The Rigakubu News

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Impact of Global Warming on Marimo

Masaru Kono (Assistant Professor, Kanagawa University) *

Marimo (freshwater filamentous green alga, scientific name: Aegagropila linnaei) has three life forms: an epiphytic form attached to rocks, a planktonic form floating in the water, and an aggregate form in which the filamentous bodies are entangled to form spheres. Lake Akan Chu-Urui Bay in eastern Hokkaido is known for its spherical aggregate-type marimo. Although aggregate-type marimo used to be found growing in many lakes and marshes around the world, many of them have disappeared or declined due to changes in their habitat since the 1900s. Lake Akan is the only lake in the world where giant marimo (up to 30 cm in diameter) can be observed, and is designated as a special natural monument by the Japanese government for its rarity and protection.

The water temperature in Lake Akan drops to 1-4°C in winter, and sunlight rarely reaches the water due to freezing and snow accumulation. In recent years, due to global environmental changes, the duration of Lake Akan's freezing period has been shortening, and there are concerns that this may affect the marimo. If the lake loses its winter freezing period due to global warming, light intensity will increase due to direct sunlight incident on the water, while the temperature of the lake water is expected to remain relatively low due to the loss of insulation effect by the ice formation. Exposure to intense light at low temperatures promotes the production of reactive oxygen species (ROS), which are dangerous to cells. If too many of these ROS are produced, they can damage the photosynthetic system. However, few studies have focused on photosynthesis in marimo, and even the basic photosynthetic dynamics of marimo throughout the year are still unknown. In this study, we sought to elucidate the actual photosynthesis of marsh eelgrass at low temperatures in order to investigate the effects of the loss of ice on the photosynthetic system of marsh eelgrass.

On a clear day in March when Lake Akan was frozen over, we drilled a hole approximately 3 m square in the ice and measured the water temperature and light intensity directly above the marimo colony. Based on these results, we conducted a verification experiment using marimo collected with permission from the Agency for Cultural Affairs. When the marimo were exposed to intense light at a water temperature of 2°C, photosynthesis was easily inhibited after a short period of exposure, but was quickly restored to its original level by subsequent exposure to relatively weak light. This indicates that marimo has an unknown repair mechanism that works at low temperatures, since it has been believed that photosynthesis is not easily repaired at low temperatures when the photosynthetic system is damaged. On the other hand, when the marimo was placed under the pseudo-natural light environment expected in the habitat after the loss of ice, it died.

The results indicate that marimo can tolerate low temperatures and intense light for a certain period of time, but cannot tolerate low temperatures and intense light for a long period of time, which is expected when the ice is lost. This study, which suggests that ice formation is important for the survival of marimo, raises alarm about the effects of global warming on lake organisms.

(a) Marimo in Lake Akan, (b) Lake Akan after the ice formation, (c) images of Lake Akan when protected from strong sunlight by the ice formation and snow cover (left) and when exposed to low temperature and strong light after the ice formation disappeared (right)

This study was published in A. Obara et al. Int. J. Mol. Sci. 24 (1), 60 (2023).

*Former Project Assistant Professor, Department of Biological Sciences (at the time of the research)

(Press release, December 23, 2022)

Published in Faculty of Science News, May 2023

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