Arctic microalgae photosynthesize in near-darkness, study finds

Arctic microalgae photosynthesize in near-darkness, study finds
by Robert Schreiber
Berlin, Germany (SPX) Sep 05, 2024

Photosynthesis can occur even under extremely low light conditions, according to an international study that examined Arctic microalgae's development at the conclusion of the polar night. Conducted during the MOSAiC expedition at 88 degrees northern latitude, the study found that microalgae can build up biomass through photosynthesis as early as the end of March, despite the sun barely rising above the horizon. During this time, the microalgae's environment under the snow and ice of the Arctic Ocean remains almost completely dark. The study, now published in 'Nature Communications', reveals that photosynthesis in the ocean is possible under much lower light conditions and can occur at much greater depths than previously believed.

Photosynthesis, the process of converting sunlight into biologically usable energy, is fundamental to all life on Earth. However, previous measurements of the light needed for this process have been significantly higher than the theoretically possible minimum. The new study in 'Nature Communications' demonstrates that biomass accumulation can occur with light levels close to this minimum.

The research team utilized data from the international MOSAiC research project, which involved freezing the German research icebreaker 'Polarstern' in the central Arctic's icepack for a year in 2019 to study the annual cycle of the Arctic climate and ecosystem. Led by Dr. Clara Hoppe from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the team focused on phytoplankton and ice algae, which are responsible for the majority of photosynthesis in the central Arctic. Remarkably, just a few days after the end of the polar night, plant biomass began to accumulate again, a process dependent on photosynthesis. The presence of extremely sensitive light sensors in the ice and water allowed the researchers to measure the minimal light available.

The findings were unexpected because photosynthesis in the Arctic Ocean occurred under snow-covered sea ice, where only a tiny fraction of sunlight penetrated. The microalgae had access to only about one hundred-thousandth of the light available on a sunny day at the Earth's surface. "It is very impressive to see how efficiently the algae can utilize such low amounts of light. This shows once again how well organisms are adapted to their environment," said Clara Hoppe.

The study was made possible through close collaboration among researchers from various disciplines. Sea ice researchers Dr. Niels Fuchs and Prof. Dirk Notz from the Institute of Marine Research at the University of Hamburg played key roles by combining measurements of the light field with biological data. "To measure such low light levels under the harsh conditions of the Arctic winter, we had to freeze special, newly developed instruments into the ice in the middle of the polar night," explained Niels Fuchs. Dirk Notz added, "It was particularly difficult to account for irregularities in the light field under the ice due to variations in ice thickness and snow, but in the end, we could be sure: There was just not more light."

The study's results have global significance. "Even though our results are specific to the Arctic Ocean, they show what photosynthesis is capable of. If it is so efficient under the challenging conditions of the Arctic, we can assume that organisms in other regions of the oceans have also adapted so well," said Clara Hoppe. This suggests that there might be sufficient light in deeper ocean regions to support photosynthesis, producing energy and oxygen, which would be beneficial to marine life such as fish. Consequently, the photosynthetic habitat in the global ocean could be much larger than previously assumed.

Research Report:Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae