Amoeba, diatoms and red algae. Polarised light micrograph of Eunotia sp. diatoms (centre-right) and a single of Arcella sp. testate (shelled) amoeba (top centre) over Batrachospermum sp. freshwater red algae (lobes, bottom). 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. The circular shell of Arcella is formed from the polysaccharide chitin. This amoeba is found in freshwater ponds and puddles, and can develop vacuoles of carbon dioxide in its body to float up to the water's surface. Magnification: x400, when printed 10 centimetres wide.