A phytoplankton bloom of coccolithophores in the Black Sea.
The Black Sea … not so black, eh?
Those coccolithophores are a key bart of regulating Earth’s climate. Like many phytoplankton, they absorb carbon dioxide from the air via the water. Only, unlike a plant, they don’t use all that carbon dioxide to make sugars. Then these little guys turn it into limestone, in intricate little “microshell” shapes, and deposit it on the bottom of the ocean or sea when they die.
Like microscopic seashells, this calcium carbonate builds up over eons and becomes what we call chalk! Prehistoric deposits of coccolithophores actually created the Cliffs of Dover, and likely line most of the Atlantic Ocean floor. Robert Krulwich from Radiolab had a wonderful post about that recently.
Here’s what their skeletons look like (via Wikipedia):