Polarised light micrograph of Volvox sp. green algae with daughter colonies and rotifer (light blue). Green algae are essential for photosynthesis, producing oxygen and removing carbon dioxide, thus supporting life. There are around 22,000 species of green algae, many of which exist primarily as single cells. Volvox sp. algae form spherical colonies that contain up to 60,000 cells embedded in a gelatinous wall. They live in a variety of freshwater habitats. Rotifers, also called wheel animals, are common in freshwater environments. They use their cilia (microscopic hairs) and two toe-like projections for locomotion. Magnification: 93x when printed 10 centimetres wide.