From The Aquarium Wiki
What is it?
Archaea is a single-celled micro-organism that lives underwater and in soil.
A single individual or species is called an archaeon (sometimes spelled "archeon"). Archaea, like bacteria, are prokaryotes. They have no cell nucleus or any other organelles within their cells. In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, they are now classed into their own group.
They have been found in a broad range of habitats, such as soils, lakes, oceans, and marshlands. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. These prokaryotes are now recognized as a major part of life on Earth and may play an important role in both the carbon cycle and nitrogen cycle. No clear examples of archaeal pathogens or parasites are known.
In early 2009 it came to light that species of archaea living underwater actually consume ammonia and turn it into nitrite and may be a signification organism in the correct cycling of aquarium tanks as well as the bacteria of course.