Michael Levandowsky

Pattern formation in a suspension of swimming microorganisms: equations and stability theory

A model for collective movement and pattern formation in layered suspensions of negatively geotactic micro-organisms is presented. The motility of the organism is described by an average upward swimming speed U and a diffusivity tensor D. It is shown that the equilibrium suspension is unstable to infinitesimal perturbations when either the layer depth or the mean concentration of the organisms exceeds a critical value. For deep layers the maximum growth rate determines a preferred pattern size explicitly in terms of U and D. The results are compared with observations of patterns formed by the ciliated protozoan Tetrahymena pyriformis.

Chemosensory Responses of Acanthamoeba castellanii: Visual Analysis of Random Movement and Responses to Chemical Signals

A visual assay slide chamber was used in conjunction with time‐lapse videomicroscopy to analyze chemotactic behavior of axenically grown Acanthamoeba castellanii. Data were collected and analyzed as vector scatter diagrams and cell tracks. Amebas responded to a variety of bacterial products or potential bacterial products by moving actively toward the attractant. Responses to the chemotactic peptide formyl‐methionyl‐leucyl‐phenylalanine (fMLP), lipopolysaccharide, and lipid A were statistically significant (P≤ 0.03), as was the response to fMLP benzylamide (P≤ 0.05). Significant responses to cyclic AMP, lipoteichoic acid, and N‐acetyl glucosamine were also found.

Sources and History of Heavy Metal Contamination and Sediment Deposition in Tivoli South Bay, Hudson River, New York

Persistent inorganic constitutents preserved in sediments of aquatic ecosystems record temporal variability of biogeochemical functioning and anthropogenic impacts.210Pb and137Cs dating techniques were used to study the past variations of heavy metals (Pb, Cu, and Zn) and accumulation rates of sediments for Tivoli South Bay, in the Hudson River National Estuarine Research Reserve ecosystem. South Bay, a tidal freshwater embayment of the Hudson, may play an important role in the sediment dynamics of this important river. The measured sedimentation rate range of 0.59 to 2.92 cm yr−1 suggests that rapid accumulation occurred during the time period represented by the length of the cores (approximately the past 50 yr). Direct measurements of sediment exchange with the Hudson River reveal high variability in the sediment flux from one tidal cycle to the next. Net exchange does not seem to be adequate to explain sediment accumulation rates in the bay as measured by210Pb and137Cs. The difference may be supplied from upland streams or the Hudson River during storm events. Concentrations of the metals Pb, Cu and Zn were found to be well correlated with each other within individual cores at five of six sites tested. This suggests a common proximate source for the three metals at a specific site. The evidence is consistent with mixing in some environmental compartment before delivery to the bay. While metals self-correlate within individual cores, absolute concentrations, depth distribution patterns, and ratios of the metals to each other vary among the cores collected at different locations within the bay. Organic matter, Fe content, and particle size distribution of sediments do not account for the intercore variations in metal concentration. It is likely that cores collected from different sites may have derived metals from different sources, such as watershed streams and tidal exchange with the Hudson River.

Microzooplankton Feeding Behavior and the Levy Walk

We propose a Levy random walk model for the grazing behavior of swimming microzooplankton. In this model the path of the organism consists of straight line segments which are traversed at constant speed. The segments are at random angles, which correspond to sudden changes in direction, and the lengths of the line segments are random, with probability density given by the Levy stable law. The Levy law is chosen because of its role in the generalized Central Limit Theorem: it is a possible limit law for sums of independent identically-distributed random variables. We show that for values of the Levy parameter β below 1, the small amount of overlap in successive excursions due to the relatively sparse distribution of turning events would lead to a more efficient feeding pattern than for a Brownian motion or a Levy walk with larger value of β. Possible physiological mechanisms are discussed, as well as an approach to testing for Levy walk behavior patterns.

Exploring the Sulfur Nutrient Cycle Using the Winogradsky Column

fully understand the workings of the biological world, it is important that students have a fundamental sense of the natural cycles that provide the nutrients and energy that power life, as well as a sense of how these systems evolved. Many teachers cover carbon cycles and emphasize microbial processes when reviewing the complexities of nitrogen cycling, but often the sulfur cycle, if covered, is done so briefly. There may be many reasons for this: time limitations, the element is less prevalent than others as a biological constituent, or the topic is thought to be too complex. However, teaching the sulfur cycle in conjunction with the classic Winogradsky column exercise presents the opportunity to cover several important topics simultaneously. The exercise links microbial processes, concepts of biodiversity, inorganic chemistry, biogeochemical cycling, evolution, microbiology, and microbial ecology to help meet the many demands and standards that are part of todays science classes. The Winogradsky column is a glass or clear plastic column, filled with enriched soil or sediment. When developed, it has an anaerobic lower zone and aerobic upper zone that allow growth of microorganisms under conditions similar to those found in sediments and water rich in nutrients (Sylvia et al., 1998). Often teachers simply convey the message that different microorganisms exist in different strata of the column and that some live in the aerobic and some in anaerobic zones. However, this is really where the discovery begins rather than ends! Explaining the complexity that lies within the depths of the ecosystem allows deeper insights into the microbial world. In the laboratory, the Winogradsky column demonstrates how the metabolic diversity of prokaryotes transforms sulfur, an essential constituent of living matter and an abundant element in the Earths crust (Stanier et al., 1976), to different forms with varying redox states, thus supplying nutrients and/or energy to the organism. The microbial assemblage that develops in the column spatially separates organisms into distinct layers several T

