Diatoms and cyanobacteria. Polarised light micrograph of Tabellaria sp. diatoms (green, square) amongst filamentous Cyanobacteria (long, thin). 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. They form an important part of the plankton at the base of the marine and freshwater food chains. Cyanobacteria (also known as blue-green algae) are bacteria that obtain their energy through photosynthesis. Magnification: x400 when printed 10 centimetres wide.