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Cable bacteria (CB) perform electrogenic sulfur oxidation (e-SOx) by spatially separating redox half reactions over centimetre distances. For freshwater systems, the ecology of CB is not yet well understood, partly because they proved difficult to cultivate. This study introduces a new 'agar pillar' approach to selectively enrich and investigate CB populations. Within sediment columns, a central agar pillar is embedded, providing a sediment-free gradient system in equilibrium with the surrounding sediment. We incubated freshwater sediments from a streambed, a sulfidic lake and a hydrocarbon-polluted aquifer in such agar pillar columns. Microprofiling revealed typical patterns of e-SOx, such as the development of a suboxic zone and the establishment of electric potentials. The bacterial communities in the sediments and agar pillars were analysed over depth by PacBio near-full-length 16S rRNA gene amplicon sequencing, allowing for a precise phylogenetic placement of taxa detected. The selective niche of the agar pillar was preferentially colonized by CB related to Candidatus Electronema for surface water sediments, including several potentially novel species, but not for putative groundwater CB affiliated with Desulfurivibrio spp. The presence of CB was seemingly linked to co-enriched fermenters, hinting at a possible role of e-SOx populations as an electron sink for heterotrophic microbes. These findings add to our current understanding of the diversity and ecology of CB in freshwater systems, and to a discrimination of CB from surface and groundwater sediments. The agar pillar approach provides a new strategy that may facilitate the cultivation of redox gradient-dependent microorganisms, including previously unrecognized CB populations.Novel agar pillar gradient columns selectively enrich for cable bacteria and differentiate electrogenic populations from surface and groundwater sediments.

Cable bacteria are multicellular microorganisms that are capable of long distance electron transport (LDET) along their length. This electron transport is the result of oxidation of hydrogen Sulfide (H2S) in the sulfidic sediment layer where electrons are conducted up through cable filament aided by cell-to-cell transfer in the oxic layer thus reducing oxygen by gaining electrons. Cable bacteria behave as dipoles where anaerobic zones interfere with oxic zones for example oil/tar pollution site and can generate enough natural SP fields as a function of redox mechanism that can be measured on the surface. This study focuses on the theoretical analysis of Self-Potential (SP) signals resulting due to the presence of dipole current source under different conductivity structures in the subsurface. To investigate the behavior of SP signals, four different types of forward models are synthesized by varying resistivity of subsurface layers and changing the depth of the dipole beneath the surface. The dipole has a default current density of 20 mA/m2. In the first model, a rectangular pollution patch carrying a dipole of the same shape is placed between two homogeneous layers where the top layer resistivity is swept from 10-1000 ohm-m while keeping the resistivity of bottom layer constant. In the second model, the pollution patch is placed between an inhomogeneous layer with low, intermediate, and high resistivity contrasts and a homogeneous layer. In this model, half of the patch lies in lower conducting region whereas the other part is in the high conductivity region. The third model is an extension of the second one, where the inhomogeneous layer is sandwiched between two homogeneous layers. In the last model, the pollution patch was moved beneath the surface to a depth where the SP signal cannot be observed at the surface. In this model, the depth is observed for three different pollution sources with current density values equal to 2, 20 and 200 mA/m2 respectively. The results showed that SP anomaly caused by the patch when the conductivity of upper layer is high is smaller as compared to the anomaly due to the less conducting upper layer. Next two models with inhomogeneous layer, correlate well with the first model showing high SP anomaly caused by dipole when it is present in the lower conducting region and low values when in high conducting region. Fourth model demonstrates when the depth of pollution patch is increased beneath the surface, SP signal decreases and is not observed beneath a depth of around 10 m, even when the source has current density value as high as 200 mA/m2. This study explicitly demonstrates the behavior of SP anomaly and will help in improved interpretation of SP technique where inhomogeneity will be present beneath the surface.

