Marimo have age rings! ~Algae balls’ growth depends on the world’s smallest scale nutrient cycle~

A group of Japanese researchers 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.

The 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.

Main Points

• Using MRI, the researchers discovered that marimo have annual growthrings similar to trees, which enabled them to successfully estimate their age based on the width of the rings.
• This research showed that marimo growth is sustained by what could be the smallest scale nutrient cycle on Earth.

Research Background
Marimo (Aegagropila linnaei) is a species of freshwater green algae in the Pithophoraceae family (Figure 1). 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. 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.

Lake Akan marimo have been exposed to various environmental changes from the first half of the 20^th century until recently, including logging, hydropower plant development and tourists visiting the surrounding area. Previously there were four marimo colonies but now only two remain due to sediment inflow, reductions in the lake’s water level and eutrophication resulting from the aforementioned environmental changes.

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

Research Methodology and Findings
In order to properly investigate the inner structure of marimo, it is necessary to perform a non-destructive experiment that avoids cutting or dissecting them. Consequently, the researchers decided to use magnetic resonance imaging (MRI), which is normally used to examine the human body, to investigate the inside of marimo.

It is known that a cavity forms inside marimo balls because the areas farthest from the surface of the ball cannot perform photosynthesis. Using the MRI, the researchers confirmed that there are cavities located 4cm~5cm below the surface (Figure 2). In addition, the researchers discovered that there were growth rings in the area 4cm~5cm from the surface of every marimo. These are similar to the annual growth rings seen on tree stumps.

It is thought that marimo are rotated and oscillated by wind-driven currents during the seasons when the water isn’t frozen, which polishes their surfaces. When the lake is frozen, the marimo hardly move and their appearance becomes ruffled and unkempt. Therefore, it can be said that marimo growth rings are a record of their age, similarly to trees’ growth rings. 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).

The streams that flow into the marimo habitats do not contain a high level of nutrients, yet aquatic plants that require many nutrients grow nearby. Therefore, there is an unanswered question as to how marimo absorb nutrients. The research group illuminated one answer: the important role played by the small detached clusters inside the marimo (which appear as white spots on the MRI image in Figure 2).

These clusters of detached, thin algae strands inside marimo enable it to recycle nutrients. Oxygen concentration is low in the submerged interior of the marimo, and these conditions cause these thin strands to become separated. These interior fragments decompose and elute nutrients. This research showed that the water inside and outside the marimo is exchanged approximately every 105 hours, causing the nutrients inside the marimo to be slowly released into the surrounding water (Figure 3). The marimo continue to grow by reusing these nutrients and performing photosynthesis, and these two methods give them an adequate balance of nutrients. Another example of a nutrient cycle is when fish species such as salmon and trout bring nutrients from the sea to the land when they swim upstream, and rainwater in turn transports nutrients from the land to the sea. Incidentally, Shiretoko (which is also in Hokkaido) was designated a world heritage site because this nutrient exchange cycle between land and sea occurs there. The nutrient exchange between the marimo’s interior and exterior revealed by this study could be the smallest scale nutrient cycle on Earth.

Further Developments
Previous research has shown that the rotation and oscillation of marimo by wind-driven currents removes photosynthesis-inhibiting matter stuck to their surface, thus allowing them to grow, and that this movement polishes the surface, forming them into sphere-like structures. In addition, the current research study has illuminated their growth rate and nutrient supply mechanism. However, the delicate balance that maintains the sphere formation and nutrient cycle may be lost due to global warming and other factors in the future. Researching the formation process of spherical marimo in a more detailed physical environment will contribute towards future efforts to maintain their habitats.

Acknowledgements
This research was supported by the Japan Society for the Promotion of Science (grant number 18H01545).