Under the sea ice during the Arctic’s pitch-black polar night, cells power photosynthesis on the lowest light levels ever observed in nature.

Most of life’s engines run on sunlight. Photons filter down through the atmosphere and are eagerly absorbed by light-powered organisms such as plants and algae. Through photosynthesis, the particles of light power a cellular reaction that manufactures chemical energy (in the form of sugars), which is then passed around the food web in a complex dance of herbivores, predators, scavengers, decomposers and more.

On a bright, sunny day, there’s a wealth of photons to go around. But what happens at low light? Biologists have long been curious about just how little light photosynthesis can run on — or how many photons need to arrive, and how quickly, for a cell’s photosynthetic machinery to process carbon dioxide into oxygen and energy. Calculations have suggested a theoretical minimum of around 0.01 micromoles of photons per square meter per second, or less than one-hundred-thousandth of the light of a sunny day.

For decades, this calculation was theoretical, given the difficulties of studying photosynthesis under low light. No one could confirm it in the field, though there are plenty of places on Earth that light barely reaches. Every winter in the high Arctic, for example, the sun, hidden by the tilt of the Earth, vanishes for months. Meters of snow blanket the sea ice and block incoming light, leaving the frigid ocean below as dark as the inside of a tomb. There, biologists assumed, photosynthesizing microalgae that live in the water and ice power down for the season and wait for warmth and light to return.

...read more at quantamagazine.org