UTILIZATION OF Fe3 + BY THE INSHORE COLORLESS MARINE DINOFLAGELLATE CRYPTHECODINIUM COHNII*

A Puerto Rican isolate of the colorless dinoflagellate Crypthecodinium cohnii was grown in a defined marine medium. Fe was added as Fe(NH4)2(SO3)2 - 6H2O (2.0 mg%), FeCl3 - 6H2O (1.0 mg%) or a particulate slurry prepared from FeCl3 + KOH, along with varying concentrations of several chelators. Heavy growth at pH 7.5-7.7 occurred with salicylhydroxamic acid, aurintricarboxylic acid, EDTA, NTA, and humic acid; and at pH 7.9-8.1 with SHAM and ATA. Moderate growth occurred at pH 7.5-7.7 with sulfosalicylic acid, dipicolinic acid, pyrocatecholdisulfonic acid, hexanohydroxamic acid, L-histidine, and at pH 7.9-8.1 with 1-naphthohydroxamic acid, EDTA, NTA. Slight growth occurred at pH 7.5-7.7 with benzohydroxamic acid, 1-naphohydroxamic acid, 2.6-dipicolinic acid N-oxide, salicylic acid, rhodotorulic acid, Na oxalate, EDDHA, sorbohydroxamic acid, gamma-pyrone-2, 6-decarboxylic acid, and at pH 7.9-8.1 with hexanohydroxamic acid, benzohydroxamic acid. Some ecological and physiological implications are discussed.

Endosymbionts, biogenic amines, and a heterodyne hypothesis for circadian rhythms.

Much of the evidence for the endosymbiont hypothesis of the origin of eukaryote cells arises from their spatial complexity: the heterogeneous array of distinct organelles suggests very naturally a colonial evolutionary history. Less studied, in this context, is the rich temporal structure of eukaryote physiology. Beyond the traditional four parts of the cell cycle, a fine structure of sequential metabolic events can now be seen. In yeasts and several flagellates, enzymes appear during the cycle in an invariant, orderly succession of “peaks” and “steps,” and temperature-sensitive mutants of these can serve as markers in the genetic analysis of temporal organization. In the yeast, Saccharumyces cereviseae, about 30 of these biochemical events have been pinpointed.= In addition to the qualitative temporal organization seen in such orderly sequences, there are also many indications of a quantitative structur-ie., regular metabolic oscillations. Well-known examples are the glycolytic oscillations in many and the regular pulses of CAMP secretion by certain cells during cellular slime-mold aggregati~n.~. 5 , Many examples of such oscillations are now known and various models have been proposed to account for them. In recent years the realization has grown that they must be the result of very nonlinear processes and therefore are probably limit cycles. That is, after perturbation, the oscillation always returns asymptotically to a stable cycle of given amplitude, wave form, and period.T Two important features of most of these biochemical oscillations are a relatively short period, of the order of several minutes, and a strong temperature dependence-eg., a Q,, of about 4 in the case of the glycolytic oscillation of yeasts. Perhaps the most striking feature of eukaryotic temporal architecture, however, is the much slower, temperature-insensitive circadian cycle-the biological clock.8 Since this phenomenon is usually considered a peculiarity of eukaryotes, it seems reasonable to enquire into its possible bearing on the endosymbiont theory of eukaryote origins.+

A White Queen Speculation

than the product of an engineering God? Certainly many felt and continue to feel that life cannot be understood on the basis of chemistry and physics alone. The emergence of Darwinism is seen by Smith as addressing part of the problem. Darwinism provided a clear answer to the problem of human origins, and the science of genetics gave Darwinism the mechanisms it required to support the theory. But Darwinism, even when fused with Mendelism, has not completely overcome opposition to a teleological interpretation of Shelleys question. In the last two chapters, Smith extends the mechanistic view of life to embryology and neurobiology. Both of these areas, especially the latter, have always been major obstacles in the progress of mechanistic biology. Many of the shrouds covering development have been removed as we come to understand more and more about gene regulation. But neurobiology has not yet permitted a purely mechanistic analysis. Few believe that major paradigms in neurobiology will not be forthcoming, but for now they remain obscured. As the book concludes, we see that the dichotomy still exists today in our understanding of life. We tend to view the world mechanistically, but we view ourselves more teleologically. We still have no satisfactory answer to Shelleys question, but the search continues, and . Smiths eminently readable and thoughtprovoking essay can only help to give us pause in our routine and inspire us to ponder the issues. Considering the objectives set forth by the author, this book succeeds with distinction. It is destined to become a classic.

Unity and Diversity in Biochemistry

A consideration of A Biochemical Phylogeny of the Protists, by M.A. Ragan and D.J. Chapman (Academic Press, 1978) and Biochemical Adaptation to Environmental Change, ed. by R.M.S. Smellie and J.F. Pennock (the Biochemical Society, 1976), within some thoughts on biochemical evolution.

Utility vs Beauty: Darwin, Wallace and the Subsequent History of the Debate on Sexual Selection

We examine the origins of the disagreement of Alfred Russel Wallace and Charles Darwin regarding the significance and mechanism of sexual selection and relate this to differences in their views of human evolution, and of cognitive ability and esthetic sensibilities of various human and nonhuman populations. We trace subsequent versions of these differing views into the twentieth century, and the controversy between R.A. Fisher’s Darwinian “runaway” model of sexual selection by female choice (the “sexy son” model), and Wallacean models of sexual selection based on signs of greater fitness of males (the “healthy gene” hypothesis). Models derived from the latter, the “honest signal” and “handicap” models, are discussed, and we note that these different models, based on utility or beauty, are not necessarily mutually inconsistent.