When we talk about unicellular organisms, the image of tiny cells that cannot be seen with the naked eye comes to mind. However, among the micro-world of the aquarium, there are representatives that deserve special attention.
Size & appearance
Spirostomum are true giants of the world of unicellular organisms, reaching 3 mm in length and inhabiting freshwater aquariums. Spirostomum is a genus of ciliates containing 8 species of unicellular organisms belonging to the class Heterotrichea. Due to the tubular, slightly flattened shape of the cell and the nature of its movement, they can be mistaken for worms. At high magnification of the microscope, we can see that the entire cell is covered with rows of cilia, which allow Spirostomum to slowly slide on the substrate or swim freely above it.
Spirostomums live at the bottom of bodies of water, in an environment rich in organic deposits such as fish feces, food remnants, and dead vegetation. There they find the necessary nutrients, like small unicellular organisms and bacteria, they can consume. However, such deposits often form oxygen-free zones where Spirostomums, surprisingly, can survive. Scientists suggest that this is possible thanks to symbiotic algae inside the cell that release oxygen during photosynthesis, as well as a series of biochemical processes that occur within the cell. Species of the Spirostomum genus are used as model organisms for studying human pathogenic bacteria and are also bioindicators for determining water quality, as they demonstrate sensitivity to toxic substances such as heavy metals like copper, mercury and zinc. Therefore, if you find these microorganisms in your aquarium, you can be sure that the water quality is good. If, on the other hand, their population is too high, this may indicate overpopulation or overfeeding of fish.