Biology subjects

Tsang, A. C. H.

Publications and source records attributed to Tsang, A. C. H..

4 recordsLinked to original sources

bioRxiv2025

Light-dependent switching of circling handedness in microswimmer navigation

Many swimming microorganisms navigate their environment by modulating the curvature of their swimming trajectories in response to external cues. Here, we show that the biflagellate alga Chlamy-domonas reinhardtiiswims in circles and actively switches its trajectory handedness in response to orthogonal illumination: the cell swims counterclockwise at low light intensities yet clockwise at high light intensities. This handedness switching arises from light-dependent modulation of flag-ellar beating, including changes in beat extension, phase, and--crucially--beat plane orientation. Using high-speed imaging and hydrodynamic modeling, we reveal that this beat plane reorientation is critical for Chlamydomonas to swim orthogonally to light as well as to dynamically modulate its trajectory curvature, enabling transitions between global exploration and localized searching in spatially structured light fields. Our results establish beat plane reorientation as a novel mechanism for curvature control in microswimmer navigation.

biophysics↗

bioRxiv2024

Adaptation-induced oscillatory phototaxis and emergence of ordered density bands in the microswimmer Chlamydomonas reinhardtii

Biological microswimmers exhibit versatile taxis behaviors and switch between multiple behavioral states to navigate the environment and search for physiologically favorable regions. Here, we report a striking oscillatory phototaxis observed in Chlamydomonas reinhardtii, where cells swim back-and-forth under a constant, unidirectional light stimulus due to alternation between positive and negative phototaxis. This oscillatory phototaxis at the individual cellular level further leads to the emergence of a highly ordered, propagating band structure formed by high density Chlamydomonas cells collectively. We experimentally verify a unified phototaxis mechanism that couples light detection, light adaptation, flagella dynamics and cell reorientation, showing that transition between phototaxis modes is achieved by switching of flagella waveforms and modulation of flagella phase difference. Oscillatory phototaxis emerges as a semi-stable state in an overlapping light intensity regime for positive and negative phototaxis, where adaptation shifts the light intensity thresholds over times. This adaptation mechanism over multiple time scales enables phototactic microswimmers to effectively expand the survival range of light intensity and provide collective photoprotection for the colonies through the formation of dynamic band structures with high density.

biophysics↗

bioRxiv2023

Flagellar beat state switching in microswimmers to select between positive and negative phototaxis

Microorganisms have evolved various sensor-actuator circuits to respond to environmental stimuli. However, how a given circuit can select efficiently between positive vs. negative taxis under desired vs. undesired stimuli is poorly understood. Here, we investigate how the cellular mechanism by which the chiral microswimmer Euglena gracilis can select between positive vs. negative phototaxis under low vs. high light intensity conditions, respectively. We propose three general selection mechanisms for microswimmer phototaxis. A generic biophysical model demonstrates the effectiveness of all mechanisms, but which varies for each depending on specific conditions. Experiments reveal that only a photoresponse in-version mechanism is compatible with E. gracilis phototaxis. Specifically, a light-intensity dependent transition on the sub-second time scale between two flagellar beat states responsible for forward swimming vs. sideway turning ultimately generates positive phototaxis at low light intensity via a run-and-tumble strategy and negative phototaxis at high light intensity via a helical klinotaxis strategy. More generally, a picture emerges where a variety of E. gracilis behaviors over a wide range of light intensities as reported in the literature can be explained by the coordinated switching between just these two flagellar beating states over time. These results provide design principles for simple two-state switching mechanisms in natural and synthetic microswimmers to operate under both noisy and saturated stimulus conditions. LAY ABSTRACTOur experimental and theoretical results explain how the single cell Euglena gracilis achieves both positive and negative phototaxis. Our insights are then able to synthesise a larger number of previously described observations on E. gracilis photoresponses and photobehaviors due to a concise two-state model of flagellar beating. These insight will likely inform the behaviors of other natural microswimmers as well as the design of synthetic ones.

biophysics↗

bioRxiv2018

Polygonal motion and adaptable phototaxis via flagellar beat switching in Euglena gracilis

Biological microswimmers exhibit versatile strategies for sensing and navigating their environment 1-7, e.g., run-and-tumble 2 and curvature modulation 3. Here we report a striking behavior of Euglena gracilis, where Euglena cells swim in polygonal trajectories due to exposure to increasing light intensities. While smoothly curved trajectories are common for microswimmers 3, 8, such quantized ones have not been reported previously. This polygonal behavior emerges from periodic switching between the flagellar beating patterns of helical swimming 6, 9 and spinning 10 behaviors. We develop and experimentally validate a biophysical model that describes the phase relationship between the eyespot, cell orientation, light detection, and cellular reorientation, that accounts for all three behavioral states. Coordinated switching between these behaviors allows ballistic, superdiffusive, diffusive, or subdiffusive motion 11,12 (i.e., the tuning of the diffusion constant over 3 orders of magnitude) and enables navigation in structured light fields, e.g., edge avoidance and gradient descent. This feedback-control links multiple system scales (flagellar beats, cellular behaviors, phototaxis strategies) with implications for other natural and synthetic microswimmers 13.

biophysics↗