Nitrous oxide is an important greenhouse gas and there is a need for sensitive techniques to study its distribution in the environment at concentrations near equilibrium with the atmosphere (9.6 nM in water at 20 °C). Here we present an electrochemical sensor that can quantify N2O in the nanomolar range. The sensor principle relies on a front guard cathode placed in front of the measuring cathode. This cathode is used to periodically block the flux of N2O towards the measuring cathode, thereby creating an amplitude in the signal. This signal amplitude is unaffected by drift in the baseline current and can be read at very high resolution, resulting in a sensitivity of 2 nM N2O for newly constructed sensors. Interference from oxygen is prevented by placing the front guard cathode in oxygen-consuming electrolyte. The sensor was field tested by measuring an N2O profile to a depth of 120 m in the oxygen minimum zone of the Eastern Tropical North Pacific Ocean (ETNP) off the coast of Mexico.

MILAN was a multidisciplinary, international study examining how the diel variability of sea-surface microlayer biogeochemical properties potentially impacts ocean-atmosphere interaction, in order to improve our understanding of this globally important process. The sea-surface microlayer (SML) at the air-sea interface is < 1 mm deep but it is physically, chemically and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all ocean-atmosphere exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the air-sea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterised. MILAN (sea-surface MIcroLAyer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability, e.g. the influence of changing solar radiation on the quantity and quality of organic material, and diel changes in wind intensity primarily forcing air-sea CO2 exchange, underline the value and the need of multidisciplinary campaigns for integrating SML complexity into the context of air-sea interaction.

The biodegradation of organic pollutants in aquifers is often restricted to the fringes of contaminant plumes where steep countergradients of electron donors and acceptors are separated by limited dispersive mixing. However, long-distance electron transfer (LDET) by filamentous ‘cable bacteria’ has recently been discovered in marine sediments to couple spatially separated redox half reactions over centimeter scales. Here we provide primary evidence that such sulfur-oxidizing cable bacteria can also be found at oxic–anoxic interfaces in aquifer sediments, where they provide a means for the direct recycling of sulfate by electron transfer over 1–2-cm distance. Sediments were taken from a hydrocarbon-contaminated aquifer, amended with iron sulfide and saturated with water, leaving the sediment surface exposed to air. Steep geochemical gradients developed in the upper 3 cm, showing a spatial separation of oxygen and sulfide by 9 mm together with a pH profile characteristic for sulfur oxidation by LDET. Bacterial filaments, which were highly abundant in the suboxic zone, were identified by sequencing of 16S rRNA genes and fluorescence in situ hybridization (FISH) as cable bacteria belonging to the Desulfobulbaceae. The detection of similar Desulfobulbaceae at the oxic–anoxic interface of fresh sediment cores taken at a contaminated aquifer suggests that LDET may indeed be active at the capillary fringe in situ. The biodegradation of organic pollutants in aquifers is often restricted to the fringes of contaminant plumes where steep countergradients of electron donors and acceptors are separated by limited dispersive mixing. However, long-distance electron transfer (LDET) by filamentous ‘cable bacteria’ has recently been discovered in marine sediments to couple spatially separated redox half reactions over centimeter scales. Here we provide primary evidence that such sulfur-oxidizing cable bacteria can also be found at oxic–anoxic interfaces in aquifer sediments, where they provide a means for the direct recycling of sulfate by electron transfer over 1–2-cm distance. Sediments were taken from a hydrocarbon-contaminated aquifer, amended with iron sulfide and saturated with water, leaving the sediment surface exposed to air. Steep geochemical gradients developed in the upper 3 cm, showing a spatial separation of oxygen and sulfide by 9 mm together with a pH profile characteristic for sulfur oxidation by LDET. Bacterial filaments, which were highly abundant in the suboxic zone, were identified by sequencing of 16S rRNA genes and fluorescence in situ hybridization (FISH) as cable bacteria belonging to the Desulfobulbaceae. The detection of similar Desulfobulbaceae at the oxic–anoxic interface of fresh sediment cores taken at a contaminated aquifer suggests that LDET may indeed be active at the capillary fringe in situ.

