Helen C. Matsos

Computerized in vitro test for chemical toxicity based on Tetrahymena swimming patterns

An apparatus and a method for rapidly determining chemical toxicity have been evaluated as an alternative to the rabbit eye initancy test (Draize). The toxicity monitor includes an automated scoring of how motile biological cells (Tetrahymena pyriformis) slow down or otherwise change their swimming patterns in a hostile chemical environment. The method, called the Motility Assay (MA), is tested for 30 s to determine the chemical toxicity in 20 aqueous samples containing trace organics and salts. With equal or better detection limits, results compare favorably to in vivo animal tests of eye irritancy.

A bioassay for monitoring cadmium based on bioconvective patterns

The effects of cadmium, one of the most lethal bivalent heavy metals, tend to concentrate in protozoa far above natural levels and therein begin transferring through freshwater food chains to animals and humans. A simple assay using the toxic response of the protozoa, Tetrahymena pyriformis, is described. The assay relies on macroscopic bioconvective patterns to measure the toxic response, giving a sensitivity better than 1 μg/1 and a toxicity threshold to 7μg/1 for Cd+2. Unlike previous efforts, this method does not require electronic or chemical analyses to monitor toxicity.

Phase Partitioning in Space and on Earth

In aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system consisting of a PEG-rich phase floating on top of a dextran-rich phase. Biological particles and macromolecules tend to partition differentially between the phases and the liquid-liquid phase interface in these systems. Bioparticle partitioning has been shown to be related to physiologically important surface properties such as membrane charge or lipid composition. Affinity partitioning into the PEG-rich phase can be accomplished by coupling PEG to a ligand having affinity for specific cells or macromolecules. Subpopulations can be identified or separated using multi-step countercurrent distribution (CCD). Incomplete understanding of the influence of gravity on the efficiency and quality of the impressive separations achievable by partitioning, and appreciation for the versatility of this efficient technique, have led to its study for low-gravity biomaterials processing. On Earth, two-phase systems rapidly demix because of density differences between the phases. In low-gravity, demixing has been shown to occur primarily by coalescence. Polymer surface coatings, developed to control localization of demixed phases in low-g, have been found to control electroosmosis which adversely affects electrophoretic separation processes on Earth and in space. In addition PEG-derivatized antibodies have been synthesized for use in immunoaffinity cell partitioning.

Preferred negative geotactic orientation in mobile cells: Tetrahymena results

For the protozoan species Tetrahymena a series of airplane experiments are reported, which varied gravity as an active laboratory parameter and tested for corresponding changes in geotaxic orientation of single cells. The airplane achieved alternating periods of low (0.01 g) and high (1.8 g; g = 980 cm/s) gravity by flying repeated Keplerian parabolas. The experimental design was undertaken to clearly distinguish gravity from competing aerodynamic and chemical gradients. In this way, each culture served as its own control, with gravity level alone determining the orientational changes. On average, 6.3% of the Tetrahymena oriented vertically in low gravity, while 27% oriented vertically in high-gravity phases. Simplified physical models are explored for describing these cell trajectories as a function of gravity, aerodynamic drag, and lift. The notable effect of gravity on turning behavior is emphasized as the biophysical cause of the observed negative geotaxis in Tetrahymena. A fundamental investigation of the biological gravity receptor (if it exists) and improved modeling for vertical migration in important types of ocean plankton motivate the present research.

The effects of variable biome distribution on global climate

In projecting climatic adjustments to anthropogenically elevated atmospheric carbon dioxide, most global climate models fix biome distribution to current geographic conditions. Previous biome maps either remain unchanging or shift without taking into account climatic feedbacks such as radiation and temperature. We develop a model that examines the albedo-related effects of biome distribution on global temperature. The model was tested on historical biome changes since 1860 and the results fit both the observed temperature trend and order of magnitude change. The model is then used to generate an optimized future biome distribution that minimizes projected greenhouse effects on global temperature. Because of the complexity of this combinatorial search, an artificial intelligence method, the genetic algorithm, was employed. The method is to adjust biome areas subject to a constant global temperature and total surface area constraint. For regulating global temperature, oceans are found to dominate continental biomes. Algal beds are significant radiative levers as are other carbon intensive biomes including estuaries and tropical deciduous forests. To hold global temperature constant over the next 70 years this simulation requires that deserts decrease and forested areas increase. The effect of biome change on global temperature is revealed as a significant forecasting factor.

