Climate change could expose Marimo algae to too much sunlight due to too thin a layer of ice on the surface of the water, which could kill them, according to a new study from the University of Tokyo, Japan.

marimo (Aegagropila linnaei) are green, fluffy, live ball-shaped algae. The largest Marimo in the world are found in Lake Akan in Hokkaido, the main island in northern Japan. Here, the algae are protected, in winter, from the sunlight by the thick layer of ice and snow. The thickness of the ice sheet, however, is thinning due to global warming, according to EurekAlert.

The researchers found that the algae could survive bright light for up to four hours and recover if placed under moderate light for 30 minutes. However, the algae died when exposed to bright light for six hours or more. The research was published in MDPI.

The biggest lump of Marimo in the world and its popularity in Japan

Marimo are green, fluffy clumps of underwater algae that have become popular among nature lovers and aquarium owners. They are between the size of a pea and a basketball and form naturally when floating strands of algae Aegagropila linnaei they are grouped together by the gentle undulating motion of the water.

This form of choice only exists in a few countries, and the largest lump of Marimo, which is found in Lake Akan, can reach up to 30 centimeters in diameter. In Japan, they are so popular that they have their own annual festival and even mascots. However, Marimo are an endangered species and globally their numbers are generally in decline.

Marimo relies on nutrients and photosynthesis to survive. Their decline is usually attributed to human intervention that alters or pollutes the freshwater lakes in which they live.

“We know Marimo can survive bright sunlight in warm summer waters, but their photosynthetic properties in cold winter temperatures have not been studied, so we were fascinated by this. We wanted to find out if Marimo can tolerate it and how they respond to a low-temperature, high-light environment,” said project assistant professor Masaru Kono of the University of Tokyo’s Graduate School of Science.

How long can Marimo algae withstand sunlight?

Kono and his team visited Churui Bay in Lake Akan during the winter to measure the temperature and light intensity underwater, both with and without ice.

They drilled a small hole in the ice 80 meters offshore to take measurements. The researchers also carefully collected by hand several 10-15 cm algae balls.

Back in Tokyo, the team recreated the environmental conditions using ice trays. Algal filaments were removed from the balls and tested for normal photosynthetic capacity. They were then placed in containers on ice under artificial light, which was adjusted to shine at different intensities for different periods of time.

“We demonstrated a new hypothesis that damaged cells in Marimo can repair themselves even after exposure to simulated bright daylight for up to four hours at low temperatures (2-4 degrees Celsius), when followed by exposure to moderate light for just 30 minutes. This moderate light had a restorative effect that did not occur in the dark. 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,” he explained Kono.

Next, the team wants to find out what would happen to the whole Marimo algae balls and whether the result would be the same as with the smaller threads.

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