Were you dreaming of a pink Christmas?

The song may refer to a white Christmas, but snow can also be pink and is sometimes called watermelon snow. The coloration is caused by a species of ice-loving algae, Chlamydomonas nivalis. This organism contains chlorophyl (green), but also has a carotenoid, which is reddish (think carrots). When the snow melts, the algae activates the carotenoid, using it as a sunscreen. (This isn’t new. Aristotle wrote about seeing pink snow in 300 BC.)