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.”