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Marimo algae balls threatened by climate change

Marimo are fluffy, squishy green balls of underwater algae that can be found in just a few countries and have recently become highly popular with tourists, aquarium owners, and nature enthusiasts. They range in size from a pea to a basketball, and form when floating strands of Aegagropila linnaei are bundled together through the rolling motion of lake waters. In Japan, they have become so popular that they have their own yearly festival, merchandise, and even a mascot.

Unfortunately, recent years have witnessed significant declines in their population, mainly due to human intervention altering or polluting their freshwater habitats. However, according to a new study led by the University of Tokyo, climate change could also play a significant role in their decline, by overexposing them to sunlight.

“We know that marimo can survive bright sunlight in warm summer waters, but the photosynthetic properties in marimo at low winter temperatures have not been studied, so we were fascinated by this point,” said study senior author Masaru Kono, a biologist at the University of Tokyo. “We wanted to find out whether marimo could tolerate it and how they respond to a low-temperature, high light-intensity environment.”  

By analyzing how exposure to extreme light affected samples of marimo algae collected from Lake Akan in Japan, the researchers found that they could survive bright light for up to four hours and would recover if placed afterwards under a moderate light for half an hour. However, when exposed to bright light for six hours or more, the algae died.

“We demonstrated that damaged cells in marimo can repair themselves even after exposure to simulated strong daylight for up to four hours at cold temperatures (two-four degrees Celsius), when followed by moderate light exposure for just 30 minutes. This moderate light had a restorative effect which did not occur in the dark,” Kono explained. 

“However, when exposed to strong daylight for six hours or more, certain cells involved in photosynthesis were damaged and the algae died, even after being treated with moderate light. These results suggest that photoinhibition (the inability to photosynthesize due to cell damage) would be a serious threat to marimo in Lake Akan, which receives more than 10 hours of sunlight a day in winter, if global warming proceeds and ice cover recedes.”

In future studies, the scientists aim to investigate what would happen to whole marimo balls if exposed to bright light for an extended period of time, and whether the outcome would be similar to that observed in the smaller threads. “In the present study, we used dissected filamentous cells, so we did not consider the effects of the structure of the spherical marimo and how it might protect against exposure to bright light. However, if damage to the surface cells increases under longer exposure to the direct sunlight, in an extreme case, this may affect the maintenance of their round bodies and lead to the disappearance of giant marimo. So, we need to constantly monitor the conditions at Lake Akan in the future,” Kono concluded.

The study is published in the International Journal of Molecular Sciences.

By Andrei Ionescu, Staff Writer

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