Diatoms. Coloured scanning electron micrograph (SEM) of an arrangement of diatoms to show their diversity. Diatoms are planktonic unicellular algae. They have a mineralised cell wall (frustule) divided into two halves. Diatoms are considered one of the most diverse and ecologically important phytoplanktonic. Marine diatoms rose to prominence about 100 million years ago and today generate most of the organic matter that serves as food for life in the sea. They exist in a dilute world where compounds essential for growth are recycled and shared, and they greatly influence global climate, atmospheric carbon dioxide concentration and marine ecosystem function.