Photosynthesis can take place in nature even at extremely low light levels, a study that investigated the development of Arctic microalgae at the end of the polar night shows. The measurements were carried out as part of the MOSAiC expedition and revealed that microalgae can build up biomass through photosynthesis in the Arctic Ocean as early as the end of March, when the sun is barely above the horizon. The results now published in Nature Communications show that photosynthesis in the ocean is possible under much lower light conditions, and can therefore take place at much greater depths, than previously assumed.