Smaller than a speck of dust and shaped like tiny discs, coccolithophores are microscopic ocean organisms with a big climate job. They draw carbon out of seawater, help produce oxygen, and their calcite plates sink to form chalk and limestone that preserve Earth’s climate history. Today, five European research organisations launched an initiative to make 10 October International Coccolithophore Day, highlighting their crucial role in regulating the planet’s carbon balance, producing oxygen, and sustaining the ocean ecosystems that underpin all life.