Light control of the flow of phototactic microswimmer suspensions

Some micro-algae are sensitive to light intensity gradients. This property is known as phototaxis: the algae swim toward a light source (positive phototaxis). We use this property to control the motion of micro-algae within a Poiseuille flow using light. The combination of flow vorticity and phototaxis results in a concentration of algae around the center of the flow. Intermittent light exposure allows analysis of the dynamics of this phenomenon and its reversibility. With this phenomenon, we hope to pave the way toward new algae concentration techniques (a bottleneck challenge in hydrogen algal production) and toward the improvement of pollutant bio-detector technology

Physically Embedded Genetic Algorithm Learning in Multi-Robot Scenarios: The PEGA algorithm

We present experiments in which a group of autonomous mobile robots learn to perform fundamental sensor-motor tasks through a collaborative learning process. Behavioural strategies, i.e. motor responses to sensory stimuli, are encoded by means of genetic strings stored on the individual robots, and adapted through a genetic algorithm (Mitchell, 1998) executed by the entire robot collective: robots communicate their own strings and corresponding fitness to each other, and then execute a genetic algorithm to improve their individual behavioural strategy. The robots acquired three different sensormotor competences, as well as the ability to select one of two, or one of three behaviours depending on context ("behaviour management"). Results show that fitness indeed increases with increasing learning time, and the analysis of the acquired behavioural strategies demonstrates that they are effective in accomplishing the desired task

Artificial Light Increases Local Predator Abundance, Predation Rates, and Herbivory.

Human activity is rapidly increasing the radiance and geographic extent of artificial light at night (ALAN) leading to alterations in the development, behavior, and physiological state of many organisms. A limited number of community-scale studies investigating the effects of ALAN have allowed for spatial aggregation through positive phototaxis, the commonly observed phenomenon of arthropod movement toward light. We performed an open field study (without restricted arthropod access) to determine the effects of ALAN on local arthropod community composition, plant traits, and local herbivory and predation rates. We found strong positive phototaxis in 10 orders of arthropods, with increased (159% higher) overall arthropod abundance under ALAN compared to unlit controls. The arthropod community under ALAN was more diverse and contained a higher proportion of predaceous arthropods (15% vs 8%). Predation of immobilized flies occurred 3.6 times faster under ALAN; this effect was not observed during the day. Contrary to expectations, we also observed a 6% increase in herbivory under ALAN. Our results highlight the importance of open experimental field studies in determining community-level effects of ALAN

Re-enterant efficiency of phototaxis in Chlamydomonas reinhardtii cells

Phototaxis is one of the most fundamental stimulus-response behaviors in biology wherein motile micro-organisms sense light gradients to swim towards the light source. Apart from single cell survival and growth, it plays a major role at the global scale of aquatic ecosystem and bio-reactors. We study photoaxis of single celled algae Chalmydomonas reinhardtii as a function of cell number density and light stimulus using high spatio-temporal video microscopy. Surprisingly, the phototactic efficiency has a minimum at a well-defined number density, for a given light gradient, above which the phototaxis behaviour of collection of cells can even exceed the performance obtainable from single isolated cells. We show that the origin of enhancement of performance above the critical concentration lies in the slowing down of the cells which enables them to sense light more effectively. We also show that this steady state phenomenology is well captured by a modelling the phototactic response as a density dependent torque acting on an active Brownian particle

Phototaxis beyond turning: persistent accumulation and response acclimation of the microalga Chlamydomonas reinhardtii

Phototaxis is an important reaction to light displayed by a wide range of motile microorganisms. Flagellated eukaryotic microalgae in particular, like the model organism Chlamydomonas reinhardtii, steer either towards or away from light by a rapid and precisely timed modulation of their flagellar activity. Cell steering, however, is only the beginning of a much longer process which ultimately allows cells to determine their light exposure history. This process is not well understood. Here we present a first quantitative study of the long timescale phototactic motility of Chlamydomonas at both single cell and population levels. Our results reveal that the phototactic strategy adopted by these microorganisms leads to an efficient exposure to light, and that the phototactic response is modulated over typical timescales of tens of seconds. The adaptation dynamics for phototaxis and chlorophyll fluorescence show a striking quantitative agreement, suggesting that photosynthesis controls quantitatively how cells navigate a light field.Comment: Six pages, three figures, plus supplementary materia

