Non-linear bioconvection in a deep suspension of gyrotactic swimming micro-organisms

 The non-linear structure of deep, stochastic, gyrotactic bioconvection is explored. A linear analysis is reviewed and a weakly non-linear analysis justifies its application by revealing the supercritical nature of the bifurcation. An asymptotic expansion is used to derive systems of partial differential equations for long plume structures which vary slowly with depth. Steady state and travelling wave solutions are found for the first order system of partial differential equations and the second order system is manipulated to calculate the speed of vertically travelling pulses. Implications of the results and possibilities of experimental validation are discussed. Received: 26 May 1997 / Revised version: 10 May 1998     

Axisymmetric bioconvection in a cylinder

In three-dimensional bioconvection, the regions of rising and sinking fluid are dissimilar. This geometrical effect is studied for axisymmetric bioconvection in a cylindrical cell with stress-free (i.e. normal velocity and tangential stress vanish) lateral and top boundaries, and rigid bottom boundary. Using the continuum model of Pedley et al. (1988, J. Fluid Mech.195, 223-237) for bioconvection in a suspension of swimming, gyrotactic microorganisms, the structure and stability of an axisymmetric plume in a deep chamber are investigated. The system is governed by the Navier-Stokes equations for an incompressible fluid coupled with a microorganism conservation equation. These equations are solved numerically using a conservative finite-difference scheme. Comparisons are made with two-dimensional bioconvection.     

Sheared bioconvection in a horizontal tube

The recent interest in using microorganisms for biofuels is motivation enough to study bioconvection and cell dispersion in tubes subject to imposed flow. To optimize light and nutrient uptake, many microorganisms swim in directions biased by environmental cues (e.g. phototaxis in algae and chemotaxis in bacteria). Such taxes inevitably lead to accumulations of cells, which, as many microorganisms have a density different to the fluid, can induce hydrodynamic instabilites. The large-scale fluid flow and spectacular patterns that arise are termed bioconvection. However, the extent to which bioconvection is affected or suppressed by an imposed fluid flow and how bioconvection influences the mean flow profile and cell transport are open questions. This experimental study is the first to address these issues by quantifying the patterns due to suspensions of the gravitactic and gyrotactic green biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow. With no flow, the dependence of the dominant pattern wavelength at pattern onset on cell concentration is established for three different tube diameters. For small imposed flows, the vertical plumes of cells are observed merely to bow in the direction of flow. For sufficiently high flow rates, the plumes progressively fragment into piecewise linear diagonal plumes, unexpectedly inclined at constant angles and translating at fixed speeds. The pattern wavelength generally grows with flow rate, with transitions at critical rates that depend on concentration. Even at high imposed flow rates, bioconvection is not wholly suppressed and perturbs the flow field.     

Bioluminescent monitoring of turbulent bioconvection.

Under adjusted experimental conditions, open-to-air cultures of lux gene-engineered Ralstonia eutropha (wholecell biosensors of copper) exhibit bioconvection, which accounts for fluctuating bioluminescence. The power spectrum of bioluminescence intensity fluctuations recorded from a cylindrical sample 9 mm in diameter and approximately 10 mm in height is characterized by a dominant low-frequency oscillation (with a characteristic period of approximately 8-12 min), which is occasionally accompanied by a few weaker oscillations. The corresponding spectral peaks emerge on a high-noise background. The spectra of bioluminescence intensity fluctuations qualitatively resemble the spectra of temperature or fluid velocity fluctuations in an appropriate turbulent thermal convection system. It has been suggested that in a bioconvective system, like in thermal convection systems, the emergence of oscillation reflects the large-scale convective circulation that spans the height of the cylindrical cell. The velocity of large-scale bioconvective circulation was estimated to be 37-48 microm/s. The occasional emergence of weaker-than-dominant oscillations was explained through the coexistence and interaction of the large-scale circulation with, presumably, a gene-expression-related cyclic process (with a characteristic period of approximately 25-50 min).     

