Biomedical subjects

N P Revsbech

Publications and source records attributed to N P Revsbech.

27 records · Page 2Linked to original sources

PubMed1988

Combined oxygen and nitrous oxide microsensor for denitrification studies.

The construction of a microsensor which can be used to measure O(2) and N(2)O simultaneously is described. The microsensor exhibited a linear response to both O(2) and N(2)O, and the response to N(2)O was independent of the O(2) concentration and vice versa. The N(2)O detection limit of a microsensor with a tip diameter of 20 mum was around 1 mumol liter. The signals for O(2) and N(2)O were affected by hydrogen sulfide, but other interfering agents were not observed in the biofilms and sediments analyzed. Microprofiles of O(2) and N(2)O were measured in a biofilm which was exposed to acetylene to block the N(2)O reductase activity of denitrifying bacteria. O(2) penetrated about 0.5 mm into the biofilm and was not affected by acetylene, but the N(2)O concentration at 1.4 mm depth increased from 32 to 411 mumol liter after the addition of the inhibitor. The shape of the N(2)O profile after the addition of acetylene showed that denitrification (denitrifying activity) was detectable in all anoxic layers of the biofilm.

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PubMed1986

Adaptation to Hydrogen Sulfide of Oxygenic and Anoxygenic Photosynthesis among Cyanobacteria.

Four different types of adaptation to sulfide among cyanobacteria are described based on the differential toxicity to sulfide of photosystems I and II and the capacity for the induction of anoxygenic photosynthesis. Most cyanobacteria are highly sensitive to sulfide toxicity, and brief exposures to low concentrations cause complete and irreversible cessation of CO(2) photoassimilation. Resistance of photosystem II to sulfide toxicity, allowing for oxygenic photosynthesis under sulfide, is found in cyanobacteria exposed to low H(2)S concentrations in various hot springs. When H(2)S levels exceed 200 muM another type of adaptation involving partial induction of anoxygenic photosynthesis, operating in concert with partially inhibited oxygenic photosynthesis, is found in cyanobacterial strains isolated from both hot springs and hypersaline cyanobacterial mats. The fourth type of adaptation to sulfide is found at H(2)S concentrations higher than 1 mM and involves a complete replacement of oxygenic photosynthesis by an effective sulfide-dependent, photosystem II-independent anoxygenic photosynthesis. The ecophysiology of the various sulfide-adapted cyanobacteria may point to their uniqueness within the division of cyanobacteria.

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PubMed1986

Transition from Anoxygenic to Oxygenic Photosynthesis in a Microcoleus chthonoplastes Cyanobacterial Mat.

Benthic cyanobacterial mats with the filamentous Microcoleus chthonoplastes as the dominant phototroph grow in oxic hypersaline environments such as Solar Lake, Sinai. The cyanobacteria are in situ exposed to chemical variations between 200 mumol of sulfide liter at night and 1 atm pO(2) during the day. During experimental H(2)S to O(2) transitions the microbial community was shown to shift from anoxygenic photosynthesis, with H(2)S as the electron donor, to oxygenic photosynthesis. Microcoleus filaments could carry out both types of photosynthesis concurrently. Anoxygenic photosynthesis dominated at high sulfide levels, 500 mumol liter, while the oxygenic reaction became dominant when the sulfide level was reduced below 100 to 300 mumol liter (25 to 75 mumol of H(2)S liter). An increasing inhibition of the oxygenic photosynthesis was observed upon transition to oxic conditions from increasing sulfide concentrations. Oxygen built up within the Microcoleus layer of the mat even under 5 mmol of sulfide liter (500 mumol of H(2)S liter) in the overlying water. The implications of such a localized O(2) production in a highly reducing environment are discussed in relation to the evolution of oxygenic photosynthesis during the Proterozoic era.

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PubMed1986

Microoxic-Anoxic Niche of Beggiatoa spp.: Microelectrode Survey of Marine and Freshwater Strains.