In marine sediments cathodic oxygen reduction at the sediment surface can be coupled to anodic sulfide oxidation in deeper anoxic layers through electrical currents mediated by filamentous, multicellular bacteria of the Desulfobulbaceae family, the so-called cable bacteria. Until now, cable bacteria have only been reported from marine environments. In this study, we demonstrate that cable bacteria also occur in freshwater sediments. In a first step, homogenized sediment collected from the freshwater stream Giber Å, Denmark, was incubated in the laboratory. After 2 weeks, pH signatures and electric fields indicated electron transfer between vertically separated anodic and cathodic half-reactions. Fluorescence in situ hybridization revealed the presence of Desulfobulbaceae filaments. In addition, in situ measurements of oxygen, pH, and electric potential distributions in the waterlogged banks of Giber Å demonstrated the presence of distant electric redox coupling in naturally occurring freshwater sediment. At the same site, filamentous Desulfobulbaceae with cable bacterium morphology were found to be present. Their 16S rRNA gene sequence placed them as a distinct sister group to the known marine cable bacteria, with the genus Desulfobulbus as the closest cultured lineage. The results of the present study indicate that electric currents mediated by cable bacteria could be important for the biogeochemistry in many more environments than anticipated thus far and suggest a common evolutionary origin of the cable phenotype within Desulfobulbaceae with subsequent diversification into a freshwater and a marine lineage.

Filamentous, multicellular bacteria of the Desulfobulbaceae family form a biogeobattery in marine sediments by mediating an electric coupling between sulfide oxidation in deeper anoxic layers and oxygen reduction at the sediment surface. The electric fields generated in such biogeobatteries were determined with microelectrodes and correlated well with geochemical indicators of microbial electrogenic activity in the top 2 cm of the sediment. The electric fields collapsed within less than a minute when oxygen was removed and reestablished when oxygen was reintroduced, demonstrating a direct coupling between aerobic microbial activity and presence of electric fields. Modeling of the electric potential distribution was used to quantify and localize electron sources and sinks. Most anodic oxidation was confined to the lower part of the anoxic zone with nondetectable free sulfide, suggesting that iron sulfide was the main sulfide source and had already been depleted in the upper part of the anoxic zone. The flow of electrons from anoxic sediment layers fuelled up to 80% of the aerobic activity of the sediment. Responses of the electric field to the presence and absence of nitrate, nitrite, or nitrous oxide in the overlying water readily demonstrated potentials for cathodic reduction of nitrite or nitric oxide and possibly nitrate but not nitrous oxide.

The currents in the lowest few millimeters of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes such as oxygen transfer into the sediment. So far, no high-resolution flow velocity profiles close to the sediment - water interface have been reported for such systems because a suitable flow meter was lacking. This article introduces a novel sensor for the measurement of extremely lowflow velocities. The sensor is based on a gas transducer surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas outside the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s(-1), with a spatial resolution of 100 to 250 mu m. The flow measurements prove to be insensitive to temperature changes between 5 and 15 C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high- resolution in situ measurements at the bottom of a pre- alpine lake with shear velocities as low as 0.13 +/- 0.02 cm s(-1). The velocity profile nicely resolves the transition zone between the viscous and the logarithmic boundary layer.

In clinical practice a method for assessment of tissue vitality is a sought - after tool. We have developed a new sensor principle, which is able to register changes in tissue concentration of O2 and tissue flow. The technique is based on diffusion of inert gases and mass spectrometer detection of gaseous metabolites. It was hypothesized that the new sensor could register changes in vital parameters after induction and release of an ischaemic insult to muscular tissue. The sensor performance was evaluated in ten anaesthetized pigs subjected to local muscular ischaemia. Preliminary data from this study indicate the validity of registered hypoxia and reduction in tissue flow as a consequence of compromised blood supply. It was concluded that although precise calibration of the technique is not yet established, it holds promise as a technique that can be used to monitor changes in tissue vitality.

