In the nitrogen cycle, which of the following steps involves removal of nitrogen from the atmosphere by bacteria?
Select one:
a. ammonification b. denitrification c. nitrification d. nitrogen fixation
The correct answer and explanation is:
The correct answer is d. nitrogen fixation.
Explanation:
Nitrogen fixation is the process by which nitrogen gas (N2) from the atmosphere is converted into a form that can be used by plants and other organisms, primarily ammonium (NH4+). This is carried out by certain bacteria, often in symbiotic relationships with leguminous plants, although free-living nitrogen-fixing bacteria in the soil and water also perform this function. These bacteria possess an enzyme called nitrogenase, which is capable of breaking the strong triple bond in nitrogen molecules, making nitrogen available to plants in the form of ammonium.
Once nitrogen is fixed into ammonium, plants can take it up and incorporate it into essential molecules like amino acids and proteins. This process is a crucial part of the nitrogen cycle because it ensures that nitrogen, which is a key component of life on Earth, is accessible to living organisms. Without nitrogen fixation, most organisms would be unable to obtain the nitrogen needed for protein and nucleic acid synthesis.
In contrast, the other processes listed are different stages in the nitrogen cycle:
- Ammonification is the process where organic nitrogen compounds from dead organisms or waste products are converted into ammonium by decomposers (such as bacteria and fungi).
- Denitrification is the process where nitrates (NO3-) are reduced back to nitrogen gas (N2), releasing it back into the atmosphere, and thus completing the nitrogen cycle.
- Nitrification involves the conversion of ammonium into nitrites (NO2-) and then nitrates, which can be taken up by plants.
Nitrogen fixation is essential for introducing usable nitrogen into ecosystems, making it the key step in removing nitrogen from the atmosphere for biological use.