Some Nitrosomonas species exhibit significant anaerobic growth - we have even found them in anaerobic digesters

From: Bagchi, S. et al. Critical Reviews in Environmental Science and Technology 42:1353–1418 August 2012

For years, I have always been told that Nitrosomonas sp are obligate aerobic bacteria. We all accepted that Nitrosomonas  used oxygen to convert ammonia into nitrite. That was the extent of their metabolic capabilities. However, Nitrosomonas have now been found to have a few other metabolic tricks available to survive in anoxic and anaerobic environments. How well does Nitrosomonas grow under D.O. limited environments, surprisingly well. In fact, Aster Bio's MCA tests on multiple anaerobic digesters has found significant Nitrosomonas populations. Here is what Nitrosomonas does in aerobic, anoxic, and anaerobic environments:

• Aerobic - this is where Nitrosomonas exhibits most rapid growth and is the best-known metabolic process. Using Ammonia + Oxygen to produce Nitrite.
  
  
• Anaerobic - Nitrosomonas can function as an ANAMMOX culture. In this pathway, Ammonium and Nitrite are the inputs with the final products being Nitrogen gas and H2O.  
  
  
• Anoxic - Nitrosomonas uses nitrite as the terminal electron acceptor and can use various cell intermediates and a few organic compounds to provide energy for growth. Technically, the anaerobic pathways listed above can be included with the anoxic systems as both require nitrite.

What does this mean for wastewater treatment? Well you may have noticed much emphasis on ANAMMOX technology where ammonia oxidation and denitrification happen in the same step. This saves on energy for aeration and makes denitrification more efficient. You do have to account for the slower growth under anoxic/anaerobic conditions, but Nitrosomonas may turn out to be one of the most important ANAMMOX cultures in wastewater systems as it also provides the NO2 used in ANAMOX during aerobic growth.