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Trending Science: Scientists ‘accidentally’ discover the world’s oldest and smallest eyeball

Biologists from Queen Mary, University of London have discovered how a single-cell organism acts like an eyeball in order to sense light and move towards it.
Trending Science: Scientists ‘accidentally’ discover the world’s oldest and smallest eyeball
After observing a specific species of cyanobacteria, called Synechocystis, that lives in water and can form a slippery green ‘pond slime,’ the researchers discovered how incoming rays are bent by the organism’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 organism then moves towards the light.

The bacteria, only 0.003 mm in diameter, are able to do this because the entire cell body acts like a lens, producing an effect equivalent to a microscopic eyeball or camera.

Within minutes the bacteria grow tiny tentacle-like structures called pili that pulls the bacteria along the surface they’re attached to and towards the source of the light.

Cyanobacteria evolved around 2.7 billion years ago and get their energy from photosynthesis, which explains their enthusiasm for bright light.

An accidental observation

Prof. Conrad Mullineaux, one of the co-authors of the study published in the journal ‘eLife,’ commented: ‘It [the bacteria] 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.’

He continued by admitting that the team only noticed the solution accidentally through a chance observation through a microscope.

‘We had cells on a surface and we were shining light from one side, in order to watch the movement towards the light,’ Prof. Mullineaux continued. ‘We suddenly saw these focused bright spots... immediately it was pretty obvious what was going on.’

To confirm and describe this single-cell ‘vision,’ he worked with colleagues in the UK, Germany and Portugal on a series of experiments.

Laser beam experiment to probe further

As well as studying the bacteria's focusing ability with different types of microscope, the team also used a laser beam to probe exactly how such focused light affected the organism’s behaviour.

With the laser beam trained steadily on the centre of a dish, the team shone a bigger, separate light on the Synechocystis cells from one side.

This drew the bacteria across the surface in the usual way, using their pili to pull themselves towards the light.

But the moment any of the bacteria moved into the laser beam, there was an abrupt u-turn.

‘When they hit it [the laser], they bounced off it,’ Prof Mullineaux said. ‘As soon as the laser was hitting one side of the cell, the cells moved away. They switched direction.’

In other words, bright light focused on one side of the bacterium definitely does drive it to run the other way - which under normal circumstances takes it towards the source of the light.

Because the same amount of light is hitting the cell from all around, the team says that each microbe will have a ‘360-degree image’ of its surroundings focused on the inside of its cell membrane.

Angular resolution for distinguishing detail

A Synechocystis cell is about half a billion times smaller than the human eye. As with the retina in the human eye, the image on the rear of the cell will be upside down.

Moreover, the ability of optical objects to distinguish fine detail is determined by ‘angular resolution.’ In a human eye this is an impressive 0.02 degrees, but the research team estimate that in Synechocystis it is about 21 degrees, meaning that the image for the bacteria is very fuzzy.

This though is more than enough for the bacteria’s photoreceptor molecules, embedded in the cell membrane, to guide its movement.

Source: Based on media reports

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  • United Kingdom
Record Number: 124806 / Last updated on: 2016-02-11
Category: Trending Science
Provider: EC