Spatially separated electron donors and acceptors in sediment can be exploited by the so-called “cable bacteria.” Electric potential microelectrodes (EPMs) were constructed to measure the electric fields that should appear when cable bacteria conduct electrons over centimeter distances. The EPMs were needle-shaped, shielded Ag/AgCl half-cells that were rendered insensitive to redox-active species in the environment. Tip diameters of 40 to 100 µm and signal resolution of approximately 10 μV were achieved. A test in marine sediments with active cable bacteria showed an electric potential increase by approximately 2 mV from the sediment-water interface to a depth of approximately 20 mm, in accordance with the location and direction of the electric currents estimated from oxygen, pH, and H2S microprofiles. The EPM also captured emergence and decay of electric diffusion potentials in the upper millimeters of artificial sediment in response to changes in ion concentrations in the overlying water. The results suggest that the EPM can be used to track electric current sources and sinks with submillimeter resolution in microbial, biogeochemical, and geophysical studies.

A microscale biosensor for continuous measurement of methane partial pressure based on a novel counterdiffusion principle is presented. Methane-oxidizing bacteria placed in the microsensor utilize oxygen from an internal oxygen reservoir when methane from the exterior diffuses through the tip membrane. The transducer is an internal oxygen microsensor with its tip positioned between the oxygen reservoir and the sensor tip membrane. The external partial pressure of methane determines the rate of bacterial oxygen consumption within the sensor, which in turn is reflected by the signal from the transducer. Tip diameters were down to 20 mu m, enabling us to study methane distribution on a microscale. The microscale construction also results in a low stirring sensitivity and a 95% response time down to 20 s. By tailoring the geometry, sensors can be made to exhibit a linear response in the full range of 0-1 atm partial pressure of methane or, alternatively, to exhibit a linear response only at lower concentrations, improving the sensitivity to below 0.1 kPa, corresponding to similar to 1 mu M in aqueous solution. Temperature, oxygen, and H2S interfere with the signal; no interferences were detected from H-2, NH3, CO2, or acetate.

The water bathing postlarval abalone often lies within the diffusive boundary layer (DBL) so its chemistry is greatly influenced by the metabolism of the biofilm on which the abalone feed. This study used microelectrodes to investigate the influence of water velocity and diatom morphology on dissolved oxygen and pH in the DBL. Decreasing water velocity increased the thickness of the DBL, thereby increasing the amplitude of changes in oxygen concentration. Over a film of the prostrate diatom Nitzschia ovalis Arnot, DBL thickness averaged 71, 139, 177, and 406 mu m at water velocities of 78, 15, 7, and 1 mm s(-1) respectively. Corresponding oxygen concentrations at the biofilm surface under moderate light (75 mu E m(-2) s(-1)) and temperature (15 degrees C) averaged 111%, 120%, 125%, and 151 % of air saturation respectively, at the four velocities. The presence of a 1-mm tall diatom canopy (Achnanthes longipes Agardh) over a Nitzschia ovalis film thickened the DBL by 3-fold at 1 mm s(-1) and 6-fold at similar to 80 mm s(-1). The thickened DBL and higher diatom biomass generated extreme conditions at the biofilm surface. Dissolved oxygen concentrations as high as 440% of air saturation, and pH Lis high as 9.8 were recorded beneath the canopy in moderate light (105 mu E m(-2) s(-1)) and temperature (15 degrees C) at a water velocity of 1 mm s(-1). Changes during darkness were less extreme, with 53% oxygen saturation and pH 7.7 the minima recorded. These measurements demonstrate the extreme water chemistry that can develop in the microhabitat of postlarval abalone. The changes will be amplified by the presence of filamentous diatoms, by increased light intensity, and by lack of water movement. Standard aeration will greatly reduce the extremes experienced by postlarvae by generating water movement sufficient to thin the DBL.

