Final answer:
Rhizobium fixes nitrogen, Nitrosomonas initiates nitrification by converting ammonia to nitrite, Nitrobacter continues nitrification by converting nitrite to nitrate, and Pseudomonas performs denitrification, converting nitrates back to nitrogen gas.
Step-by-step explanation:
The roles of Rhizobium, Nitrosomonas, and Nitrobacter in the nitrogen cycle are specific and crucial. Rhizobium is a group of soil bacteria that fix nitrogen after becoming established inside root nodules of legumes, converting atmospheric nitrogen (N₂) into a form that is usable by plants. Nitrosomonas bacteria are involved in the nitrification process, which oxidizes ammonia (NH3) to produce nitrite (NO₂). Nitrobacter bacteria then take over in the nitrification process to convert nitrite into nitrate (NO₃). Conversely, Pseudomonas, another type of bacterium, is involved in denitrification, the process of reducing nitrates back to nitrogen gas (N₂), thereby closing the nitrogen cycle loop.
Given the provided information and roles of these bacteria, the correct statement regarding their roles in the nitrogen cycle is that Rhizobium fixes nitrogen, Nitrosomonas nitrifies, and Nitrobacter also nitrifies, but later in the nitrification process than Nitrosomonas. Note that among the provided information, there is a typo or incorrect statement that says nitrification converts nitrites to ammonium ion (NH4+), which is incorrect; nitrification involves the conversion of ammonia to nitrites and nitrates, not to ammonium ion.