Algae discovered to make polygonal shapes

23 January 2019

A Croucher scholar has revealed mathematical secrets behind the movement of the much-studied microorganism Euglena gracilis with some possible implications for robotics.

The single-celled alga has been observed under the microscope by generations of biology students. However, no one had noticed that it swims in precise polygons when exposed to increased light until postdoctoral scholar Dr Alan Tsang (Croucher Fellowship 2017) observed this behaviour in a computer model he developed.

Tsang and a team of bioengineers at Stanford University discovered that under increased light intensity, the Euglena halts its forward movement and begins tracing out elaborate anticlockwise polygons — triangles, squares and pentagons — in a mathematically defined effort to find a better environment.

In its natural pond-dwelling setting, this would enable it to navigate away from bright sunlight.

The discovery, described in a paper published in Nature Physics, could help scientists design micro-scale robots that are more efficient and effective in manoeuvring through watery environments, including the human bloodstream.

The team was initially surprised by Tsang’s observation and thought there was something wrong with the model’s coding. But when they checked under the microscope, increasing light levels as in the simulation, they saw the polygons.

When Tsang simulated increased light, the alga began tracing out the polygons. The shapes arise from how Euglena gracilis navigates the world. Because the organism normally rolls through the water on its long axis, the eyespot rotates to survey 360 degrees of light. In steady light conditions, which are normal under a microscope, it meanders along in a relatively straight path.

Euglena makes sharp turns to avoid strong light.

Tsang explained that when the eyespot detects increased light intensity, the Euglena makes a hard turn, but when the light is absent it swims straight again. Since it is constantly rotating, after a full cycle it makes another strong turn. After making more straight lines followed by sharp turns, a triangle is born.

Tsang also noticed that over the course of about 30 seconds the Euglena adapted to the stronger light and made less sharp turns, resulting in the expanding polygons — squares, then pentagons — until, finally, the microorganism headed in a relatively straight line.

The phenomenon has not been noticed before because researchers rarely alter light levels while observing Euglena under a microscope. Tsang, in contrast, was specifically trying to model how the organism moved in relation to light, which prompted the behaviour to appear.

Dr Alan Tsang, Croucher Fellowship 2017

He explained that the mathematical model involved in this movement could be useful in microscale robotics, given the emerging field where people are trying to engineer and programme microscopic swarm robotics in areas such as microsurgery or drug delivery. “I definitely see people looking for efficient control mechanisms at the microscale,” Tsang said. Nature in a simple, single-celled form may have offered a solution.

Dr Alan Cheng Hou Tsang received the Croucher Fellowship in 2017. He completed his BEng and MPhil in Mechanical Engineering from the University of Hong Kong in 2009 and 2011, and his PhD in Mechanical Engineering from the University of Southern California in 2016. His research interests lie at the interface of microfluidics and biophysics, with particular focus on the collective behaviour and taxis mechanisms of swimming microorganisms. He is currently a Postdoctoral Fellow in the Bioengineering Department at Stanford University.

To view Dr Tsang’s Croucher profile please click here.