Ocean-atmosphere CO2 exchange: An accessible lab simulation for considering biological effects

Phytoplankton is considered a key component mediating the ocean-atmospheric exchange of carbon dioxide and oxygen. Lab simulations which model biological responses to atmospheric change are difficult to translate into natural settings owing in part to the vertical migration of phytoplankton. In the sea this vertical migration acts to regulate actual carbon dioxide consumption. To capture some critical properties of this vertical material transfer, we monitored the effects of atmospheric CO2 on dense suspensions of bioconvecting microorganisms. Bioconvection refers to the spontaneous patterns of circulation which arise among such upwardly swimming cells as alga, protozoa, zoospore and large bacteria. Gravity, phototaxis and chemotaxis have all been implicated as affecting pattern-forming ability. The ability of a biologically active suspension to detect atmospheric changes offers a unique method to quantify organism adjustment and vertical migration. With increasing CO2, bioconvection patterns in alga (P. parva) and protozoa (T. pyriformis) lose their robustness, and surface cell populations retreat from the highest CO2 regions. Cell movement (both percent motile and mean velocity) generally diminishes. A general program of image analysis yields statistically significant variations in macroscopic migration patterns; both fractal dimension and various crystallographic parameters correlate strongly with carbon dioxide content.

Microbial diffraction gratings as optical detectors for heavy metal pollutants

As a significant industrial pollutant, cadmium is implicated as the cause of itai‐itai disease. For biological detection of cadmium toxicity, an assay device has been developed using the motile response of the protozoa species, Tetrahymena pyriformis. This mobile protozoa measures 50 μm in diameter, swims at 10 body lengths per second, and aggregates into macroscopically visible patterns at high organism concentrations. The assay demonstrates a Cd+2 sensitivity better than 1 μM and a toxicity threshold to 5 μM, thus encouraging the study of these microbial cultures as viable pollution detectors. Using two‐dimensional diffraction patterns within a Tetrahymena culture, the scattered light intensity varies with different organism densities (population counts). The resulting density profile correlates strongly with the toxic effects at very low dosages for cadmium (<5 ppm) and then for poison protection directly (with nickel and copper antagonists competing with cadmium absorption). In particular, copper dosa...

Bioconvective percolation on an incomplete Voronoi grid

Bioconvection is a fluid instability common to many biological organisms including swimming bacteria, alga and protozoa. The statistics of bioconvective pattern formation is tested against percolation models for space-filling. A percolation threshold is found (p=0.63) and compared to theoretical point distributions for random tesselations. Simulations reveal that a percolation backbone defines a complete path across the observation window but remains incomplete as an equal partitioning grid (Voronoi diagram). The generic development of incomplete Voronoi grids and their yet unknown statistical properties captures some interest as an alternative to traditional point-based lattices.

Bioconvection patterns, topological phase transitions and evidence of self-organized critical states

Abstract Bioconvection is the name given to a fluid instability which arises spontaneously in aqueous suspensions of upwardly swimming, heavy biological organisms such as algae, bacteria, etc. The resulting pattern consists of arrayed poxes (“dots”) or polygons, the selection of which depends on suspension depth, organism count and swimming speed. We report observations and analysis of a dramatic ordering transition which occurs as pox patterns self-organize into connected polygonal nets. This enhanced geometric order, referred to here as topological solidification, does not progress via local or random interconnections between pox patterns, but proceeds via a triggered series of global restructurings or “avalanches” between barely stable or critical states. The signature of such a process entails power law behavior and over a decade and a half in size, the observed distributions in bioconvection pattern sizes D(S) scale with size S as, D(S)∼S −2.3 . This observed scaling is consistent with models proposed elsewhere for self-organized criticality, actual experimental examples of which still remain few but growing.

Organic chelation of cadmium using Di-PDMS: a bioconvective test for protective effects

Abstract Cadmium is a major industrial effluent and the cause of itai‐itai disease. As a biological detector for cadmium toxicity, an assay using the motile response of the protozoa species, Tetrahymena pyriformis, has been described. The assay relies on macroscopic bioconvective patterns to score the toxic response, giving a sensitivity better than 1 μM and a toxicity threshold to 5 ( μM for Cd+2. Using pattern formation as the toxicity monitor, the organic chelating agent D/‐PDMS is investigated individually for toxic effects at low dosages (without cadmium) and for poison protection directly (with cadmium). Dosages of 0.1–0.5 μM Di‐PDMS show protective chelation of cadmium and enhance pattern formation capabilities for 1 μM Cd+2.