Evaluation of a novel method for measuring tissue blood flow and gases.

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.

Methane microprofiles in a sewage biofilm determined with a microscale biosensor.

Microprofiles of the methane concentration in a 3.5-mm-thick sewage outlet biofilm were measured at high spatial and temporal resolution using a microscale biosensor for methane. In the freshly collected biofilm, methane was building up to a concentration of 175 mumol l-1 at 3 mm depth with a total methanogenesis of 0.14 mumol m-2 s-1, as compared to an aerobic respiration (including methane oxidation) of 0.80 mumol m-2 s-1. A model biofilm was established by homogenisation of an in situ biofilm and 12 days of incubation with surplus sodium acetate. The homogenised biofilm was able to maintain 50% of the methanogenic activity in the absence of external electron donor. Oxygen had only a minor effect on the methane production, but aerobic respiration consumed a substantial part of the produced methane and was thus an important control on methane export from the biofilm. A concentration of 2 mmol l-1 nitrate was shown to inhibit methanogenesis only in the upper layer of the biofilm, whereas a further addition of 2 mmol l-1 sulphate inhibited methanogenesis in the entire biofilm. The study demonstrated the power of the methane microsensor in the study of microhabitats with concurrent production and consumption of methane.

Distribution of sulfate-reducing and methanogenic bacteria in anaerobic aggregates determined by microsensor and molecular analyses.

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 x 10(-9) mmol s(-1) per aggregate) was located in a surface layer of 50 to 100 microm thick. The sulfidogenic aggregates contained a wider sulfate-reducing zone (the first 200 to 300 microm from the aggregate surface) with a higher activity (1 to 6 mmol of S(2-) m(-3) s(-1) or 7 x 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 x 10(-9) mmol s(-1) per aggregate) was located more inward, starting at ca. 100 microm 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 microm, 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 microm, and methanogens were distributed over the inner part in clusters with syntrophic bacteria.

Use of an oxygen-insensitive microscale biosensor for methane to measure methane concentration profiles in a rice paddy.

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 degrees C, heterotrophic oxygen consumption prevented oxygen from diffusing further than 170 mum into the capillary, whereas atmospheric levels of oxygen were consumed within 50 mum. The capacity of the oxygen guard for scavenging oxygen decreased with decreasing temperature, and atmospheric levels of oxygen caused oxygen penetration to 200 mum at 5 degrees C. The sensors could be manufactured with tip diameters as small as 25 mum, 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.

A microscale biosensor for methane containing methanotrophic bacteria and an internal oxygen reservoir.

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 µm, 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 ∼1 µM in aqueous solution. Temperature, oxygen, and H(2)S interfere with the signal; no interferences were detected from H(2), NH(3), CO(2), or acetate.