Slime can see: Living lenses use micro-optics to sense direction of light

Slime can see: Living lenses use micro-optics to sense direction of light
Some cyanobacteria are tiny, photosynthetic Beholders , according to an international team of scientists.

, according to an international team of scientists. We already knew that Synechocystis bacteria are pretty good at managing their photonutritive needs. They can move preferentially towards a light source of the right color and intensity, which implies that they can “see” the difference. But how can such a tiny cell, simpler than the simplest eye, accurately detect where light is coming from? New research has shown that Synechocystis, an old friend to research, uses its whole body to act like a camera eye.

Bacteria are optical objects, each cell acting like a microscopic eyeball or the world’s oldest and smallest camera eye. Image: eLife

Synechocystis are spheroid bacteria whose shape actually lets the whole single-celled organism function as a camera eye. The light incident on the bacterium is focused by its spherical shape, such that it forms an inverted image on the interior face of the cell, where it’s picked up by photopigments just like in the human eyeball. With an angular resolution of about 20°, Synechocystis can’t see anywhere near as well as we can — the angular resolution of the human eye clocks in at about 0.2° — but even their blurry optics are enough to give the bacteria a clear sense of where the light is best, and what to do about it.

(a) False color image of fluorescence from the photosynthetic pigments, with the laser spot indicated by the red circle. The broken white lines indicate the approximate cell boundaries and the white arrows highlight examples of the focused images of the light source at the rear edge of the cell. Cells approaching the laser spot show strong selective excitation of the leading edge of the cell. (b) Direction switching triggered by contact with the edge of the laser spot. The arrowed lines indicate net displacements of representative cells over time windows of 132 s before and after closest approach to the laser spot. The mean orientation of the tracks is shown. (c) Light intensity required to reverse the path of Synechocystis cells. Image: eLife

The bacterium moves away from the interior spot of focused light by extending foot-like pili on the side opposite the focused dot, to shuffle along like the Blob toward places where the light conditions are better. We know this because we tested it by herding pond slime with a laser. It scooted away from the spot of focused light created by the laser dot. This demonstrates that the movement of Synechocystis is based on direct perception, rather than a biased random walk.

What’s more, the image on the inside of the bacterium is focused within an area smaller than the wavelength of the incident light. This moves Synechocystis to a whole new echelon of sophistication. The researchers doing this experiment found that “the observed near-field light pattern could be accurately reproduced by an FDTD simulation approximating the cell as a dielectric sphere with a diameter of 3 µm and a refractive index of 1.4.”

In human contexts, this kind of nanomachinery is referred to as a photonic nanojet (PDF), and it can breeze past the theoretical resolving power of an optical microscope by using a dielectric microsphere to narrowly focus light with a beam width that’s only a fraction of its wavelength. This can be done with spheres or cylinders, and in fact the researchers believe that they’ll find other, rod-shaped bacteria acting like an optical fiber while doing this kind of sophisticated visual sensing. Where synthetic microspheres or microcylinders could be used in sub-wavelength nanolithographyor ultra-high-density optical storage, these living lenses could find applications in biophotoreactors or a wholly new class of solar cells.

Original: http://dx.doi.org/10.7554/eLife.12620