The disposal of sulfide mine tailings in an environmentally sound, yet cost-effective, manner is an issue facing most metal mines. Subaqueous tailing disposal is considered an attractive option for disposal that limits oxygen (O-2) availability within sulfide mine tailings and controlling sulfide oxidation and the resultant acid mine drainage (AMD). Assuming that O-2 profiles represent steady-state conditions, we aim to evaluate the depth-dependent and temperature-dependent rates of O-2 consumption in saturated mine tailings. Measurements include microscale O-2 gradients and diffusivity profiles within columns representing undisturbed mine tailing profiles from an impoundment near Nanisivik Mine in northern Canada. Measurements were made across the diffusive boundary layer (DBL) above the tailing-water interface as well as in the tailings below. Laboratory measurements of O-2 profiles are compared to in situ profiles. From the laboratory results it is possible to evaluate the O-2 flux across the DBL and the depth-integrated O-2 uptake. The results are compared with the average sulfate production rate over 3 months. O-2 uptake in saturated tailings is discussed in relation to O-2 uptake measured in columns after free drainage. The methods applied provide consistent O-2 consumption rates as well as reliable predictions for controlling AMD by keeping tailings under water. Copyright (C) 2001 Elsevier Science Ltd.

A microscale biosensor for nitrate/nitrite was used for in-situ monitoring in an activated sludge plant. The biosensor is based on diffusion of nitrate/nitrite through a tip membrane into a dense mass of bacteria converting the ions into nitrous oxide with subsequent electrochemical detection. The biosensor signal was in very close agreement with the nitrate/nitrite concentration in the plant as analysed by conventional laboratory techniques. The concentration of the only interfering substance, nitrous oxide, was so low that it did not have any effect. The response time of the applied biosensors was less than 30 s, and the detection limit was less than 50 mu g N l(-1). Detachment of the ion-permeable membrane in the tip ol these first-generation biosensors limited the period with continuous monitoring to 5 days. (C) 2000 Elsevier Science Ltd. All rights reserved.

An oxygen-insensitive microscale biosensor for methane was constructed by furnishing a previously described biosensor with an oxygen guard. The guard consisted of a glass capillary containing heterotrophic bacteria, which consumed oxygen diffusing through the tip membrane, thus preventing it from diffusing into the methane-sensing unit. Oxygen microprofiles were measured through the oxygen guard capillary, demonstrating the principle and limitations of the method. When the tip of the guard capillary was exposed to 100% oxygen at 21°C, heterotrophic oxygen consumption prevented oxygen from diffusing further than 170 μm into the capillary, whereas atmospheric levels of oxygen were consumed within 50 μm. The capacity of the oxygen guard for scavenging oxygen decreased with decreasing temperature, and atmospheric levels of oxygen caused oxygen penetration to 200 μm at 5°C. The sensors could be manufactured with tip diameters as small as 25 μm, and response times were about 1 min at room temperature. Pore water profiles of methane concentrations in a rice paddy soil were measured, and a strong correlation between the depths of oxygen penetration and methane appearance was observed as a function of the light regimen; this finding confirmed the role of microbenthic photosynthesis in limiting methane emissions from surfaces of waterlogged sediments and soils.

The effect of respiration on the positive phototactic movement of swarming agar colonies of the facultative phototroph Rhodospirillum centenum was studied. When the electron flow was blocked at the be, complex level by myxothiazol, the oriented movement of the colonies was totally blocked. Conversely, inhibition of respiration via the cytochrome c oxidase stimulated the phototactic response. No phototaxis was observed in a photosynthesis deficient mutant (YB707) lacking bacteriochlorophylls. Analyses of the respiratory activities as monitored by a oxygen microelectrode in single agar colonies during light/dark transitions showed a close functional correlation between the photosynthetic and respiratory apparatuses. The respiratory chain of Rsp. centenum was formed by two oxidative pathways: one branch leading to a cytochrome c oxidase inhibited by low cyanide concentrations and a second pathway formed by an oxidase less-sensitive to cyanide that also catalyzes the light-driven respiration. These results were interpreted to indicate that (1) there is a cyclic electron transport, and (2) photoinduced cyclic electron flow is required for the phototactic response of Rsp. centenum. Furthermore, under oxic conditions in the Light, reducing equivalents may switch from photosynthetic to respiratory components so as to reduce both the membrane potential and the rate of locomotion.

