Spring sunlight triggers photoinhibition and recovery in marimo

The marimo (Aegagropila brownii), a nationally designated Special Natural Monument of Japan, inhabits Lake Akan in Hokkaido, where environmental conditions fluctuate drastically with the seasons. Of particular concern is the period immediately after ice melt in early spring, when low water temperatures coincide with strong sunlight, posing a risk of severe damage to photosynthetic activity.

In this study, a research team led by the Astrobiology Center conducted a detailed assessment of marimo photosynthetic performance during this critical transition period, combining field observations with laboratory experiments. The results revealed that while marimo maintains healthy photosynthetic capacity in both summer and ice-covered winter conditions, their activity significantly declines just after ice melts. However, it was also found that marimo can recover this function over the following 20 to 30 days.

These findings provide valuable insight into the seasonal vulnerability of marimo and highlight the importance of spring as a critical period for conservation. The study was published in the journal Phycological Research on September 28, 2025.

This study, for the first time, reveals, based on field data, that the most vulnerable period for marimo throughout the year is immediately after the ice thaws. Environmental changes during this period could have a significant impact on the long-term survival of marimo.

Marimo (Aegagropila brownii) is a freshwater green alga known for forming beautiful spherical aggregations. In particular, the large marimo found in Lake Akan, Hokkaido, are designated as a Special Natural Monument of Japan and represent a globally unique natural heritage.

During winter, Lake Akan is covered by thick ice and snow, which act like a “sunshade,” protecting marimo from harsh solar radiation. However, during seasonal transitions, which are just before ice formation and immediately after ice melt, the lake bottom experiences a harsh environmental condition characterized by low water temperatures (1–4°C) combined with strong sunlight.

This creates what is known as a low water temperature and high light (LT-HL) environment, which can severely stress photosynthetic organisms. Under such conditions, marimo may suffer “photoinhibition,” a physiological impairment where photosystem II, the protein complex responsible for photosynthesis, is damaged due to excess light energy.

While the risk of photoinhibition under LT-HL conditions has been theoretically suggested, it remained unclear how marimo are actually affected in natural lake environments during these transitional periods. This study aimed to fill that knowledge gap through detailed field observations and laboratory experiments.

The research team conducted seasonal monitoring of water temperature and light conditions in Churui Bay of Lake Akan, along with seasonal sampling of marimo. Their photosynthetic performance was evaluated using a method called PAM chlorophyll fluorescence measurement.

The results showed that marimo collected in summer (August) and in midwinter when the lake was fully ice-covered (March) maintained high photosynthetic activity, with Fv/Fm values—an indicator of photosystem II performance—around 0.6, indicating healthy physiological states.

In contrast, marimo collected immediately after ice melted (early April) exhibited significantly reduced Fv/Fm values, dropping to approximately 0.27 on the sun-exposed surface, clearly demonstrating severe photoinhibition. This suggests that marimo, which had acclimated to the dark conditions under ice, experienced substantial physiological stress due to sudden exposure to intense sunlight after thawing.

Nevertheless, marimo also demonstrated a remarkable capacity for recovery. By early May, 20–30 days after ice melt, Fv/Fm values had rebounded to around 0.55 as water temperatures began to rise. This recovery process was also verified in laboratory experiments, which showed that damaged marimo cells placed under low light began to recover their photosynthetic capacity.

In recent years, due to the effects of climate change, a trend of delayed ice formation and earlier thawing has been reported in Lake Akan. This does not simply mean a longer warm period. Solar radiation is extremely strong on clear days from early spring (January to March), and under normal circumstances, a thick layer of ice and snow protects the marimo. However, if the ice thaws earlier, the marimo are exposed to this intense light for a longer period while water temperatures remain low. This raises concerns that the duration of exposure to the harsh LT-HL environment will be extended, making it easier for light-induced damage to accumulate.

If the recovery from photoinhibition cannot keep pace, the entire marimo population could weaken, potentially jeopardizing its future survival. Furthermore, the ice-covered period itself is a crucial element that supports the entire lake ecosystem, and its shortening or loss is expected to have a profound impact on the interactions among diverse organisms.

To pass this precious natural heritage on to future generations, it is crucial not only to protect the physical habitat but also to establish science-based conservation strategies that focus on the physiological impacts of climate change on marimo, especially the light stress during this vulnerable period. The findings of this research extend beyond the specific biological community of marimo in Lake Akan; they also hold significance as a model case for the universal challenge of how to conserve rare aquatic species facing complex stressors like climate change in other lake ecosystems, both in Japan and internationally.

This research also offers important insights from an astrobiological perspective. Elucidating the life strategies of marimo, which endure and recover from sudden intense light stress upon ice thawing after dwelling in the darkness beneath the ice, provides critical clues for understanding the mechanisms by which life might survive in extreme extraterrestrial environments, such as icy celestial bodies.

Furthermore, investigating the impact of Earth’s climate change on this unique ecosystem offers a valuable case study for considering how life might respond and leave its traces on planets that have experienced dramatic environmental shifts. This study is an attempt to address the fundamental astrobiological question, “How does life survive and evolve in extreme planetary environments?” by examining life forms here on Earth.