06/10/2026
How did an alga that senses light lead to this year’s Nobel Prize in Physiology or Medicine?
In the early 1990s, Peter Hegemann – awarded the 2026 medicine prize – asked a simple question: How can the unicellular alga Chlamydomonas react so rapidly to light?
Chlamydomonas is famous for its ability to swim towards light. When researchers stir this green alga into a petri dish, the liquid turns a faint green colour. If they then illuminate one side of the dish, the green colour shifts towards the light source, as the microscopic algae swim towards it (see figure). The algae sense the light with an eyespot, a tiny orange dot on the cell’s surface that contains a light-capturing molecule, retinal.
Hegemann investigated the algal eyespot. Using minuscule electrodes, he measured the electrical signals that arose in Chlamydomonas when the eyespot was illuminated. The algae reacted incredibly quickly. Just half a millisecond after light reached the eyespot, an electrical impulse occurred.
Compared to other biological systems that researchers knew of at the time, this was remarkably fast. For example, light reaching a human eye triggers a complex chemical chain reaction that involves many different steps. The final protein in this chain – an ion channel – forms an opening in the cell surface through which charged ions can flow, creating an electrical signal. The entire process takes at least 10 milliseconds. So how could the response to light be more than twenty times faster in Chlamydomonas than in our own eyes?
Hegemann could only think of one explanation for this ultrafast response: the process must be much simpler in Chlamydomonas than in a human eye. In the early 1990s, he formulated the hypothesis that a single protein complex both captured light and acted as an ion channel. However, his proposal was met with scepticism. At the time, researchers knew of many different types of ion channels, but none of them could react to light on their own.
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