The processes producing and consuming the trace gases N2O and CH4 are often taking place in microenvironments characterized by steep chemical gradients. The chemical microenvironment and the transformation rates within such stratified communities can be studied by use of microsensors, which can resolve chemical gradients with a spatial resolution of a few micrometers. Reviews on the general use of microsensors in microbial ecology have previously been published (Revsbech and Jorgensen 1986, Revsbech 1994). At present, microsensors for both N2O (Revsbech et al. 1988) and CH4 (LR Damgaard and NP Revsbech, unpublished) have been developed, but the N2O microsensor has almost exclusively been used to quantify denitrification while N2O reductase is inhibited with acetylene, and the CH4 microsensor is so new that no work using this sensor has been published. In the present paper we describe the limited work on trace gas transformation presently done by use of microsensors, but it should be emphasized that this is only the beginning of a very detailed description of these processes now enabled by microsensor development.

Microbiosensors for nitrate and methane have been constructed with diameters down to 30 μm. Both sensors are characterized by linear responses to their substrates and 90% response times from 10–120 s. The efficient diffusional transport at a micrometer scale makes it possible to utilize new concepts in the construction of biosensors.

Noninvasive H-1-nuclear magnetic resonance (NMR) imaging was used to investigate the diffusive properties of microbial mats in two dimensions. Pulsed field gradient NMR was used to acquire images of the H2O diffusion coefficient, D-s,D- and multiecho imaging NMR was used to obtain images of the water density in two structurally different microbial mats sampled from Solar Lake (Egypt). We found a pronounced lateral and vertical variability of both water density and water diffusion coefficient, correlated with the laminated and heterogeneous distribution of microbial cells and exopolymers within the mats. The average water density varied from 0.5 to 0.9, whereas the average water diffusion coefficient ranged from 0.4 to 0.9 relative to the values obtained in the stagnant water above the mat samples. The apparent water diffusivities estimated from NMR imaging compared well to apparent O-2 diffusivities measured with a diffusivity microsensor. Analysis of measured O-2 concentration profiles with a diffusion-reaction model showed that both the magnitude of calculated rates and the depth distribution of calculated O-2 consumption/production zones changed when the observed variations of diffusivity were taken into account. With NMR imaging, diffusivity can be determined at high spatial resolution, which can resolve inherent lateral and vertical heterogeneities found in most natural benthic systems.

Using molecular techniques and microsensors for H 2 S and CH 4 , we studied the population structure of and the activity distribution in anaerobic aggregates. The aggregates originated from three different types of reactors: a methanogenic reactor, a methanogenic-sulfidogenic reactor, and a sulfidogenic reactor. Microsensor measurements in methanogenic-sulfidogenic aggregates revealed that the activity of sulfate-reducing bacteria (2 to 3 mmol of S 2− m −3 s −1 or 2 × 10 −9 mmol s −1 per aggregate) was located in a surface layer of 50 to 100 μm thick. The sulfidogenic aggregates contained a wider sulfate-reducing zone (the first 200 to 300 μm from the aggregate surface) with a higher activity (1 to 6 mmol of S 2− m −3 s −1 or 7 × 10 −9 mol s −1 per aggregate). The methanogenic aggregates did not show significant sulfate-reducing activity. Methanogenic activity in the methanogenic-sulfidogenic aggregates (1 to 2 mmol of CH 4 m −3 s −1 or 10 −9 mmol s −1 per aggregate) and the methanogenic aggregates (2 to 4 mmol of CH 4 m −3 s −1 or 5 × 10 −9 mmol s −1 per aggregate) was located more inward, starting at ca. 100 μm from the aggregate surface. The methanogenic activity was not affected by 10 mM sulfate during a 1-day incubation. The sulfidogenic and methanogenic activities were independent of the type of electron donor (acetate, propionate, ethanol, or H 2 ), but the substrates were metabolized in different zones. The localization of the populations corresponded to the microsensor data. A distinct layered structure was found in the methanogenic-sulfidogenic aggregates, with sulfate-reducing bacteria in the outer 50 to 100 μm, methanogens in the inner part, and Eubacteria spp. (partly syntrophic bacteria) filling the gap between sulfate-reducing and methanogenic bacteria. In methanogenic aggregates, few sulfate-reducing bacteria were detected, while methanogens were found in the core. In the sulfidogenic aggregates, sulfate-reducing bacteria were present in the outer 300 μm, and methanogens were distributed over the inner part in clusters with syntrophic bacteria.

