Light micrograph (polarized light) of Cosmarium sp. Cosmarium are photosynthesizing microscopic algae that inhabit the upper sunlit layer of almost all bodies of freshwater on earth. Cosmarium are purely planktonic and cannot propel themselves. They are wanderers that drift with the currents. Cosmarium, along with other microscopic green algae, serve as the base of the aquatic food web, providing an essential ecological function for all aquatic life. There are more than 400 different species of Cosmarium. Magnification: x650 when printed at 10 centimetres wide at its longest edge.