Using a new high-throughput video-tracking platform to assess behavioural changes in Daphnia magna exposed to neuro-active drugs

© 2019. ElsevierOne of the major challenges that faces today regulatory risk assessment is to speed up the way of assessing threshold sublethal detrimental effects of existing and new chemical products. Recently advances in imaging allows to monitor in real time the behaviour of individuals under a given stress. Light is a common stress for many different organisms. Fish larvae and many invertebrate species respond to light altering their behaviour. The water flea Daphnia magna as many other zooplanktonic species has a marked diel vertical phototactic swimming behaviour against light due to fish predation. The aim of this study was to develop a high throughput image analysis to study changes in the vertical swimming behaviour to light of D. magna first reproductive adult females exposed to 0.1 and 1 µg/L of four psychiatric drugs: diazepam, fluoxetine, propranolol and carbamazepine during their entire life. Experiments were conducted using a new custom designed vertical oriented four 50 mL chamber device controlled by the Noldus software (Netherlands). Changes in speed, preferred area (bottom vs upper areas) and animal aggregation were analysed using groups of animals under consecutive periods of dark and apical light stimulus of different intensities. Obtained results indicated that light intensity increased the speed but low light intensities allowed to better discriminate individual responses to the studied drugs. The four tested drugs decreased the response of exposed organisms to light: individuals move less, were closer to the bottom and at low light intensities were closer each other. At high light intensities, however, exposed individuals were less aggregated. Propranolol, carbamazepine and fluoxetine were the compounds effecting most the behaviour. Our results indicated that psychiatric drugs at environmental relevant concentrations alter the vertical phototactic behaviour of D. magna individuals and that it is possible to develop appropriate high-throughput image analysis devices to measure those responses.Peer ReviewedPostprint (author's final draft

Natural History Note Differential Response To Circularly Polarized Light By The Jewel Scarab Beetle Chrysina Gloriosa

Circularly polarized light is rare in the terrestrial environment, and cuticular reflections from scarab beetles are one of the few natural sources. Chrysina gloriosa LeConte 1854, a scarab beetle found in montane juniper forests of the extreme southwestern United States and northern Mexico, are camouflaged in juniper foliage; however, when viewed with right circularly polarizing filters, the beetles exhibit a stark black contrast. Given the polarization-specific changes in the appearance of C. gloriosa, we hypothesized that C. gloriosa can detect circularly polarized light. We tested for phototactic response and differential flight orientation of C. gloriosa toward different light stimuli. Chrysina gloriosa exhibited (a) positive phototaxis, (b) differential flight orientation between linear and circularly polarized light stimuli of equal intensities, and (c) discrimination between circularly polarized and unpolarized lights of different intensities consistent with a model of circular polarization sensitivity based on a quarter-wave plate. These results demonstrate that C. gloriosa beetles respond differentially to circularly polarized light. In contrast, Chrysina woodi Horn 1885, a close relative with reduced circularly polarized reflection, exhibited no phototactic discrimination between linear and circularly polarized light. Circularly polarized sensitivity may allow C. gloriosa to perceive and communicate with conspecifics that remain cryptic to predators, reducing indirect costs of communication.Integrative Biolog

A tale of three taxes: photo-gyro-gravitactic bioconvection

The term bioconvection encapsulates the intricate patterns in concentration, due to hydrodynamic instabilities, that may arise in suspensions of non-neutrally buoyant, biased swimming microorganisms. The directional bias may be due to light (phototaxis), gravity (gravitaxis), a combination of viscous and gravitational torques (gyrotaxis) or other taxes. The aim of this study is to quantify experimentally the wavelength of the initial pattern to form from an initially well-mixed suspension of unicellular, swimming green algae as a function of concentration and illumination. As this is the first such study, it is necessary to develop a robust and meticulous methodology to achieve this end. The phototactic, gyrotactic and gravitactic alga Chlamydomonas augustae was employed, with various red or white light intensities from above or below, as the three not altogether separable taxes were probed. Whilst bioconvection was found to be unresponsive to changes in red light, intriguing trends were found for pattern wavelength as a function of white light intensity, depending critically on the orientation of the illumination. These trends are explored to help unravel the mechanisms. Furthermore, comparisons are made with theoretical predictions of initial wavelengths from a recent model of photo-gyrotaxis, encouragingly revealing good qualitative agreement