Option 1: Pseudomonas. Pseudomonas is a genus of bacteria that is known for its wide range of metabolic capabilities. However, Pseudomonas does not have a specific role in converting nitrates to free nitrogen. It is involved in various other processes such as biodegradation, production of antibiotics, and nitrogen fixation, but it does not directly convert nitrates to free nitrogen.

Option 2: Nitrosomonas. Nitrosomonas is a genus of bacteria that is responsible for the first step of nitrification, which is the oxidation of ammonia to nitrite. This process does not involve the conversion of nitrates to free nitrogen.

Option 3: Nitrobacter. Nitrobacter is another genus of bacteria involved in the nitrification process. It is responsible for the second step of nitrification, which is the oxidation of nitrite to nitrate. While Nitrobacter plays a crucial role in the nitrogen cycle, it does not convert nitrates to free nitrogen.

Option 4: Rhizobium. Rhizobium is a genus of bacteria that forms a symbiotic relationship with legume plants. It is responsible for nitrogen fixation, which is the conversion of atmospheric nitrogen to a form