Moss balls, lake balls, Cladophora balls, marimo: Whatever you call them, they’re strange — and they’re beautiful.
These mysterious green orbs occupy cool lakes in the Northern Hemisphere in places like Scotland, Iceland and Ukraine. In Japan, they are a protected species and official national treasure.
Under certain conditions, marimo form when a type of algae bind together and roll in the currents in the bottom of shallow lakes, collecting in large colonies. Sometimes the orbs — which can grow to nearly a foot in diameter — rise to the surface during the day, and then sink back down at night.
Marimo contain many mysteries. But now, at least one has been solved. In a paper published this week in Current Biology, biologists at the University of Bristol in England described how this fuzzy freshwater friend floats and sinks. The algae regulate photosynthesis with a biological clock, and in the process, release oxygen bubbles that carry them toward the sun.
Carl Linnaeus first collected samples of these weird spheres in Sweden in 1753. But they weren’t called “marimo” until Takuya Kawakami, another botanist, found them along Lake Akan in Hokkaido, Japan, more than a century later. Now they are a protected national treasure, and every October (this year Oct. 8 to 10), Japan’s Ainu people host a festival in which they first welcome the marimo to Lake Akan, collect them and then return them in a sacred ceremony.
Collecting them from Lake Akan is otherwise prohibited, but some keep marimo, man-made or collected elsewhere, as aquarium pets — which is how they captured the attention of Dora Cano-Ramirez, the lead author of the paper.
After purchasing her own marimo, she noticed that bubbles covered them at some times of day and not others. And during the day, a few moved to the top of her tank. Because she studied how biological clocks control photosynthesis in land plants, she wondered if the same kinds of processes might be at play in her light-harvesting algae balls. She took her idea to Antony Dodd, a biologist and head of her lab.
“The rationale behind the work was a little whimsical,” Dr. Dodd said. “But in science sometimes you get interested in things, and if you have a chance, it’s worth pursuing them just to find out more about the world.”
They put the marimo in water with a chemical that stops photosynthesis. The oxygen bubbles disappeared and the balls didn’t float. But why did they float during the day — did light trigger them, or were they following an internal clock?
To find out, they trained the algae to sync their clocks in a fake night and day environment, where light switched on and off every twelve hours. Then the researchers put the marimo in a room too dim for them to photosynthesize and make bubbles, but just bright enough to keep their newly synchronized internal clocks ticking.
When the researchers stimulated the marimo with a brighter light and measured how quickly they photosynthesized, they found that the marimo were quicker to photosynthesize at their dawn than their midday. This suggested that the marimo used an internal clock and light to rise toward the sun, kind of like how a sunflower follows it on land.
If land plants use biological clocks, and some aquatic creatures that live symbiotically with sun-loving algae try to maximize light exposure, Dr. Dodd said it’s possible that floating may allow marimo to capture more light. Sinking may also protect them from cold surface water or ice, he said. Right now it’s unclear.
In the meantime, the researchers hope to bring attention to this natural enigma, which is disappearing worldwide. In the past 30 years, the algae have been missing from half the lakes where they were once spotted. The decline may be caused by the aquarium trade, tourism or pollution and agricultural runoff blocking the light that marimo need to survive.