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Arctic microalgae photosynthesis in darkness at theoretical minimum

NEWS - Microalgae require only one hundred thousandth of the amount of light available on Earth's surface for their growth. Photosynthesis can occur in nature even at very low light levels, allowing algae to build biomass when the sun is barely above the horizon.

Arctic microalgae photosynthesis in darkness at theoretical minimum

An international team of researchers used data from the MOSAiC expedition to investigate algae growth at the end of the polar night in northern latitudes and revealed that microalgae can build biomass through photosynthesis in the near-darkness of habitats beneath the snow and ice of the Arctic Ocean.

Photosynthesis in the Arctic Ocean occurs beneath the snow-covered sea ice, which allows only a few photons of incoming sunlight to pass through, but microalgae only have about one hundred thousandth of the amount of light available on Earth's surface for their growth.

"It is impressive to see how efficiently algae utilize such low amounts of light. This shows how well the organisms are adapted to their environment," says Clara Hoppe from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

Photosynthesis converts sunlight into energy, which is the basis of all life on the planet. However, previous measurements of the amount of light required for this have always been well above the theoretical minimum. The researchers show that biomass build-up can actually occur with light levels close to this minimum.

The research team froze the German icebreaker Polarstern in the central Arctic ice sheet for 12 months in 2019 to investigate the annual cycle of Arctic climate and ecosystems. Hoppe and his team focused on phytoplankton and ice algae, which are responsible for most of the photosynthesis in the central Arctic.

"To measure the very low light levels under the harsh Arctic winter conditions, we had to freeze a newly developed special instrument into the ice in the middle of the polar night," says Niels Fuchs from the University of Hamburg.

The measurements show that just a few days after the end of the month-long polar night, plant biomass builds up again, which is crucial for photosynthesis. Highly sensitive light sensors in the ice and water make it possible to measure the amount of available light.

The study was made possible by close collaboration between researchers from different disciplines to combine light field measurements with biological measurements. It is very difficult to account for irregularities in the light field under ice due to variations in the thickness of ice and snow.

"But in the end we can be sure that there is no more light," says Dirk Notz from the University of Hamburg.

The results of the study are important for the entire planet. Light for the production of energy and oxygen that can be used through photosynthesis in deeper ocean areas may also be available to fish. Suitable photosynthetic habitats in the global ocean could therefore be much larger than previously assumed.

"Although our results are specific to the Arctic Ocean, they show what photosynthesis is capable of. If photosynthesis is so efficient under challenging Arctic conditions, we assume that marine organisms in other regions are also very well adapted," says Choppe.

Original research

Hoppe, C.J.M., Fuchs, N., Notz, D. et al. Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae. Nature Communications 15, 7385 (2024), DOI:10.1038/s41467-024-51636-8

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