Arctic Microalgae Thrive in Near-Total Darkness, Defying Traditional Beliefs

Sunlight is a fundamental energy source for most life forms on Earth, providing the necessary photons for photosynthesis, a process whereby plants and algae convert light into chemical energy. This energy fuels various ecological interactions, sustaining a complex food web.

While abundant light is available during sunny days, scientists have long pondered the minimum light levels required for photosynthesis to occur. Previous calculations estimated that photosynthesis could theoretically function on as little as 0.01 micromoles of photons per square meter per second, a minuscule fraction compared to the light available on a bright day.

For years, this theory remained untested in real-world conditions, particularly in regions where light is scarce. In the Arctic, for example, prolonged periods of winter darkness make it difficult to study the effects of low light on photosynthesis. The assumption was that microalgae, which typically thrive in brighter conditions, would cease to function when the sun disappeared for months.

However, a recent study challenges this perception. A research team conducted an expedition during the polar night, revealing that some microalgae can continue to grow and reproduce even under extremely low light conditions. The findings were published in the journal Nature Communications, suggesting that these organisms can remain active when previously thought impossible.

Historically, scientists viewed the Arctic winter as a dormant period for marine life, with many organisms either migrating to warmer areas or hibernating until conditions improved. The research led by a biogeochemist sought to uncover the truth about life in the dark depths of the polar ocean. Previous expeditions had already shown unexpected activity in certain organisms, including clams, during the winter months.

During the 2020 polar night expedition, researchers inhabited the RV Polarstern, an icebreaker ship that drifted through the Arctic ice, allowing them to gather extensive data on the ecosystem's winter dynamics. They collected seawater samples from a designated site, referred to as Ocean City, and analyzed the microalgae present.

The team conducted experiments to observe the microalgae's photosynthetic abilities under controlled conditions, simulating the low light levels typical of the polar night. Their observations indicated that the microalgae had the potential to perform photosynthesis at light levels close to theoretical minimums, thereby demonstrating a remarkable adaptation to their harsh environment.

As spring approached and light began to return, the researchers noted a significant increase in the microalgae's chlorophyll levels and overall biomass. This uptick coincided with the first rays of sunlight, suggesting that these organisms were primed to respond to the return of light more quickly than anticipated.

Additional measurements taken by physicists on the same expedition confirmed that very few photons penetrated the ice during the darkest months. Despite this, the microalgae appeared to remain metabolically active, able to utilize minimal energy to survive until conditions improved.

The researchers propose that this ability to endure and even thrive in low light may be crucial for the Arctic ecosystem, enabling certain organisms to jumpstart the food web when sunlight returns in the spring.

These findings could have broader implications, suggesting that similar mechanisms may exist for phytoplankton in other dark ocean regions, potentially expanding our understanding of marine life in extreme conditions. The ability of these organisms to persist during prolonged darkness could redefine assumptions about their life cycles and interactions within the ecosystem.