Huge ‘marimo’ moss balls at risk of deadly winter sunburn

Aegagropila linnaei algae can live as free-floating fibers, grow on rocks, transform into their signature ball shape, and form flat balls when crushed, depending on their environment. Ball growth is slow at about 5 millimeters per year and they can live for centuries. Credit: 2022 Yoichi Oyama

Climate change can kill rare underwater “marimo” algae balls by overexposing them to sunlight, according to a new study from the University of Tokyo. Marimo are living fluffy balls of green algae. The world’s largest marimo can be found in Lake Akan on Japan’s northern main island of Hokkaido.

Here they are shielded from a lot of winter sunlight by a thick layer of ice and snow, but due to global warming the ice is getting thinner. The researchers found that the algae can survive in bright light for up to four hours and then recover if kept in moderate light for 30 minutes. But the algae died when exposed to bright light for six hours or more. The team hopes this discovery will highlight the climate change threat to this endangered species and the urgent need to protect their habitats.

Some people have pet cats, others have pet rocks, but what about pet moss? Marimo are fluffy, fluffy green balls of underwater algae that have become popular with tourists, nature lovers and aquarium owners. They range in size from peas to basketballs and are formed naturally when floating filaments of Aegagropila linnaei algae come together with the gentle rolling action of lake water.

They are found in only a few countries, and the largest marimo found in Lake Akan can grow up to 30 centimeters. They are so popular in Japan that they have their own annual festival, merchandise, and even a mascot. However, the marimo is an endangered species and their numbers are generally declining globally.

Marimo relies on nutrients and photosynthesis to survive. Their decline is often attributed to human intervention altering or polluting the freshwater lakes they inhabit. However, not much research has been done on the impact of changing access to sunlight.

“We know Marimo can survive in warm summer waters in bright sunlight, but marimo’s photosynthetic properties at low winter temperatures have not been studied, so we were fascinated by this point,” said Project Assistant Professor Masaru Kono from the Graduate School of Natural and Applied Sciences. at the University of Tokyo. “We wanted to find out if Marimo can tolerate it and how they respond to a low-temperature, high-light-intensity environment.”


Too much sun cannot be processed in cold weather and instead results in the formation of harmful, reactive chemicals. This harms the marimo’s ability to photosynthesize and repair itself. Credit: 2022 Akina Obara

Kono and his team visited Churui Bay of Akan Lake in winter to measure temperature and light intensity both under ice-covered and ice-free water. First, they drilled a small hole in the ice 80 meters offshore, and then carved a large 2.5-by-2.5-metre square to take the readings. They also carefully hand-picked several marimo balls the size of a shot put (10-15 cm).

Back in Tokyo, the team recreated the environmental conditions using an ice machine and ice trays made with white LED lights. Algae strips were extracted from marimo balls and tested for their normal photosynthetic abilities. They were then placed in containers on ice under artificial light set to glow at different intensities for different times.

“We show a new finding that damaged cells in Marimo are able to repair themselves even after exposure to simulated strong daylight for up to four hours at cold temperatures (2-4 degrees Celsius) and then only after 30 minutes of moderate light exposure. However, when exposed to strong sunlight for six hours or more, certain cells involved in photosynthesis were damaged and the algae died even after treatment with moderate light,” explains Kono.

“These results suggest that photoinhibition (inability to photosynthesize due to cell damage) in Lake Akan, which receives more than 10 hours of sunlight per day in winter, will pose a serious threat to marimo if global warming progresses and ice cover decreases.”


A researcher takes samples of seaweed fibers from a marimo ball. The ball consists entirely of green algae and there is no other substance in its core. Credit: 2022 Akina Obara

Next, the team wants to know what will happen to all the marimo balls and whether the result will be the same with smaller strings.

“In the current study, we used fragmented filamentous cells, so we did not consider the effects of the spherical marimo structure and how it might protect against exposure to bright light. However, if the damage to the surface cells increases with longer exposure to direct sunlight, in an extreme case, this will prevent the maintenance of their rounded bodies. “It could affect the population and cause the disappearance of giant marimos. That’s why we need to constantly monitor conditions in Lake Akan in the future.”


Thanks to the ice and snow cover, temperatures underwater are kept relatively constant, around 1-4 degrees Celsius. But above ground, they range from minus 18 degrees to 1 degree Celsius. Credit: 2022 Asami Fujita

Kono hopes this research will help both local and national governments understand the urgent need to protect Japan’s unique marimos and their habitats. “We also hope this will be an opportunity for all people to think seriously about the effects of global warming.”

Paper published International Journal of Molecular Sciences.

More information:
Akina Obara et al, Effects of High Irradiance and Low Water Temperature on Photoinhibition and Repair of Photosystems in Marimo (Aegagropila linnaei) in Lake Akan, Japan, International Journal of Molecular Sciences (2022). DOI: 10.3390/ijms24010060

Provided by the University of Tokyo

Quotation: Huge ‘marimo’ moss balls at risk of fatal winter sunburn, retrieved from on December 23, 2022 (2022, 23 December)

This document is subject to copyright. No part may be reproduced without written permission, except in fair trade for personal study or research purposes. The content is for informational purposes only.

Leave a Reply

Your email address will not be published. Required fields are marked *