Researchers Discover Age Rings on Marimo

The marimo in the photo on the right has a diameter of approximately 20cm. Photographs adapted with permission from Yoichi Oyama.

A group of Japanese researchers from Kobe University’s Graduate School of Engineering have discovered how to estimate the age of marimo. They also successfully illuminated a mechanism that is vital for the algae balls’ growth. These findings will be useful for maintaining marimo populations.

Marimo (Aegagropila linnaei) is a species of freshwater green algae in the Pithophoraceae family. They are widely distributed in lakes and other bodies of water at high latitudes in the northern hemisphere. Marimo are also known as “lake balls” or “algae balls” because they form into almost true spherical shapes.

Marimo balls are clusters made up of countless thin algae (each around 3cm long). These algae form into balls as they rotate to absorb the necessary sunlight for photosynthesis through their surface layers. This rotation has been confirmed by research, yet many aspects of marimo, such as their growth rate, nutrient supply system and the physical factors underlying their formation, were not fully understood.

There are only two known places in the world where marimo grow into spheres over 10cm in diameter: Lake Mývatn in Iceland and Lake Akan in Hokkaido, Japan. However, it was reported in 2013 that the majority of marimo had disappeared from Lake Mývatn due the effects of eutrophication and other factors.

Research methodology and findings

Using MRI, the researchers were able to see inside the marimo without dissecting them. They discovered that marimo have annual growth rings — much like trees — which enabled them to estimate their age based on ring width. Each growth ring is between 4.5mm to 6.3mm wide, and by converting this into diameter, it was revealed that marimo grew approximately 9mm to 12.6mm each year. In other words, marimo start to form into spheres at 5cm and it takes between 20 to 28 years for them to become a so-called giant marimo (which are 30cm in diameter).

Marimo are rotated and oscillated by wind-driven currents when lakes aren’t frozen, which polishes their surfaces. When the lake is frozen, the marimo hardly move and their appearance becomes ruffled and unkempt.

How the nutrient cycle operates has remained a mystery until the research team showed that due to low oxygen concentration on the interior of the balls, the interior strands become separated and die. Their decomposition slowly releases nutrients into the surrounding water. The outer strands both take up the nutrients and perform photosynthesis. Between the two methods they receive adequate nutrition. The cycle occurs approximately every 105 hours and researchers believe this could be the smallest scale nutrient cycle on Earth.

Sadly, the delicate balance that maintains the sphere formation and nutrient cycle is in peril due to global warming and other factors. Research in a more detailed physical environment will contribute towards future efforts to maintain their habitats.

The research group consisted of researchers from Kobe University’s Graduate School of Engineering (Professor NAKAYAMA Keisuke, Professor KUMAMOTO Etsuko and PhD. student OGATA Keisuke), Kushiro Board of Education’s Marimo Research Center (Dr. OYAMA Yoichi), Kitami Institute of Technology’s Faculty of Engineering (Professor KOMAI Katsuaki), Kobe University Hospital’s Center of Radiology and Radiation Oncology (Mr. HORII Shintarou and Mr. SOMIYA Yuichiro), Nishimuragumi Co. Ltd. (Mr. YAMADA Toshiro) and Housui Engineering Consultants Co. Ltd. (Dr. SATO Yukinobu and Mr. SANO Fumiya). The research results were first published in Scientific Reports on November 11, 2021.

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