Arctic microalgae show photosynthesis is possible in near darkness

In nature, photosynthesis can take place even when light levels are extremely low. This is the result of an international study that investigated the development of Arctic microalgae at the end of the polar night. The measurements were carried out as part of the MOSAiC expedition at 88° north latitude and revealed that even at this distance, microalgae can accumulate biomass through photosynthesis as early as the end of March.

At this time, the sun is barely above the horizon, so it is still almost completely dark in the microalgae habitat under the snow and ice cover of the Arctic Ocean. The results of the study are now published in the journal Nature Communications demonstrate that photosynthesis in the ocean is possible under much lower light conditions and can therefore take place at much greater depths than previously thought.

Photosynthesis converts sunlight into biologically usable energy and thus forms the basis of all life on our planet. However, measurements so far of the amount of light required for this transformation have always been well above the theoretically possible minimum. The study shows that biomass formation can indeed take place with an amount of light close to this minimum.

The researchers used data from the international research project MOSAiC for their work. As part of the expedition, they froze the German research icebreaker Polarstern in the ice floes of the central Arctic for a year in 2019 to study the annual cycle of the Arctic climate and ecosystem. The team led by Dr. Clara Hoppe from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), focused on studying phytoplankton and ice algae.

These are responsible for most of the photosynthesis in the central Arctic. Unexpectedly, the measurements showed that just a few days after the end of the month-long polar night, the plant biomass had been rebuilt, which is absolutely essential for photosynthesis. Highly sensitive light sensors placed in the ice and water made it possible to measure the amount of light available.

The results are particularly surprising because photosynthesis in the Arctic Ocean takes place under snow-covered sea ice, which allows only a few photons of incident light to pass through: the microalgae had only about one hundred thousandth of the amount of light available to them for growth as in a sunny day on the Earth’s surface.

“It is very impressive how efficiently algae can exploit such small amounts of light. This shows once again how well adapted the organisms are to their environment,” says Hoppe.

The study was made possible by a close collaboration between researchers from different disciplines. Sea ice researchers Dr. Niels Fuchs and Professor Dirk Notz from the Institute for Marine Research at the University of Hamburg were tasked with combining the light field measurements with the biological measurements.

“To measure such low light levels in 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,” says Fuchs. His colleague Dirk Notz adds that it was particularly difficult to take into account the irregularities in the light field under the ice due to the varying thickness of the ice and snow. “But in the end, we were able to be sure that there was simply no more light.”

The results of the study are important for the entire planet. “Although our results relate specifically to the Arctic Ocean, they show what photosynthesis is capable of. If it is so efficient in the harsh conditions of the Arctic, we can assume that organisms in other regions of the oceans have also adapted well there,” says Hoppe.

This means that in deeper areas of the oceans there could be enough light to produce usable energy and oxygen through photosynthesis, which would then be available to fish, for example. The corresponding photosynthetic habitat in the global ocean could therefore be significantly larger than previously thought.