gas flux, and make the distribution of dissolved oxygen and pH around the interface change in horizontal and vertical direction. Based on planar sensing film with highly spatial and temporal resolution which can provide the characteristics of two-dimensional distribution information, we carry out the simulation experiment of raindrops about oxygen and pH distribution in air–water interface using double parameters planar optode. The experimental data are analyzed from the gas transfer velocity, the kinetic energy flux and the sea-air flux of oxygen. The results show that rainfall process plays an important role in adjusting dissolved oxygen and pH of the surface water, the raindrop can break the balance of micro surface of water–gas interface mechanism, increase the gas transfer velocity and promote the dissolution of oxygen in the atmosphere in water to make a average increase of about 2.3 mg/L in vertical direction 23 mm. The impact of rainfall on pH of the water surface within 12 mm is relatively obvious, the pH value decreased by an average of 0.2–0.4 units, indicating that the raindrop promoted the migration of the atmosphere in the air–water interface, and the dissolved CO2 caused the surface water acidification. This study provides a novel technical method for understanding the influence of raindrops on the dissolved oxygen concentration and pH of the surface water in low wind-impacting area and static-water area. This study provides a novel technical method for understanding the influence of raindrops on the dissolved oxygen concentration and pH of the surface water in low wind impacting area and static water area.

Oxygen loss from seagrass roots coincides with colonisation of sulphide-oxidising cable bacteria and reduces sulphide stress.

Seagrasses thrive in anoxic sediments where sulphide can accumulate to phytotoxic levels. So how do seagrasses persist in this environment? Here, we propose that radial oxygen loss (ROL) from actively growing root tips protects seagrasses from sulphide intrusion not only by abiotically oxidising sulphides in the rhizosphere of young roots, but also by influencing the abundance and spatial distribution of sulphate-reducing and sulphide-oxidising bacteria. We used a novel multifaceted approach combining imaging techniques (confocal fluorescence in situ hybridisation, oxygen planar optodes, and sulphide diffusive gradients in thin films) with microbial community profiling to build a complete picture of the microenvironment of growing roots of the seagrasses Halophila ovalis and Zostera muelleri. ROL was restricted to young root tips, indicating that seagrasses will have limited ability to influence sulphide oxidation in bulk sediments. On the microscale, however, ROL corresponded with decreased abundance of potential sulphate-reducing bacteria and decreased sulphide concentrations in the rhizosphere surrounding young roots. Furthermore, roots leaking oxygen had a higher abundance of sulphide-oxidising cable bacteria; which is the first direct observation of these bacteria on seagrass roots. Thus, ROL may enhance both abiotic and bacterial sulphide oxidation and restrict bacterial sulphide production around vulnerable roots, thereby helping seagrasses to colonise sulphide-rich anoxic sediments.