From the 1940s to the 1970s, several studies indicated that a microbe was missing from nature that could anaerobically oxidize ammonium, with nitrate or nitrite, to dinitrogen gas and that the nitrogen cycle (Fig. 1) thus contained more reactions than was known at that time (8, 29, 73). This hypothesis was based on field observations that much less ammonium accumulated in anoxic water bodies than was expected from Redfield stoichiometry and thermodynamic calculations.