Beggiatoa spp. grow optimally in media containing opposed gradients of oxygen and soluble sulfide, although some strains also require an organic substrate. By using microelectrodes, we characterized oxygen and sulfide gradients during their initial development in uninoculated media and in cultures of marine and freshwater strains. In gradient media, Beggiatoa strains always grew some distance below the air/agar interface as a dense "plate" of constantly gliding filaments with sharply demarcated upper and lower boundaries. Within established plates, the maximum oxygen partial pressure was 0.6 to 6.0% of air saturation and not significantly lower if filaments were fixing nitrogen. Oxygen penetrated only 100 to 300 mum into the plate, and the anoxic fraction increased from less than 10% to approximately 90% during later stages of growth. For lithoautotrophically grown marine strains, the linearity of the oxygen profile above the plate plus its drop to zero therein indicated that oxygen uptake for the entire tube occurred only within the Beggiatoa plate. Consequently, oxygen consumption could be predicted solely from the distance between the air/agar interface and the top of a plate, given the diffusion coefficient for oxygen. By contrast, for freshwater strains grown heterotrophically (with sulfide also in the medium), oxygen profiles were frequently nonlinear because of nonbiological reaction with sulfide which had diffused past the aggregated filaments. For all strains tested, microoxic aggregation also occurred in the absence of sulfide, apparently reflecting a step-up phobic response to oxygen.

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PubMed1986

Growth Pattern and Yield of a Chemoautotrophic Beggiatoa sp. in Oxygen-Sulfide Microgradients.

Recently developed techniques involving opposed, gel-stabilized gradients of O(2) and H(2)S permit cultivation of a marine Beggiatoa strain as a chemolithoautotroph which uses gliding motility to precisely track the interface between H(2)S and O(2). In the current study with microelectrodes, vertical profiles of H(2), O(2), and pH were measured in replicate cultures grown for various intervals. After an initial period of exponential biomass increase (doubling time, 11 h), linear growth prevailed throughout much of the time course. This H(2)S-limited growth was followed by a transition to stationary phase when the declining H(2)S flux was sufficient only to supply maintenance energy. During late-exponential and linear growth phases, the Beggiatoa sp. consumed a constant 0.6 mol of H(2)S for each 1.0 mol of O(2), the ratio anticipated for balanced lithoautotrophic growth at the expense of complete oxidation of H(2)S to SO(4). Over the entire range of conditions studied, this consumption ratio varied by approximately twofold. By measuring the extent to which the presence of the bacterial plate diminished the overlap of O(2) and H(2)S, we demonstrated that oxidation of H(2)S by Beggiatoa sp. is approximately 3 orders of magnitude faster than spontaneous chemical oxidation. By integrating sulfide profiles and comparing sulfide consumed with biomass produced, a growth yield of 8.4 g (dry weight) mol of H(2)S was computed. This is higher than that found for sulfide-grown thiobacilli, indicating very efficient growth of Beggiatoa sp. as a chemoautotroph. The methods used here offer a unique opportunity to determine the yield of H(2)S-oxidizing chemolithoautotrophs while avoiding several problems inherent in the use of homogeneous liquid culture. Finally, by monitoring time-dependent formation of H(2)S profiles under anoxic conditions, we demonstrate a method for calculating the molecular diffusion coefficient of soluble substrates in gel-stabilized media.

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PubMed1984

Microelectrode studies of interstitial water chemistry and photosynthetic activity in a hot spring microbial mat.

Microelectrodes were used to measure oxygen, pH, and oxygenic photosynthetic activity in a hot spring microbial mat (Octopus Spring, Yellowstone National Park), where the cyanobacterium Synechococcus lividus and the filamentous bacterium Chloroflexus aurantiacus are the only known phototrophs. The data showed very high biological activities in the topmost layers of the microbial mat, resulting in extreme values for oxygen and pH. At a 1-mm depth at a 55 degrees C site, oxygen and pH reached 900 muM and 9.4, respectively, just after solar noon, whereas anoxic conditions with a pH of 7.2 were measured before sunrise. Although diurnal changes between these extremes occurred over hours during a diurnal cycle, microbial activity was great enough to give the same response in 1 to 2 min after artificial shading. Oxygenic photosynthesis was confined to a 0.5- to 1.1-mm layer at sites with temperatures at or above about 50 degrees C, with maximum activities in the 55 to 60 degrees C region. The data suggest that S. lividus is the dominant primary producer of the mat.

