This Bacterium Acts Like a One-Cell Eyeball

Stop looking at me, microbe.

Synechocystis colonies (Conrad Mollineaux)

Your eye is an inch-wide orb that detects light by focusing it onto a retina with a lens. Now, we know that a freshwater bacterium called Synechocystis does virtually the same thing, even though it’s 10,000 times smaller and consists of just a single, spherical cell. When light hits the cell, it becomes focused on the opposite side of the sphere, allowing the bacterium to sense where it’s coming from and move in that direction. Synechocystis is effectively a living lens, and its entire boundary is a retina.

Synechocystis is part of a large family of bacteria called cyanobacteria, which can make their own food through photosynthesis. In doing so, these microbes release oxygen, and they’re the reason why the Earth’s atmosphere contains breathable amounts of this vital gas.

Photosynthesis depends on sunlight, and for more than a century, scientists have known that many bacteria can move towards sources of light. Some swim; others, like Synechocystis, shoot out long cables that end in stick tips, shuffling forward using these tiny grappling hooks. But even though their movements are well-understood, it wasn’t clear how they knew which way to go. How do they manage to head towards the light?

Some researchers had suggested that they do so in a bumbling way: They sample the strength of light at their current position, and use that to bias their otherwise random movements. Slowly but erratically, they head in the right direction. But Nils Schuerger and Annegret Wilde from the University of Freiburg showed that this couldn’t be true. When they placed Synechocystis on a surface that was lit from overhead, so that one end was bright and the other dark, the bacteria didn’t head towards the brighter end. They only did that if the light was specifically coming from that direction. They could perceive the direction of a light source.

The team originally suspected that pigments within the bacteria were shading the sides opposite the light. “My contribution was to point out that this was pretty much impossible,” says Conrad Mullineaux from Queen Mary University of London, who visited the group on sabbatical. “The cells are so tiny that they only absorb a small percentage of the light that goes through them, so they’re the same brightness on the rear side as the front side. We were really puzzled.”

Schuerger solved the mystery by accident. He had set up a microscope to illuminate the cells from just one side, to give them a signal to move. Then, he noticed that the cells had an exceptionally bright spot on the opposite edge to the light source. They were focusing the light! “It’s very obvious when you see it but no one had noticed it before,” says Mullineaux.

The team tested their idea by using a laser to shine a focused spot of light onto one edge of the cells. Sure enough, they started moving in the opposite direction. How the cell responds to that spot of light is still an open question, though. In our eyes, the retina sends electrical signals to the brain, but that’s clearly not what happens in Synechocystis: It has no brain; it’s just a cell. Still, the team says that it’s “probably the world’s smallest and oldest example of a camera eye”—a simpler version of those that you’re now using to read these words.

“That’s going too far,” says Dan-Eric Nilsson from Lund University, an expert in eye evolution. The similarities to our eyes are there, but they’re not exact. He makes a comparison with the purple sea urchin, which has light-detecting cells in the hundreds of “tube feet” that protrude from its body. “That makes the entire urchin act a bit like a compound eye, but no one would claim sea urchins have compound eyes,” says Nilsson (er, except me, that one time).

He also notes that true vision is about producing an image of the world by integrating information about light from different directions. The bacteria are merely focusing light to sense its direction. “This is not vision,” says Nilsson, “but it beautifully demonstrates how little you need to acquire the functions that can evolve into vision.”

But Mullineaux argues that Synechocystis meets Nilsson’s criteria. If he shines two lights upon the bacteria, most of them move towards a point midway between the sources. “They’re acquiring and integrating spatial information, and responding. I’d say that’s the definition of seeing.”

“I tried to alter a photograph of me to see what would I look like to Synechocystis,” he adds, “and they should be able to see the outline of my head if I was peering down at them on a Petri dish,” says Mullineaux. “They can store reasonably complicated visual information.”

Semantics aside, it’s clear that microbes are doing more advanced things with light than we gave them credit for. There are algae called warnowiids that, despite also consisting of just one cell, have components that act like lenses, irises, corneas, and retinas. Some of these seem to be made from domesticated bacteria, which were swallowed by the warnowiids’ ancestor and have since become repurposed into light-processing cellular compartments.

And Mullineaux thinks that many bacteria can carry out Synechocystis’s light-focusing trick. “The lensing effect might be optimized in Synechocystis, but any bacterium will act as a sort of lens,” he says. “Whether they respond to it or not, we don’t know. But there should be other bacteria out there that can see.”