Pattern swimming of Phytophthora citricola zoospores: an example of microbial bioconvection

The genus Phytophthora, belonging to the class Oomycota, comprises a group of over fifty fungus-like plant pathogens in both managed and unmanaged ecosystems. A unique feature of the oomycete lifecycle is a zoosporic stage in which motile, unicellular propagules, serving as the primary agents of dispersal, are produced and released in the presence of water. In Petri dish suspensions, zoospores frequently exhibit 'pattern swimming', whereby they spontaneously form concentrated swimming masses, visible to the naked eye, even in the absence of a chemical or electrical gradient. The nature of this behaviour is unclear, but is of interest because of the potential for auto-attraction and implications for cohort recruitment during infection. Similar behaviour observed in a variety of motile bacteria, algae, and protists is attributed to 'bioconvection' that results from instability in fluid density due to the organisms' upward-swimming tendency and greater-than-water density. In this investigation, we determined that Phytophthora citricola zoospore 'pattern swimming' is unrelated to phototaxis, surface tension-driven (Marangoni) convection, or auto-attraction and that the observed convective pattern, directional swimming, and depth- and concentration dependence are consistent with bioconvection.     

Plasmonic Electro-Optical Switches Operating at Telecom Wavelengths

Electro-optical (EO) switches with a subwavelength device length based on metal–dielectric–metal nanocavities waveguide combined with organic EO materials have been proposed and numerically investigated. The finite difference time domain (FDTD) method with perfectly matched layer absorbing boundary condition is adopted to simulate and study their properties. The FDTD simulation results reveal that these structures filled with EO materials can realize the function of switch with low-driving voltage. The wavelength conversion switch structure might become a choice for the design of integrated architectures for optical computing and communication, especially in WDM systems in the nanoscale.     

Effects of experience on parasitoid movement in odour plumes

Abstract. Insects commonly improve the effectiveness with which they locate biotic resources through learning, but the mechanism by which experience exerts its effects has rarely been studied in detail. The effect of oviposition experience on upwind movement of the eucoilid parasitoid, Leptopilina heterotoma (Thomson) (Hym.: Eucoilidae), in odour plumes of host microhabitats, was quantified with the use of a Kramer-type locomotion compensator. A 2h exposure to host Drosophila melanogaster larvae in either fermenting apple-yeast or decaying mushroom substrate (known to affect their preference for these odours in glasshouse and field choice experiments) had a number of effects on movement in plumes of each substrate. Females experienced with a particular substrate walked faster and straighter, made narrower turns and spent more time in upwind movement (i.e. toward the source) in a plume of odour from that substrate than in odour from an alternative substrate. Inexperienced females, by contrast, generally showed little or no significant difference in responses to alternative odours. In addition to affecting the mean values of movement parameters, experience also affected variability around those means. When walking speed or path straightness in an odour plume was increased by experience, variability among individuals was correspondingly decreased. The consequences of odour learning for microhabitat choice is discussed briefly.     

A Super Meta-Cone Absorber for Near-Infrared Wavelengths

We present a meta-cone absorber based on metamaterials which can absorb nearly all incident light in the near-infrared spectrum. The absorber has an ultrahigh absorption with a broad receiving angle and independence of polarization state. This absorption enhancement can be attributed to the excitation of slow light mode and localized surface plasmon resonances (LSPR). In addition, we use slow light theory to explain why incident light with different wavelengths are trapped at different positions. We believe our work will provide a promising candidate as absorbing elements in technical applications and scientific research.     

Effects of meteorological conditions on spore plumes

Fungal spores are an ever-present component of the atmosphere, and have long been known to trigger asthma and hay fever symptoms in sensitive individuals. The atmosphere around Tulsa has been monitored for airborne spores and pollen with Burkard spore traps at several sampling stations. This study involved the examination of the hourly spore concentrations on days that had average daily concentrations near 50,000 spores/m(3) or greater. Hourly concentrations of Cladosporium, Alternaria, Epicoccum, Curvularia, Pithomyces, Drechslera, smut spores, ascospores, basidiospores, other, and total spores were determined on 4 days at three sites and then correlated with hourly meteorological data including temperature, rainfall, wind speed, dew point, air pressure, and wind direction. On each of these days there was a spore plume, a phenomenon in which spore concentrations increased dramatically over a very short period of time. Spore plumes generally occurred near midday, and concentrations were seen to increase from lows around 20,000 total spores/m(3) to highs over 170,000 total spores/m(3) in 2 h. Multiple regression analysis of the data indicated that increases in temperature, dew point, and air pressure correlated with the increase in spore concentrations, but no single weather variable predicted the appearance of a spore plume. The proper combination of changes in these meteorological parameters that result in a spore plume may be due to the changing weather conditions associated with thunderstorms, as on 3 of the 4 days when spore plumes occurred there were thunderstorms later that evening. The occurrence of spore plumes may have clinical significance, because other studies have shown that sensitization to certain spore types can occur during exposure to high spore concentrations.     

