Bacteria can see by utilizing a single-celled eye of slime, a new discovery from an international team of researchers reveals. This answers a question that goes back 300 years — how these microscopic beings are able to detect the presence of light.

Unlike human beings, cyanobacteria are not able to see images but are capable of detecting light. They are then driven toward the bright source by pili, an action known as phototaxis.

"The fact that bacteria respond to light is one of the oldest scientific observations of their behavior. 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," Conrad Mullineaux from Queen Mary University of London said.

The new study reveals that the body of the bacteria acts like a lens, focusing the light that falls upon it. Tentacle-like pili then grow on the side of the bacteria where the light is striking and attach onto any surface they can reach, pulling the organism toward the energy source.

Synechocystis is one of many species of cyanobacteria that commonly live in water bodies, often seen as a slick, green slime on rocks and small stones.

Cyanobacteria, once known as blue-green algae, were first seen on Earth 2.7 billion years ago. They were responsible for the Great Oxygenation Event, the Earth's first great extinction, which took place 2.5 billion years ago. This change in climate also brought about the first ice age in the history of the Earth.

Human eyes can see detail as small as 1/50th of a degree — roughly 1/25th of the diameter of the full moon. Researchers who carried out this latest investigation believe cyanobacteria can only see details as small as 21 degrees from side-to-side.

Eyes of human beings also use a curved surface to focus light. This, researchers state, is an example of convergent evolution — when two distant species evolve similar techniques to carry out a task.

"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," said Annegret Wilde of the University of Freiburg in Germany.

Results of the study revealing the process by which bacteria detect light were published in the journal eLife.

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