It could never be found until recently, in a fish tank a few floors below a university microbiology department: one single organism able to perform the complete process of nitrification. Microbiologists used to think that two distinct groups of bacteria were responsible for the stepwise oxidation of ammonia to nitrate via nitrite.
http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature16459.html
http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature16459.html
