While both *Nitrosomonas* and *Nitrobacter* are important bacteria in the nitrogen cycle, they are not nitrogen-fixing bacteria.

Here's why:

* Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into a usable form like ammonia (NH3). This is done by specific bacteria called nitrogen-fixing bacteria (e.g., *Rhizobium*, *Azotobacter*).

* Nitrosomonas and Nitrobacter are nitrifying bacteria. They play a vital role in converting ammonia (NH3) to nitrite (NO2-) (by *Nitrosomonas*) and nitrite to nitrate (NO3-) (by *Nitrobacter*). This is part of the nitrification process, a different step in the nitrogen cycle.

In summary: Nitrogen fixation and nitrification are distinct processes within the nitrogen cycle, and *Nitrosomonas* and *Nitrobacter* are not involved in the initial nitrogen fixation step.