Microbiology and Nitrogen Cycle

Root nodules contain both the nitrogenase enzyme (for fixation) and leghemoglobin (to protect the enzyme from oxygen).

Both Rhizobium and Frankia are soil bacteria that can fix atmospheric nitrogen when they form symbiotic relationships with host plants.

Rhizobium is the well-known symbiotic bacterium that forms root nodules on legumes, including alfalfa.

Rhodospirillum is a photosynthetic bacterium that is anaerobic and capable of nitrogen fixation.

Both Azotobacter and Beijerinckia are well-known free-living aerobic nitrogen-fixing bacteria.

Denitrification is the process of reducing nitrate to nitrogen gas, carried out by bacteria such as Pseudomonas and Thiobacillus.

Nitrosomonas and Nitrobacter are chemoautotrophic bacteria that obtain energy by oxidizing ammonia or nitrite.

Nitrification is the two-step process where ammonia is oxidized to nitrite by Nitrosomonas and then to nitrate by Nitrobacter.

Nitrogen fixation is specifically defined as the conversion of atmospheric nitrogen (N2) into ammonia (NH3).

Molybdenum (Mo) is a critical component of both nitrogenase (involved in nitrogen fixation) and nitrate reductase (involved in nitrogen assimilation).

While plants can absorb ammonium (NH4+), they primarily absorb nitrogen from the soil in the form of nitrate (NO3-).

The enzyme nitrogenase is highly sensitive to oxygen and is inactivated by it. Nitrogen-fixing organisms must create an anaerobic environment to protect the enzyme.

Bacteroids are the differentiated form of Rhizobium bacteria found within the root nodules of legumes, where they actively fix atmospheric nitrogen.

Archaebacteria are a diverse group of prokaryotes that include methanogens, thermoacidophiles, and halophiles, which are adapted to extreme environments.