Nature

Bacteria Can See Too? Scientists Accidentally Discover the “World’s Smallest Eyeballs”

February 12, 2016 | Joanne Kennell

Cyanobacteria in a petri dish
Photo credit: Queen Mary University of London

No one noticed for 340 years!

It is amazing the things people discover by accident — penicillin, the microwave, even velcro!  And now, scientists have accidentally found that bacteria see the world just like us — the bacterial cells act like microscopic eyeballs.  In other words, the world’s smallest eye!

“The idea that bacteria can see their world in basically the same way that we do is pretty exciting,” says lead researcher Conrad Mullineaux, Professor of Microbiology from Queen Mary University of London’s School of Biological and Chemical Sciences in a press release.

Researchers made the discovery by accident when studying aquatic cyanobacteria, which often form the slippery green film found on rock and pebbles.  The species used in the study, members of the genus Synechocystis, are found naturally in freshwater lakes and rivers.

SEE ALSO: Scientists Created Lenses That Work Like Insect Eyes

Cyanobacteria evolved nearly 2.7 billion years ago, and their ability to produce oxygen and fix carbon dioxide using energy from the sun — a process known as photosynthesis — is thought to have caused mass extinctions and the oldest known ice age.

Scientists also knew the bacteria could perceive the position of a light source and move towards it — a phenomenon called phototaxis — but no one understood how they did it, until now.

“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,” Mullineaux told Jonathan Webb at BBC News.  “We suddenly saw these focused bright spots [inside the cells] and we thought, ‘bloody hell!’  Immediately, it was pretty obvious what was going on.”

Further research revealed that the bacteria are able to do this because their cell bodies act like lenses — as light hits the spherical surface, it refracts into a point on the other side of the cell, triggering movement by the cell away from the focused spot.

Within minutes, the bacterium grows tiny tentacle-like structures called pili that reach out towards the light source.  The pili attach to the surface they’re on and then retract, pulling the bacterium to its desired location.

Below is a conceptual model of how the cyanobacterial cells see.

How slime sees
image credit: eLife

“The fact that bacteria respond to light is one of the oldest scientific observations of their behaviour,” said Mullineaux.  “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.”

That being said, the bacteria don’t have vision equal to that of humans — just the light-based lens mechanisms works in the same way.  According to the researchers, a Synechocystis cell is roughly half a billion times smaller than the human eye, with a much lower resolution — meaning they would only be able to see a blurred outline of objects.

“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 team member Annegret Wilde from the University of Freiburg in Germany.

The ability for eyes to distinguish an object’s detail is determined by its angular resolution.   For example, humans have an impressive 0.02 degrees, whereas the team estimates that the Synechocystis is about 21 degrees.

The ability that these bacteria possess, seeing light and using their eyes to hunt out sunshine and photosynthesize, is a process crucial for their survival.

Now, scientists finally know how they do it!

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