Coccolithophore

Coccolithophores are single-celled marine algae covered in an armor of interlocking calcium carbonate plates called coccoliths, so abundant in the world's oceans that massive blooms are visible from space as pale, milky patches of water. Over immense spans of geological time, the accumulated coccolith remains of dead coccolithophores have built up thick layers of chalk on the sea floor, including the White Cliffs of Dover, and the group plays a major role in the ocean's long-term carbon cycle by locking carbon into these mineral shells as they sink after death.