| Across art and literature, sunlight is a radiant metaphor. A happy person is a "ray of sunshine." Divinity shines down from the sky. A sunny personality looks on the bright side. But visible electromagnetic radiation doesn't just mesmerize and illuminate; it is the engine of life on Earth.
Billions of years ago, early life spun solar radiation into chemical energy. The series of reactions that we know as photosynthesis changed the Earth and its atmosphere. It released oxygen, which helped kick off the evolution of complex multicellular life, and also created the food sources to keep it alive and evolving. If the sun suddenly disappeared, most life on Earth would go extinct in short order.
Light from the sun isn't only an energy source; it also regulates the functioning of plants and animals. Almost all the cells in our body tick along on a 24-hour clock, following a natural oscillation known as a circadian rhythm. They're kept in time by a master clock in the brain, which uses cues from sunlight to sync the cells. Circadian rhythms influence processes throughout the body, including hormone release, appetite, heart rate, sleep and wakefulness.
Even bioluminescence, or light created by living creatures, and most of our artificial electric lights also ultimately derive from the energy of solar light. Life harnesses this resource in all sorts of surprising ways.
What's New and Noteworthy Moonlight — reflected light of the sun — is an important cue for some creatures, including the marine bristle worm Platynereis dumerilii, which times its reproduction with the moon's cycles. When the moon wanes, worms swim to the surface of the Bay of Naples at night to reproduce. But how can the worms distinguish moonlight from sunlight or one moon phase from another? A few years ago, researchers discovered a protein, known as L-Cry, that can detect light. Recently, they figured out how it works. When it's dark, two L-Cry molecules link up; when it's light, they break apart. This protein helps the worms, who only get one chance to mate, know when the time is right.
Plants don't have eyes, but they clearly sense light, twirling and bending to expose their leaves to as much of it as possible — a trick known as phototropism. But how plants can sense the direction light is coming from, and react to it, remained largely mysterious until recently, when a group of scientists discovered part of the secret, at least for a roadside weed often studied as a model plant. Thale cress, or Arabidopsis thaliana, has stems built of cells — and in between those cells are pockets of air. Because of the way these air pockets scatter light, they create a light gradient that the seedlings follow to grow toward the brightest sunlight.
Sometimes light is hard to come by, especially for cells living in dark polar waters in the dead of winter. Biologists recently used microalgae to confirm how many photons, or light particles, are needed to kick-start photosynthesis. Previously, theoretical calculations suggested that cells need, at minimum, around 0.01 micromoles of photons per square meter per second — or less than one-hundred-thousandth of the light of a sunny day — to power the reactions that turn light into food. Working in the months-long polar night, the researchers discovered that microalgae can grow and reproduce at or around these levels — the first experimental proof that neared the theoretical numbers.
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