Abstract
Aegagropila linnaei is a freshwater green alga, which at one time was distributed widely in the northern hemisphere. The aggregate often forms beautiful spherical shapes known as “lake balls” or “Marimo”. The population of Marimo has been rapidly decreasing worldwide, and today the large Marimo, with a diameter of more than 12 cm, exit only in Lake Akan in Japan. However, how Marimo grow and maintain their unique spherical shape in natural habitats remains unsolved. Here we show that Marimo are “polished” into spheres by the rotation induced by wind waves. Such a process enhances the water exchange between the interior and exterior of the Marimo, thereby recycling nutrients for growth. Our results provide an intriguing model of a physical environment interacting with biological processes in a self-sustaining ecosystem. We also demonstrate that Marimo have a spherical annual ring structure, and their growth rate is associated with ice cover. The balance between the ecology of Marimo and the water environment in Lake Akan is highly vulnerable and at risk of irreversible degradation. We must endeavor to rescue Marimo from the fate of a "canary in the coal mine" of global climate change.
Highlights
Aegagropila linnaei is a freshwater green alga, which at one time was distributed widely in the northern hemisphere
An investigation of A. linnaei in the Netherlands concluded that the species had disappeared in all regions of that country due to e utrophication[9], and another study reported that large Marimo (10–12 cm in diameter or more) were found only in Lake Mývatn in Iceland and Lake Akan in Japan in 2 0047
For magnetic resonance imaging (MRI) analysis of the void ratio, we placed Marimo into a polyethylene bag filled with water
Summary
Aegagropila linnaei is a freshwater green alga, which at one time was distributed widely in the northern hemisphere. An investigation of A. linnaei in the Netherlands concluded that the species had disappeared in all regions of that country due to e utrophication[9], and another study reported that large Marimo (10–12 cm in diameter or more) were found only in Lake Mývatn in Iceland and Lake Akan in Japan in 2 0047. By analyzing these video images and the accompanying meteorological data, our research team was able to verify that wind waves rotate Marimo[14,25]. We identified an additional phenomenon occurring in this species: the oscillational motion of wind waves moves the giant
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