Photosynthesis in motion
In the shallow bays of Lake Akan, a body of water within the Akan Caldera, lives a peculiar life-form: nearly perfect spheres of green algae that actively roll themselves. These are Marimo, a rare growth form of the filamentous alga Aegagropila linnaei. While the alga itself is found in other Northern Hemisphere lakes, only in a few locations like Lake Akan in Japan and Lake Mývatn in Iceland do they form these dense, velvety balls.
The motion that puzzled scientists for years is a direct result of photosynthesis. As the algae absorb sunlight, they produce oxygen bubbles that get trapped within their fine, thread-like filaments. This increases their buoyancy, causing parts of the sphere to lift off the lakebed. The gentle, wind-driven water currents then push on the buoyant section, causing the entire ball to slowly rotate. This ensures that all surfaces of the alga receive sunlight over time, maintaining its spherical shape and health. The process is so regular that the balls tend to rise during the day when photosynthesis is active and sink back to the lakebed at night.
These algal balls grow incredibly slowly, at a rate of only about 5 millimeters per year. In Lake Akan, some specimens have reached diameters of 20 to 30 centimeters (8 to 12 inches), making them centuries-old organisms. Their structure is complex; larger marimo often develop a hollow cavity as the filaments in the dark interior decompose, recycling nutrients for the living outer layers.
A cultural and conservation treasure
The marimo of Lake Akan are designated as a Special Natural Monument in Japan and have been a protected species since 1921. Their populations have faced significant threats, including water pollution and habitat disruption from logging and the construction of a hydroelectric power plant in the 1920s, which lowered the lake's water level. Climate change now presents a new challenge, as rising lake temperatures threaten their survival.
Marimo hold a special place in Japanese culture, particularly for the indigenous Ainu people. To raise awareness for conservation, the Ainu began the Marimo Matsuri (Marimo Festival) in 1950. This annual three-day event, held in early October, involves ceremonies where marimo are symbolically welcomed from the lake, honored with traditional dances, and then carefully returned to the water by Ainu elders in wooden canoes. The festival shows the connection between the people and the unique ecology of Lake Akan.
Visitors can observe these unique organisms without disturbing their habitat. A sightseeing boat on Lake Akan takes passengers to the Marimo Exhibition and Observation Center on Churui Island, where tanks replicate the lakebed environment and display marimo of various sizes.