That year, Francis and other scientists conducted independent studies on Crenarchaeota, a division of the Archaea domain that includes thermophiles and non-extremophiles. ''Crenarchaeota are everywhere-in soils, sediments, the deep subsurface and the ocean,'' he said. ''They are potentially the most abundant organism on Earth, yet we really had no idea how they survive in the ocean.''
Non-extremophilic Crenarchaeota are fascinating, according to Francis, in large part because of their potentially vital role in cycling the global supply of nitrogen. All living things need nitrogen to make proteins, DNA and other biomolecules. Nearly 80 percent of the atmosphere consists of nitrogen gas, which, unlike oxygen, cannot be absorbed by most organisms. Getting usable nitrogen turns out to be a complicated biological process. First, special ''nitrogen-fixing'' bacteria in the environment convert atmospheric nitrogen into ammonia, then ''nitrifying'' bacteria oxidize the ammonia into nitrites and nitrates, which are readily absorbed by plants or removed by other microbes. Animals and people, in turn, obtain nitrogen by eating plants and other herbivorous animals.
That was the dogma taught for decades in biology classes-until September 2005, when David Stahl of the University of Washington showed that bacteria do not have a monopoly on nitrification. In a unique laboratory experiment, Stahl and his colleagues demonstrated for the first time that archaea-in this case, marine Crenarcheaota-also oxidize ammonia into nitrite.
Less than a month later, Francis and his colleagues published a paper in the Proceedings of the National Academy of Science (PNAS) showing that ammonia-oxidizing Crenarchaeota are pervasive in water columns and sediments throughout the ocean. Then in July 2006, German microbiolog