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Just like land plants, algae use sunlight as an energy source. Many green algae actively move in the water; they can approach the light or move away from it. For this they use special sensors (photoreceptors) with which they perceive light.

The decades-long search for these light sensors led to a first success in 2002: Georg Nagel, at the time at Max-Planck-Institute of Biophysics in Frankfurt/M, and collaborators discovered and characterized two so-called channelrhodopsins in algae. These ion channels absorb light, then open up and transport ions. They were named after the visual pigments of humans and animals, the rhodopsins.

Now a third “eye” in algae is known: Researchers discovered a new light sensor with unexpected properties. The new photoreceptor is not activated by light but inhibited. It is a guanylyl cyclase which is an enzyme that synthesizes the important messenger cGMP. When exposed to light, cGMP production is severely reduced, leading to a reduced cGMP concentration – and that’s exactly what happens in the human eye as soon as the rhodopsins there absorb light. (1)

See too much light.

And your eyes will close.

It is darkness you seek.

So that your eyes open.

For only in the dead of the night, can you detect brightness…

Only there, standing alone in the complete absence of any source of light, can you realize that the only thing emitting light in this cosmos is you… And this knowledge will be the darkest knowledge you will ever have.

Cherish that knowledge.

And never seek light outside you.

If you do, you will find it.

And the whole cosmos will instantly fall into darkness…