These rotating spheres are called volvox, the genus of around 20 species of green algae that can be found in nutrient-rich freshwater environments. They were first recorded by Dutch microscopist Antonie van Leeuwenhoek in 1700.
Microscopists who are interested in observing Volvox should try to investigate water samples from ponds and puddles. It is also possible to grow Volvox at home. Volvox likes to grow in nutrient-rich water. Dilute some plant fertilizer in water and add some pond water containing Volvox (or other green algae that you want to grow). Place the container on the window sill for several days but prevent direct sunlight as this may cause overheating, and drives out the CO2 for photosynthesis from the water. Alternatively, you can also use a plankton net to catch the colonies.
Find more microscopy photos by Gougi on Flickr, including these:
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