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PubMed1983

Oxygen Microelectrode That Is Insensitive to Medium Chemical Composition: Use in an Acid Microbial Mat Dominated by Cyanidium caldarium.

A novel oxygen microelectrode with a tip diameter of 2 to 20 mum was constructed which could function satisfactorily under a variety of environmental conditions and in a variety of media, including human blood serum, citric acid at pH 2.5, moist air, and paraffin oil. Measurement of oxygen by this electrode does not require stirring of the medium. Electrodes could be made so that the 90% response time necessary to detect changes in oxygen concentration was less than 0.2 s, and response was linear with oxygen concentration. To demonstrate the performance of the electrode, oxygen and photosynthesis profiles of an acid microbial mat (pH 2.8) dominated by the eucaryotic alga Cyanidium caldarium were made. Photosynthetic rates as high as 95 mmol of O(2) dm h were measured within the most active 0.1-mm layer, which was ca. 0.2 mm below the surface of the microbial mat. The total photosynthetic activity was 47 mmol of O(2) m h. Vertical profiles of photosynthesis at different light intensities showed that the microalgae within the mat were not photoinhibited at bright sunlight (2,090 muEinsteins m s).

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PubMed1983

Colorless Sulfur Bacteria, Beggiatoa spp. and Thiovulum spp., in O(2) and H(2)S Microgradients.

The interactions between colorless sulfur bacteria and the chemical microgradients at the oxygen-sulfide interface were studied in Beggiatoa mats from marine sediments and in Thiovulum veils developing above the sediments. The gradients of O(2), H(2)S, and pH were measured by microelectrodes at depth increments of 50 mum. An unstirred boundary layer in the water surrounding the mats and veils prevented microturbulent or convective mixing of O(2) and H(2)S. The two substrates reached the bacteria only by molecular diffusion through the boundary layer. The bacteria lived as microaerophiles or anaerobes even under stirred, oxic water. Oxygen and sulfide zones overlapped by 50 mum in the bacterial layers. Both compounds had concentrations in the range of 0 to 10 mumol liter and residence times of 0.1 to 0.6 s in the overlapping zone. The sulfide oxidation was purely biological. Diffusion calculations showed that formation of mats on solid substrates or of veils in the water represented optimal strategies for the bacteria to achieve a stable microenvironment, a high substrate supply, and an efficient competition with chemical sulfide oxidation. The continuous gliding movement of Beggiatoa cells in mats or the flickering motion of Thiovulum cells in veils were important for the availability of both O(2) and H(2)S for the individual bacteria.

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PubMed1979

Diurnal cycle of oxygen and sulfide microgradients and microbial photosynthesis in a cyanobacterial mat sediment.

The diurnal variation in the microgradients of O(2), H(2)S, and Eh were studied in the benthic cyanobacterial mats of a hypersaline desert lake (Solar Lake, Sinai). The results were related to light intensity, light penetration into the mat, temperature, pH, NH(4), photosynthetic activity, pigments, and the zonation of the microbial community. Extreme diurnal variation was found, with an O(2) peak of 0.5 mM at 1 to 2 mm of depth below the mat surface during day and a H(2)S peak of 2.5 mM at 2 to 3 mm of depth at night. At the O(2)-H(2)S interface, the two compounds coexisted over a depth interval of 0.2 to 1 mm and with a turnover time of a few minutes. The photic zone reached 2.5 mm into the mat in summer, and the main CO(2) light fixation took place at 1 to 2 mm of depth. During winter, light and photosynthesis were restricted to the uppermost 1 mm. The quantitative dynamics of O(2) and H(2)S were calculated from the chemical gradients and from the measured diffusion coefficients.

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