Pond scum and the Nobel Prize in Medicine

Pond scum and the Nobel Prize in Medicine

One of the problems with Trump’s attack on scientific research is that projects that seem to have little practical value can turn out to have enormous consequences. Consider this question: how does a bit of pond scum swim toward the light it needs for photosynthesis? That question led to this year’s Nobel Prize in Physiology or Medicine. The core discovery work was done in Germany at Max Planck institutes with a contribution from a researcher at Stanford. 

The brain is incredibly complex, and so are the tools needed to study it. The most important of the tools is the brain power of the researcher. The other, optogenetics, won the 2026 Nobel Prize. It grew out of a scientist’s curiosity about how a single-celled alga manages to swim toward light.

The answer turned out to be channelrhodopsin, a protein that can make almost any cell it is placed into sensitive to light. Its discovery was said to have “jump-started the development of a whole range of new tools that let us selectively turn on and off brain cells with incredible precision. It has let us go from a Rand McNally road atlas of the brain to something more akin to Google Earth.” 

The prize is shared by Karl Deisseroth of Stanford University, Peter Hegemann of Humboldt University of Berlin and Georg Nagel of the University of Würzburg, “for their discoveries concerning light-gated ion channels and optogenetics.” Starting from the study of algae that are attracted to light, they and others built the field of optogenetics using light to control nerve cells that have been genetically modified to carry a light-sensitive protein.

Some background: nerve impulses are generated by proteins called ion channels, which sit in the cell membrane and let charged atoms cross it. The nervous system relies on ion channels that open only under specific conditions, such as when they detect a neurotransmitter or a change in voltage. That fine control lets each type of nerve cell fire only when it should and keeps the brain from dissolving into a haze of electrical noise. What neuroscientists lacked was a way to switch specific cells on and off at will.

The solution came from a single-celled alga called Chlamydomonas. It’s one cell is remarkably sophisticated, with two flagella for swimming and an eyespot that detects the light it moves toward. In the early 1990s Hegemann, then at the Max Planck Institute of Biochemistry near Munich, recorded from Chlamydomonas with fine electrodes and found that a flash of light triggered an electrical response within about half a millisecond. That was so fast that he suspected the light-sensing protein might itself be the ion channel.

When researchers began cataloguing the messenger RNAs made by Chlamydomonas, Hegemann spotted two genes resembling bacteriorhodopsin, a light-driven ion pump found in salt-loving archaea. Experiments that blocked these genes with RNA interference reduced the alga’s response to light, implicating them in its light sensing.

Hegemann then teamed up with Nagel, an ion-channel specialist at the Max Planck Institute of Biophysics in Frankfurt. By producing the proteins in frog egg cells, they showed that both were light-gated ion channels. The first, described in 2002, appeared to let through mainly protons (essentially hydrogen ions), while the second, described in 2003, passed a broad range of positively charged ions. The two respond best to somewhat different wavelengths of light, and, like the light receptors in our eyes, both capture light using retinal, a chemical relative of vitamin A. The new class of proteins was named channelrhodopsins.

Nagel was instrumental in showing how widely they could be used. The 2003 work showed that channelrhodopsin-2 functions in cultured human and hamster cells, and in 2005 Nagel and collaborators showed that switching it on with light in the tiny worm C. elegans changed the worms’ behavior.

Deisseroth and his team at Stanford turned this into a broadly useful technology. In 2005 they showed that channelrhodopsin-2 works in mammalian neurons: blue light made rat nerve cells fire on command, with millisecond precision. In the years that followed, his lab and others found or engineered many more light-sensitive proteins, tuned to different colors of light, including ones that move negatively charged chloride ions into the cell. Those let researchers silence nerve cells as well as activate them.

Deisseroth’s group also tackled the engineering side, developing compact light sources and thin, flexible optical fibers that could deliver light into the brains of animals that were free to move around and behave relatively normally. Working with other researchers, he used the approach to wake sleeping mice on cue. The result is a light-operated switch that can turn chosen nerve cells on or off in a living brain.

The work has already had real-world medical impact, opening up a field known as optogenetic therapy. In one trial, published in 2021, it was used to partially restore the sight of a man who had gone blind from retinitis pigmentosa, a disease that destroys the light-sensitive cells of the retina.

Again: all of this began with the question of how a bit of pond scum swims toward the light it needs for photosynthesis.

This is drawn heavily from: Ars Technica; Nobel Prize press release