Final answer:
The first step in the oxidation of NH3 by bacteria is the conversion to nitrite (NO2-) by Nitrosomonas bacteria. This step is part of the nitrification process in the nitrogen cycle.
Step-by-step explanation:
The first step in the oxidation of ammonia (NH3) by bacteria is conversion to nitrite (NO2-). Bacteria of the genus Nitrosomonas are involved in this initial process. They oxidize NH3 into nitrite as part of the nitrification process. Following this, Nitrobacter bacteria oxidize the nitrite to nitrate (NO3-). This nitrification is essential in the nitrogen cycle and provides a means to convert ammonia, which can be toxic at high concentrations, into forms that can be used by plants or further processed within the ecosystem.