A single cell's siesta: How non-moving single-celled organisms manage to avoid bright light

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Too much of a good thing is no good at all. Living organisms enjoy sunlight—in fact, they need it to stay alive—but they tend to avoid light that is too bright. Animals go to their shelter, humans have a siesta, even plants have mechanisms to avoid an overdose of light. But how do non-moving single-celled organisms deal with light that is too intense? Researchers at the University of Amsterdam have discovered the surprising answer.

Its full scientific name is Pyrocystis lunula. You may never have heard of this single-celled alga, but sailors and fishermen know its effect very well: the P. lunula algae are the organisms that occasionally make the sea glow blue. P. lunula is an example of a dinoflagellate—a single-celled organism that cannot move on its own. Its main source of energy is sunlight: similar to plants, it uses a structure known as a chloroplast to convert energy from sunlight into useable, chemical energy.

The plants around us use a clever strategy when they are exposed to light that is too bright: Their chloroplasts rearrange within their cells and collectively cover one another, in such a way that only the necessary amount of light is absorbed and damage to the cells is prevented.

P. lunula cannot use that same strategy: it has chloroplasts organized in the form of a complex network, necessitating a different form of motion to avoid bright light. Moreover, the alga cannot easily move away from the light like animals and humans can. How these organisms do manage to deal with excessive amounts of light was a scientific mystery. A mystery that has now been solved.

A flexible chloroplast

Biophysicists Nico Schramma, Gloria Casas Canales and Maziyar Jalaal devised a clever way to study what exactly happens to the chloroplast of P. lunula when it is exposed to light. Using microscopy, they captured movies of the cell and its chloroplast and then fitted a network of nodes and edges to its complex shape using computer algorithms. Doing this under conditions of changing light color and intensity, they could follow exactly what the cell's chloroplast was doing.

What the researchers found was that while the chloroplast cannot escape intense light, it can minimize its effect by shrinking. When exposed to bright white light—essentially the light of a sunny afternoon—the cell's chloroplast shrunk to a ball, reducing its size by about 40% within five minutes. When the light conditions were changed to dim red light, within half an hour the chloroplast had returned to its original size and shape.

The structure that allows the chloroplast to make these necessary changes was found to be a network of thin filaments. Together, these filaments form a material that can easily contract and expand in all directions.

The key point is the "in all directions"—most structures that we find in nature do not have this property. Step on a lemon, and while its height will dramatically decrease, its size will increase in the other directions, turning it into a disk-shaped object that still has a considerable surface area. P. lunula manages to avoid this natural behavior.

The research is published in the Proceedings of the National Academy of Sciences.

Nature's Hoberman sphere

The structure that allows the chloroplast to decrease in size in all directions is somewhat similar to that of a Hoberman sphere—a design patented by Chuck Hoberman in 1988 and used in popular children's toys. This observation connects the research of the physicists not only to biology but also to mathematics—more precisely, the branch of mathematics known as topology—and to materials design.

Lab-made materials that have exactly the surprising properties that the Hoberman sphere and P. lunula's chloroplast show, have been intensively studied recently with all sorts of applications in mind—for instance, as "smart materials" that significantly change their properties when experiencing external stimuli. Surprisingly, the clever solutions that engineers and physicists come up with in the lab, turn out to be out there in living nature.

When a single scientific question is answered, sometimes many other answers and discoveries follow. This may well be the case for the question of how P. lunula and other dinoflagellates manage to avoid bright light.

Its answer not only tells us more about this tiny one-celled organism that occasionally makes the sea glow blue—it teaches us about structures in nature, about how they apply intricate mathematics, and teaches us valuable lessons that we can apply when designing our own new materials.

More information: Nico Schramma et al, Light-regulated chloroplast morphodynamics in a single-celled dinoflagellate, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2411725121

Journal information: Proceedings of the National Academy of Sciences

Provided by University of Amsterdam