A calcifying form of marine phytoplankton that constitutes the primary ingredient of chalk.

I used to be under the impression that it was the fossilized remains of this specialized class of photosynthetic algae that formed the thousands of kilometers of chalk that line many parts of our planet, but it turns out they actively create their chalky, microscopic exoskeletons while still alive. Using ions dissolved in seawater, they produce calcium-carbonate platelets intracellularly, which are then extruded onto the surface of the cell.

While the precise utility of these exoskeletal structures is unknown—including that of the coccosphere pictured above—scientists speculate that they serve as a form of armour, protecting the delicate algae within from predators, viruses, and harmful UV rays. Crucially, on a global scale, the chalk that they produce has helped to regulate the acidity of the oceans for millions of years.

Image from linked paper, 'Why Marine Phytoplankton Calcify' in Science Advances Vol 2, Issue 7, CC BY 4.0