We all know that the nitrogen cycle consists of breaking down ammonia or ammonium into nitrite and then into nitrate. For years, the microorganisms credited with performing these tasks in aquaria were bacteria, specifically from the genera Nitrosomonas and Nitrobacter. That is still true, however the notion that bacteria are exclusively responsible has recently been challenged by the scientific community. In fact, it may not be bacteria at all that perform the majority of the work, at least in freshwater aquariums.
Another microorganism called archaea, which was long thought to be a bacteria, is also a participant. However it has more recently been reclassified as a different type of prokaryote and is now thought to possibly perform the lion share of ammonia oxidation (at least in some circumstances). In several experiments, ammonia-oxidizing archaea, or AOA, was found to occupy a greater volume of filtration media in freshwater aquaria than ammonia-oxidizing bacteria, or AOB. There are, of course, certain variables that can influence this volume. These include the pH of the water, the dissolved oxygen (DO), and the water temperature. What isn’t clear is if the oxidization capacity is the same for equal volumes of AOA and AOB. In other words, a higher volume (or density) of one in filtration media may not necessarily signify it’s oxidizing the most ammonia.
Nonetheless, experienced hobbyists who continue to espouse bacteria as the only oxidizer in the nitrogen cycle may be incorrectly informing new hobbyists. For more information about archaea’s role in the nitrogen cycle, see the following journal articles:
Bagchi, S., Vlaeminck S. E, Sauder, L. A., Mosquera, M., Neufeld J.D., et al. (2014). Temporal and spatial stability of ammonia-oxidizing archaea and bacteria in aquarium biofilters. PLoS One 9(12): e113515. doi:10.1371/journal.pone.0113515.
Sauder, L. A., Engel, K., Stearns, J. C., Masella, A. P., Pawliszyn, R., et al. (2011). Aquarium nitrification revisited: Thaumarchaeota are the dominant ammonia oxidizers in freshwater aquarium biofilters. PloS one, 6(8): e23281. doi.org/10.1371/journal.pone.0023281.
Shout out to Ricky Kenerly of Ricky Kenerly Cichlids. I was actually watching a video on his YouTube channel where I first heard about archaea.