The fluid mechanics of arthropod sniffing in turbulent odor plumes

Many arthropods capture odorant molecules from the environment using antennae or antennules bearing arrays of chemosensory hairs. The penetration of odorant-carrying water or air into the spaces between these chemosensory hairs depends on the speed at which they are moved through the surrounding fluid. Therefore, antennule flicking by crustaceans and wing fanning by insects can have a profound impact on the odorant encounter rates of the chemosensory sensilla they bear; flicking and fanning are examples of sniffing. Odors are dispersed in the environment by turbulent wind or water currents. On the scale of an antenna or antennule, an odor plume is not a diffuse cloud but rather is a series of fine filaments of scent swirling in odor-free water. The spatiotemporal pattern of these filaments depends on distance from the odor source. The physical interaction of a hair-bearing arthropod antennule with the surrounding fluid affects the temporal patterns of odor concentration an animal intercepts when it sniffs in a turbulent odor plume.     

Electromagnetic Emission at Micron Wavelengths from Active Nerves

In recent years there has been experimental work and speculation bearing upon the significance in neural functioning of electromagnetic energy in the region of the spectrum between 0.3 and 10 μ. We demonstrate, in this experiment, micron wavelength electromagnetic emission from active live crab nerves as compared to inactive live and dead nerves. Further, the data indicate that the active nerve emission is caused by specific biophysical reactions rather than being simply black-body radiation.     

Antennal resolution of pulsed pheromone plumes in three moth species

Male antennae of Cadra cautella,Pectinophora gossypiella, and Spodoptera exigua were presented with 20-ms-duration pulses of their two-component pheromone at rates of 1 to 33 Hz. Fourier analyses of electroantennograms resolved the temporal structure of trains of pheromone filaments delivered at up to 33 Hz for C. cautella and S. exigua and 25 Hz for P. gossypiella. Pheromone components tested separately for each species were generally equivalent in filament resolution to complete blends. Ambient temperatures of 18, 23 and 28 °C affected filament resolution only slightly, with poorer ability to discriminate rapidly pulsed signals at 18 °C. The question of how, or indeed if, such frequencies are conserved beyond the peripheral nervous system, remains.     

Wind structure in relation to odour plumes in tsetse fly habitats

Abstract. Key characteristics of airflow were measured in the African bush in a study of host odour plume structure. Wind speed, speed variance, direction, and directional variance were measured by conventional cup anemometers plus wind-vanes and by a solid state ultrasonic anemometer, on time scales from seconds to minutes. The two technologies gave opposite relationships between wind speed and turbulence measured as rate of angular direction change in the wind (° s^-1). A positive correlation between turbulence and wind speed was observed with mechanical anemometers and wind-vanes, evidently caused by their inherent hysteresis (stalling in weak wind, overswinging after gusts). The same correlation was negative with the solid-state anemometer which, being hysteresis free, should have measured the true directional turbulence more accurately. Such fine-scale turbulence at a fixed point in space (on a scale of about ∼15 cm diam.) decreased with wind speed up to ∼1.5 m s^-1, as does large-scale (∼1m diam.) turbulence of air moving through space (Brady et al. , 1989). This decrease occurred both within vegetation and out in the open, but the slope and intercepts of the relationship depended on vegetation and topography. Variables for describing wind speed and turbulence are considered in the context of odour plume structure.     

Effects of UV-irradiation at Various Wavelengths on Sterilizing Drosophila Embryos

Embryos of Drosophila melanogaster at the early intravitelline nuclear multiplication stage were irradiated with UV light at the posterior pole. The sterility and mortality of these embryos were examined in relation to the dose and wavelength of the UV light.
Sterility, expressed either as the frequency of pole-cell-deficient embryos, or as the frequency of agametic adults, was found to be dependent on the wavelength of UV light. UV-irradiation at 280 nm was found moot effective in causing sterility on Drosophila embryos. The minimum dose of radiation to give a 100% sterility was 200 J/m² at 280 nm, and 400 J/m² at 254 nm. In contrast, mortality showed no dependency on the wavelength.
The possibility that nucleic acids in the posterior region is a target of 280 nm radiation is discussed.     

