Primary and secondary aerenchyma oxygen transportation pathways of Syzygium kunstleri (King) Bahadur & R. C. Gaur adventitious roots in hypoxic conditions

Some plant species develop aerenchyma to avoid anaerobic environments. In Syzygium kunstleri (King) Bahadur & R. C. Gaur, both primary and secondary aerenchyma were observed in adventitious roots under hypoxic conditions. We clarified the function of and relationship between primary and secondary aerenchyma. To understand the function of primary and secondary aerenchyma in adventitious roots, we measured changes in primary and secondary aerenchyma partial pressure of oxygen (pO2) after injecting nitrogen (N2) into the stem 0–3 cm above the water surface using Clark-type oxygen microelectrodes. Following N2 injection, a decrease in pO2 was observed in the primary aerenchyma, secondary aerenchyma, and rhizosphere. Oxygen concentration in the primary aerenchyma, secondary aerenchyma, and rhizosphere also decreased after the secondary aerenchyma was removed from near the root base. The primary and secondary aerenchyma are involved in oxygen transport, and in adventitious roots, they participate in the longitudinal movement of oxygen from the root base to root tip. As cortex collapse occurs from secondary growth, the secondary aerenchyma may support or replace the primary aerenchyma as the main oxygen transport system under hypoxic conditions.

Mass transfer phenomena in biofilm systems

Mathematical models allow the simulation of microorganism growth and substrate transport in biofilm systems. Nevertheless there is still a lack of knowledge about the mass transfer of substrate in the boundary layer between biofilm and bulkphase. Several biofilms were cultivated under different substrate and hydrodynamic conditions in a biofilm tube reactor. Oxygen concentration profiles were measured with oxygen microelectrodes in the biofilm and in the boundary layer. The thickness of the concentration layer was found to depend on surface structure which depends on the substrate loading and the hydrodynamic conditions during the growth phase of the biofilm. Biofilm density and maximum substrate flux were also influenced by growth conditions. An empirical function for the concentration layer thickness was formulated for biofilms grown under different conditions to describe transport phenomena in the boundary layer.

The use of oxygen microelectrodes to determine the net production by an Antarctic sea ice algal community

Oxygen microelectrodes were used to measure the photosynthetic rates of Antarctic fast ice algal mats. Using the oxygen flux across the diffusive boundary layer below the fast ice at Davis, a productivity range of 0–1.78 mg C m−2 h−1 was measured. This is at the lower end of fast ice productivity estimates and suggests that conventional 14C techniques may overestimate sea ice algal mat productivity. Photosynthetic capacity (P max) approached 0.05 mg C. (mg chl a)−1 h −1. Onset of photosynthesis saturation, E k, was found at c. 14 μmol photons m −2 s −1. The irradiance of photoinhibition onset, E inh, was c. 20 μmol photons m −2 s −1 and the irradiance at the compensation point, E c, was 4 μmol photons m −2 s −1.

Mass transfer coefficients for an autotrophic and a heterotrophic biofilm system

To evaluate the mass transfer coefficients at the interface bulk/biofilm two biofilm systems with autotrophic and heterotrophic microbes were investigated with oxygen microelectrodes over a longterm period (1 year). To determine the transfer coefficient the oxygen profiles were simulated with a mass transfer model and compared with the measured substrate flux. The oxygen profiles at flow velocities >10 cm/s yielded concentration layers of about 100 μm and transfer coefficients > 10−4 m/s. These evaluated transfer coefficients were one magnitude higher than those known from hydrodynamics without biofilm reactions. Thus, the assumption, which determines the transfer coefficient as a quotient of the diffusion coefficient through the thickness of the laminar sublayer is oversimplified.

Determination of effective oxygen diffusivity in biofilms grown in a completely mixed biodrum reactor

Direct measurement of unsteady-state variation of oxygen concentrations inside deactivated biofilm at intervals of 100 μm was conducted with oxygen microelectrodes. The diffusivity of each layer was estimated using an explicit finite-difference method. The results show that the distribution of the biofilm effective oxygen diffusivity varies from 25% Dw at the substratum of the biofilm to 90% Dw at the surface of the biofilm. This information provides experimental evidence necessary for biofilm modelling that could not be approached in the past, and will create a new dimension for evaluation of biofilm processes.

Effect of microvascular rarefaction on tissue oxygen delivery in hypertension

A mathematical model of oxygen transport in tissue was used to analyze the effects of microvessel rarefaction and nonhomogeneous oxygen consumption on tissue oxygen distribution. The model was based on the diffusion equation with a nonhomogeneous consumption term. Solutions were computed for several configurations of vessel and oxygen sink distributions on a finite domain using the finite element method. A microcirculatory unit consisting of a tissue slice of 100-microns depth and 40-microns width was chosen. Symmetry boundary conditions were applied so that the entire tissue consisted of a series of such microvascular units placed side by side. The boundary condition at the surface of the unit was chosen to simulate a tissue suffusion experiment in which the suffusion oxygen was varied from 0 to 37 mmHg. Results of the model, which were compared with direct measurements with oxygen microelectrodes, indicate that vascular oxygen delivery strongly dominates the tissue oxygen field for suffusion PO2 values of less than 20 mmHg, whereas above this level tissue oxygen is dominated by the suffusion PO2. Reduction of vessel density within the tissue was found to have the largest effect on tissue oxygen levels at low suffusion oxygen. Finally, under some configurations of oxygen sources (vessels) and sinks (mitochondria), extremely low PO2 levels may exist within the area of high consumption, which could limit the metabolic activity of the tissue.

Effect of Boundary Layers on Cutaneous Gas Exchange

Boundary layers may offer significant resistance to cutaneous oxygen uptake by amphibians in water. This hypothesis was tested by measuring resistance to oxygen uptake as a function of water velocity in bullfrogs submerged at 5 °C and by direct measurements of the boundary layer with oxygen microelectrodes. The oxygen diffusion boundary layer was easily measurable with oxygen microelectrodes. The proportion of the total resistance to oxygen uptake represented by the boundary layer increased from 35 % at a water velocity of 5 cms−1 1 to over 90% at 0.1 cms−1. At water velocities below lcms−1 oxygen uptake was limited by the resistance of the boundary layer. At 0.1 cms−1, the partial pressure of oxygen immediately adjacent to the skin was only 2 kPa (15 mmHg); placing an immobilized frog in still water was tantamount to placing it in anoxic water. Body movements disrupted boundary layers efficiently; even occasional small movements by the animal (1 min−1) were sufficient to maintainoxygen uptake in still water.