Which of the following is not an iron bacteria that occurs in well water?

Cryptosporidium is the correct answer because it is not classified as an iron bacteria. It is a protozoan parasite that can cause gastrointestinal illness in humans and animals when ingested through contaminated water. Its primary habitat is associated with fecal contamination in water sources, not with the iron-rich environments typically inhabited by iron bacteria.

On the other hand, Sphaerotilus, Leptothrix, and Gallionella are all types of iron bacteria known to thrive in iron-rich water environments, like wells. These bacteria derive energy from the oxidation of iron and can contribute to the formation of unpleasant tastes, odors, and the staining of fixtures and laundry due to their iron deposits. Each of these bacteria plays a role in the biogeochemical cycling of iron but does not share the pathogenic characteristics of Cryptosporidium. This distinction highlights the differences in the types of organisms present in well water and their implications for water quality.