Desmid and Dictyosphaerium green algae. Darkfield illuminated polarised light micrograph of a Micrasterias apiculata desmid (large, centre) amongst Dictyosphaerium ehrenbergianum green algae (small, circular). Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Micrasterias is highly adapted to nutrient poor, acidic water. Magnification: x200 when printed 10 centimetres wide.