Chemical reactions involved in bio-geo-chemical cycle are given below:
(a) NO2-→ NO3- (Nitrite to Nitrate)
(b) N2→ NH3 (Nitrogen to Ammonia)
(c) NH4+→ NO2- (Ammonium to Nitrite)
(d) NO3-→ N2 (Nitrate to Nitrogen)
The organisms associated with them are:

Question

Chemical reactions involved in bio-geo-chemical cycle are given below: 
(a) NO2-→ NO3- (Nitrite to Nitrate) 
(b) N2→ NH3 (Nitrogen to Ammonia) 
(c) NH4+→ NO2- (Ammonium to Nitrite) 
(d) NO3-→ N2 (Nitrate to Nitrogen) 
The organisms associated with them are:

Accepted Answer

The correct option is 4:
The correct option accurately matches the nitrogen cycle reactions with their primary microbial participants. The overall process you are examining is the crucial biological component of the biogeochemical nitrogen cycle, which converts nitrogen between various forms that can be utilized by organisms. Reaction (a), the conversion of nitrite ($\text{NO}\_2^{-}$) to nitrate ($\text{NO}\_3^{-}$), is the second stage of nitrification, an oxidation process performed by chemolithoautotrophic bacteria, most notably *Nitrobacter*. Reaction (b), the conversion of atmospheric nitrogen ($\text{N}\_2$) into ammonia ($\text{NH}\_3$), is known as nitrogen fixation, which is carried out by certain prokaryotes like the free-living cyanobacterium *Nostoc* (and others like *Azotobacter* or *Rhizobium*). Reaction (c), the conversion of ammonium ($\text{NH}\_4^{+}$) to nitrite ($\text{NO}\_2^{-}$), is the first and rate-limiting step of nitrification, typically carried out by genera such as *Nitrosomonas* and *Nitrosococcus*. Finally, reaction (d), the conversion of nitrate ($\text{NO}\_3^{-}$) back into inert atmospheric nitrogen ($\text{N}\_2$), is denitrification, a reductive process carried out by facultative anaerobic bacteria like *Pseudomonas*, which return nitrogen to the atmosphere, completing the cycle. The sequence in Option 4, (a)Nitrobacter (b)Nostoc (c)Nitrosomonas (d)Pseudomonas, therefore correctly aligns with the universally accepted roles of these organisms in the cycle.

The incorrect option is 1:
This option incorrectly assigns *Nitrosomonas* to reaction (a) and *Nitrobacter* to reaction (d). *Nitrosomonas* is responsible for the first step of nitrification (c), converting ammonium to nitrite, not the second step of converting nitrite to nitrate (a). Conversely, *Nitrobacter* performs reaction (a), but is incorrectly placed at (d). Reaction (d), denitrification, is carried out by *Pseudomonas*, as it involves the reduction of nitrate to nitrogen gas.

The incorrect option is 2:
Option 2 incorrectly identifies *Pseudomonas* with reaction (a) and *Nitrobacter* with reaction (b). *Pseudomonas* is a denitrifying bacteria, meaning it performs reaction (d), reducing nitrate to nitrogen gas, and is not involved in the oxidation of nitrite to nitrate (a). Furthermore, *Nitrobacter* is a nitrifying bacteria (reaction (a)), not a nitrogen fixer (reaction (b)); nitrogen fixation is the role of *Nostoc* and related species. This option fundamentally confuses the key steps of nitrification, nitrogen fixation, and denitrification.

The incorrect option is 3:
This option is incorrect because it incorrectly associates *Nostoc* with reaction (a) and *Nitrobacter* with reaction (c). *Nostoc* is a nitrogen-fixing cyanobacterium (reaction (b)) and does not perform the nitrification step of converting nitrite to nitrate (a). While *Nitrobacter* is a nitrifying bacterium, it specifically handles the second step (a), not the first step (c) which is the domain of *Nitrosomonas*. Therefore, placing *Nostoc* at (a) and *Nitrobacter* at (c) results in a complete factual mismatch of the microbial roles in the cycle.