Biologists say they have solved the riddle of how a tiny bacterium senses light and moves towards it: the entire organism acts like an eyeball.
In a single-celled pond slime, they observed how incoming rays are bent by the bug’s spherical surface and focused in a spot on the far side of the cell.
By shuffling along in the opposite direction to that bright spot, the microbe then moves towards the light.
Other scientists were surprised and impressed by this “elegant” discovery.
Despite being just three micrometres (0.003mm) in diameter, the bacteria in the study use the same physical principles as the eye of a camera or a human.
It seemed really, really obvious afterwardsProf Conrad Mullineaux, Queen Mary University of London
This makes them “probably the world’s smallest and oldest example” of such a lens, the researchers write in the journal eLife.
Cyanobacteria, including the Synechocystis species used in the study, are an ancient and abundant lifeform. They live in water and get their energy from photosynthesis – which explains their enthusiasm for bright light.
“It has a way of detecting where the light is; we know that because of the direction that it moves. But we were puzzled about this because the cells are very, very small,” said study co-author Conrad Mullineaux, from Queen Mary University of London.
He told the BBC it was a chance observation through a microscope that put his team on the right track.
“We noticed it accidentally, because we had cells on a surface and we were shining light from one side, in order to watch the movement towards the light.
“We suddenly saw these focused bright spots and we thought, ‘bloody hell!’. Immediately, it was pretty obvious what was going on.”
After more than three centuries of scientists eyeballing bugs under microscopes, Prof Mullineaux said it was remarkable that nobody had picked up on this before.
“It seemed really, really obvious afterwards.”
To confirm and describe this single-cell “vision”, he worked with colleagues in the UK, Germany and Portugal on a series of experiments.