Groundwater contamination associated with biodegradation of organic compounds may result in negative electric Self-Potential (SP) anomalies at the ground surface. This negative SP anomaly is associated with a gradient in the redox potential caused by degradation of organic matter by cable bacteria. Cable bacteria transport electrons along their filaments as a function of their metabolism at the oxic/anoxic (water and oil/hydrocarbon) interface. The cable bacteria filaments can be several centimeters long and generate a considerable current density that can be measured on the surface. To delineate polluted zones, a SP survey was planned near an old gas station in Aarhus, Denmark. Since most of the area was covered with asphalt, it was not practical to dig holes in the ground and take measurements, thus, a new approach was established for measuring surface SP data. In this approach, small holes (1.5 cm diameter) were made in the asphalt surface using an electrical dill and wooden pegs soaked in saline water were inserted in these holes. Measurements were made by placing measuring and reference electrodes in small buckets filled with wet sand covered by saline-water drenched cloth making electrical contact with the wooden pegs in the ground. Using this approach, more than 400 surface SP data points referenced with differential GPS were measured. The results indicate a large negative anomaly over a region with known contamination (Figure 1a). At the same location, three borehole SP measurements revealed the depth to the interface of probable contamination, which is no more than 1 m below the surface. SP signals measured in the study area were also affected due to the presence of different conductivity structures in the area (asphalt, grass, buried metallic tanks). To support the SP measurements, four ERT measurements were also done in the same area showing depth of the interface lying between 1-2 m additionally showing effects of different conductivity structures in the measured profile. Results of the study propose that SP method could be a fast and non-intrusive technique to delineate contaminated interfaces associated with microbial activities in a polluted area.

The currents in the lowest few millimeters of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes such as oxygen transfer into the sediment. So far, no high-resolution flow velocity profiles close to the sediment - water interface have been reported for such systems because a suitable flow meter was lacking. This article introduces a novel sensor for the measurement of extremely lowflow velocities. The sensor is based on a gas transducer surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas outside the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s(-1), with a spatial resolution of 100 to 250 mu m. The flow measurements prove to be insensitive to temperature changes between 5 and 15 C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high- resolution in situ measurements at the bottom of a pre- alpine lake with shear velocities as low as 0.13 +/- 0.02 cm s(-1). The velocity profile nicely resolves the transition zone between the viscous and the logarithmic boundary layer.

Information about muscular tissue flow is important for diagnosing, treating and monitoring patients with tissue ischemia. For this purpose an objective method which is reproducible and continuous is needed. In co-operation with the company Unisense A/S, we have developed a sensor for instantaneous and continuous monitoring of muscle tissue flow in vivo. The method is based on a flexible microsensor which emits and measures minute amounts of inert tracer gases. The objective was to evaluate the capability of the microsensor to measure varying degrees of tissue flow changes and compare these measurements with the (133)Xenon Washout Technique for measuring local perfusion rates in skeletal muscle. The Unisense microsensor was tested in the gracilis muscle of six anaesthetized pigs subjected to varying degrees of muscle ischemia. We found that both tissue flow and pO(2) declined in syncrony with reduced blood flow to the lower extremities. All the data from the Unisense microsensor show the same trend as the Xenon data and thus confirms that the microsensor can measure changes in tissue flow.