Nitrosomonas and Nitrobacter convert
A. Carbon dioxide to carbohydrates
B. Ammonium ion into nitrate
C. Nitrate ion into nitrogen molecule
D. Nitrogen into nitrate

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Hint: Nitrosomonas and Nitrobacter is a genus comprising rod-shaped, gram-negative, and chemoautotrophic bacteria. The genus Nitrosomonas is found in soil, freshwater, and on building surfaces, especially in areas that contain high levels of nitrogen compounds. The genus Nitrobacter is widely distributed in both aquatic and terrestrial environments.

Complete answer: Nitrosomonas oxidizes ammonia into nitrite as a metabolic process, known as nitritation (a step of nitrification). They are important in the nitrogen cycle as they increase the bioavailability of nitrogen to plants whilst limiting carbon fixation. Nitrobacter plays an important role in the nitrogen cycle by oxidizing nitrite into nitrate in soil and marine systems. Nitrobacter uses energy from the oxidation of nitrite ions into nitrate ions to fulfill their energy needs. In the oceans, nitrite-oxidizing bacteria such as Nitrobacter are usually found in close proximity to ammonia-oxidizing bacteria, Nitrosomonas. These two reactions together make up the process of nitrification. The nitrite-oxidation reaction generally proceeds more quickly in ocean water and therefore is not a rate-limiting step in nitrification. For this reason, it is rare for nitrite to accumulate in ocean waters.
So, the correct answer is option B-Ammonium ion into nitrate

Note: Nitrosomonas are useful in a polluted water and waste treatment technique known as bioremediation. Some Nitrosomonas species possess the enzyme urease which catalyzes the conversion of the urea into ammonia and carbon dioxide. Nitrobacter fixes carbon dioxide via the Calvin cycle for their carbon requirements.