Final answer:
Bacteria that use nitrogen directly from the atmosphere are called nitrogen fixers; they convert nitrogen gas to a form usable by plants.
Step-by-step explanation:
Some bacterial species that can use nitrogen directly from the atmosphere are known as nitrogen fixers. This function is vital for the nitrogen cycle, as these bacteria convert atmospheric nitrogen gas (N₂) into a form that can be utilized by plants, which is typically an organic compound or ammonium ion (NH4+).
Nitrogen fixation is a process carried out by certain bacteria including free-living bacteria such as Azotobacter, symbiotic bacteria like rhizobia on the root nodules of legumes, and certain autotrophic bacteria like cyanobacteria. Therefore, the correct answer is: b) Nitrogen fixer.
Nitrogen-fixing bacteria are the bacteria that can use nitrogen directly from the atmosphere. This ability is known as nitrogen fixation. These bacteria convert atmospheric nitrogen gas (N₂) into a form that plants can use, such as ammonia (NH₃). Examples of nitrogen-fixing bacteria include free-living bacteria like Azotobacter, as well as symbiotic bacteria that live in the root nodules of legumes.