Bacteria can see, say scientists, solving 300-year-old mystery
Lowly bacteria can "see" in much the same way as humans, research has shown.
Each bacterial cell acts as a microscopic eyeball or fibre optic filament, scientists said.
The discovery solves a mystery that has vexed scientists for more than 300 years - how do bugs that rely on sunshine to survive sense light?
The answer is they do it by turning their whole body into a camera-like lens that focuses light onto a particular spot. This triggers movement away from the focal point and towards the light source.
Lead researcher Professor Conrad Mullineaux, from Queen Mary University of London, said: "The idea that bacteria can see their world in basically the same way that we do is pretty exciting."
The team studied Synechocystis, a species of cyanobacteria - bugs that form the green slime on rocks and pebbles and which, like plants, photosynthesise to tap into energy from the sun.
An ability to sense light is crucial to the survival of these ancient microbes that evolved more than two billion years ago.
Previous studies have shown that they possess an ability to pinpoint the position of a light source and move towards it. The new research shows that they achieve this by using their bodies as tiny lenses.
As light hits the spherical surface, it focuses onto a point on the other side of the cell. Within minutes, the bugs grow tiny tentacle-like structures called pili that reach out and pull the bacteria towards the light source.
Prof Mullineaux added: " Our observation that bacteria are optical objects is pretty obvious with hindsight, but we never thought of it until we saw it. And no-one else noticed it before either, despite the fact that scientists have been looking at bacteria under microscopes for the last 340 years."
A Synechocystis cell is about half a billion times smaller than the human eye.
As in the case of the human eye retina, the image cast onto the back of the cell is upside down. But its resolution is much lower, producing only a blurred outline.
While the bacterium studied acts as a spherical lens, the scientists believe light-sensitive rod-shaped bacteria behave like threads of optical fibre.
The findings, published in the journal eLife, demonstrate how evolution can converge even in organisms as different as bacteria and humans.
Co-author Annegret Wilde, from the University of Freiburg in Germany, said: "The physical principles for the sensing of light by bacteria and the far more complex vision in animals are similar, but the biological structures are different."