Diatoms. Coloured scanning electron micrograph (SEM) of a Thalassiosira sp. diatoms. Thalassiosira is a genus of centric diatoms, comprising over 100 marine and freshwater species. It is a diverse group of photosynthetic eukaryotes that make up a vital part of marine and freshwater and freshwater ecosystems, in which they are key primary producers and essential for carbon cycling. Diatoms are single-celled photosynthetic algae, of which there are about 100, 000 species. They have mineralised cell walls (frustules) that contain silica and provide protection and support. Magnification: X 600 when printed at 10 centimetres wide.