How microorganisms swim without a brain and why it matters

Picture a microorganism swimming through a liquid. It moves with purpose, but it has no brain, no nerves. How does it know where to go? How does it move at all?

A team of researchers from TU Wien, the University of Vienna, and Tufts University decided to tackle these questions. They simulated microorganisms on a computer to understand how they move without having centralized control.

The findings reveal something surprising: even without a brain or any central control, microorganisms can still swim efficiently.

This behavior isn’t just interesting from a biological standpoint – it could also lead to more effective designs for nanobots that navigate through fluids to deliver drugs or clean up pollutants.

Microorganisms swim efficiently

Bacteria, amoebas, and even blood cells move without central control. Yet, they navigate effectively through fluids. The researchers wanted to know how.

Benedikt Hartl, from TU Wien and Tufts University, explained the concept using a simple analogy.

“Simple microorganisms can be imagined as being composed of several parts, a bit like a string of pearls,” he said. Each part can move relative to the next, but there is no overall command center issuing orders.

The research team created virtual microorganisms made up of interconnected beads. Each bead could exert a force to the left or right.

But here’s the catch: each bead only knew the position of its immediate neighbors. It had no idea what the rest of the body was doing. Despite this, the beads still managed to coordinate their movements to swim effectively.

Why decentralized control is effective 

Why bother with decentralized control? Why not just give these microorganisms a virtual brain?

The researchers argue that single-celled organisms don’t have brains, so it made sense to simulate movement without one. They wanted to see if simple rules could still produce coordinated movements.

“Is there a control system, a set of simple rules, a behavioral strategy that each bead can follow individually so that a collective swimming motion emerges – without any central control unit?” Hartl questioned.

Turns out, there is. The beads don’t need a central controller. Instead, they rely on simple, localized rules. If each bead reacts only to its immediate neighbors, a kind of collective intelligence emerges. The beads work together without knowing it.

Virtual microorganisms swimming

The researchers simulated these movements using a basic neural network. Each bead acted like a tiny agent with its own controller.

The neural network had 20 to 50 parameters – not much in the world of artificial intelligence. Still, it was enough to produce coordinated swimming behaviors.

On the computer screen, the virtual microorganisms swam through simulated viscous fluids. They didn’t just move randomly. They propelled themselves in specific directions, responding to simulated forces.

The most intriguing part was that they were swimming with purpose without any central control.

How microorganisms learn to swim

The researchers didn’t just program these beads to swim. They let them learn how to swim. They used neuroevolution – a process that mimics natural selection.

In each iteration, the virtual microorganisms tried different swimming strategies. The best ones survived, while the less efficient ones were eliminated.

With each new generation, the swimmers got better at moving through fluids. They became more coordinated and more efficient. They developed swimming gaits that propelled them forward with surprising speed and precision.

Applications beyond biology

As the simulations progressed, a pattern emerged. Larger microorganisms tended to adopt long-wavelength body deformations. Instead of twitching back and forth, they generated sweeping, wave-like movements. These long-wavelength movements proved to be the most efficient.

Andreas Zöttl from the University of Vienna saw potential beyond biology. If microorganisms could swim using such simple rules, so could nanobots.

Imagine a swarm of nanobots moving through the bloodstream, delivering drugs directly to cancer cells. With decentralized control, each nanobot could operate independently, yet still contribute to the overall mission.

How microorganisms adapt to cargo

The researchers didn’t stop at swimming. They wanted to know if these decentralized controllers could handle cargo. They loaded the virtual microorganisms with additional beads – representing cargo – and watched what happened.

Despite the added weight, the swimmers kept moving. They didn’t need any adjustments to their control systems. The same simple rules that allowed them to swim also enabled them to transport cargo. As the cargo got heavier, the swimmers slowed down. But they kept moving.

Robustness without reprogramming

The researchers pushed the virtual swimmers further. What if a few beads stopped working? What if some connections got severed?

Even then, the swimmers kept going. The decentralized control system was remarkably robust. If one bead failed, the others adapted.

If a connection broke, the swimmers still moved. This kind of resilience is rare in centralized systems. But in decentralized systems, it’s almost a given.

How larger microswimmers adapt

The researchers didn’t just stick to small models. They scaled up to 100 beads. Would the same rules work? Could the microorganisms still swim without central control?

Yes. In fact, the larger swimmers performed even better. They generated longer, smoother waves. They moved more efficiently without any additional programming. The same set of rules that worked for a three-bead swimmer also worked for a 100-bead swimmer.

Adapting to new challenges

The virtual swimmers didn’t just learn to swim. They also learned to adapt. The researchers varied the simulated environment. They added obstacles, changed the viscosity of the fluid, and introduced new challenges.

The swimmers adapted without any reprogramming. Their controllers adjusted to the new conditions. This adaptability hints at the power of decentralized systems. They don’t just follow orders. They learn and evolve.

Nanobots could swim like microorganisms

The implications extend far beyond microorganisms. Zöttl envisioned nanobots that could replicate these movements.

Imagine a swarm of nanobots in polluted water, seeking out oil droplets and breaking them down. Or consider medical nanobots that navigate through the bloodstream, releasing drugs at specific locations.

These nanobots wouldn’t need complex programming. They’d just follow simple rules – the same rules that let microorganisms swim without a brain.

A new way of thinking about movement

This study challenges the assumption that complex movement requires complex control systems. Instead, the researchers showed that simple, decentralized rules can produce surprisingly coordinated behaviors.

The implications for nanotechnology are significant. Autonomous nanobots could swim without central control systems, like microorganisms, thus reducing computational costs and increasing resilience.

And by learning from nature, engineers could develop systems that adapt, evolve, and keep moving – even when some parts fail.

The study is published in the journal Communications Physics.

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