A bioconvection model for viscoelastic nanofluid confined by tapered asymmetric channel: implicit finite difference simulations

As part of the growing evolution in nanotechnology and thermal sciences, nanoparticles are considered as an alternative solution for the energy depletion due to their ultra-high thermal effectives. Nanofluids reflect inclusive and broad-spectrum significances in engineering, industrial and bio-engineering like power plants, energy source, air conditioning systems, surface coatings, evaporators, power consumptions, nano-medicine, cancer treatment, etc. The present study describes the bio-convective peristaltic flow of a third-grade nanofluid in a tapered asymmetric channel. Basic conservation laws of mass, momentum, energy, and concentration as well as the microorganism diffusion equation are utilized to model the problem. The simplified form of the modeled expressions is accounted with long wavelength assumptions. For solving the resulting coupled and nonlinear equations, a well-known numerical method implicit finite difference scheme has been utilized. The graphical results describe the velocity, temperature and concentration profiles, and the density of motile microorganisms at the nanoscale. Furthermore, microorganism concentration lines are analyzed.

     

Ultraviolet-B Wavelengths Regulate Changes in UV Absorption of Cleaner Fish Labroides dimidiatus Mucus

High-energy wavelengths in the ultraviolet-B (UVB, 280-315 nm) and the UVA (315-400-nm) portion of the spectrum are harmful to terrestrial and aquatic organisms. Interestingly, UVA is also involved in the repair of UV induced damage. Organisms living in shallow coral reef environments possess UV absorbing compounds, such as mycosporine-like amino acids, to protect them from UV radiation. While it has been demonstrated that exposure to UV (280-400 nm) affects the UV absorbance of fish mucus, whether the effects of UV exposure vary between UVB and UVA wavelengths is not known. Therefore, we investigated whether the UVB, UVA, or photosynthetically active radiation (PAR, 400-700 nm) portions of the spectrum affected the UV absorbance of epithelial mucus and Fulton’s body condition index of the cleaner fish Labroides dimidiatus. We also compared field-measured UV absorbance with laboratory based high-performance liquid chromatography measurements of mycosporine-like amino acid concentrations. After 1 week, we found that the UV absorbance of epithelial mucus was higher in the UVB+UVA+PAR treatment compared with the UVA+PAR and PAR only treatments; after 2 and 3 weeks, however, differences between treatments were not detected. After 3 weeks, Fulton’s body condition index was lower for fish in the UVB+UVA+PAR compared with PAR and UVA+PAR treatments; furthermore, all experimentally treated fish had a lower Fulton’s body condition index than did freshly caught fish. Finally, we found a decrease with depth in the UV absorbance of mucus of wild-caught fish. This study suggests that the increase in UV absorbance of fish mucus in response to increased overall UV levels is a function of the UVB portion of the spectrum. This has important implications for the ability of cleaner fish and other fishes to adjust their mucus UV protection in response to variations in environmental UV exposure.     

Plasmonic Tuning at Mid-Infrared Wavelengths by Composite Arrays of Graphene Ribbons

Plasmonics - This work reports on a numerical simulation study regarding the systematic tuning of plasmonic resonance wavelength in the mid-infrared regime, by using composite arrays of graphene...     

Photochemical Yields in Ribonuclease and Oxidized Glutathione Irradiated at Different Wavelengths in the Ultraviolet

The quantum yields for the disruption of various amino acids in glutathione and ribonuclease by 229, 254, 265, and 280 nm UV photons have been determined. The results of the measurements on the destruction of tyrosine and histidine and the loss of enzymic function in RNAse and the disruption of cystine in both compounds lead to the following conclusions: (a) The photodestruction of some and perhaps many constituent amino acid residues does not cause RNAse inactivation. (b) Contrary to the basic premise of proposals made by other authors, the photochemical yields of constituent residues in a protein are not the same as that for the same amino acids in solution alone-the difference is a function of the exciting wavelength. Further, the extent of histidine destruction varies by a large factor among three proteins. (c) Consistent with previous predictions, the present results show that photons absorbed in the aromatic residues of RNAse cause the disruption of cystines elsewhere in the enzyme. (d) Although cystine disruption appears to be the most prevalent mode of RNAse inactivation by photons of the four wavelengths studied, some of the minor mechanisms leading to loss of enzymic function may vary with the UV energy.