Which of the following bacterium causes denitrification?

Question

Accepted Answer

The correct option is 4:

Pseudomonas is a common genus of bacteria that plays a significant role in the nitrogen cycle, specifically in the process of denitrification. Denitrification is a microbial process where nitrates (NO3-) are sequentially reduced to nitrogen gas (N2) through a series of intermediate gaseous nitrogen oxide products (nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O)). This process is an anaerobic respiration pathway where nitrate serves as the terminal electron acceptor instead of oxygen. Pseudomonas species are facultative anaerobes, meaning they can switch from aerobic respiration (using oxygen) to anaerobic respiration (using nitrates) when oxygen is scarce, making them highly efficient denitrifiers in various environments, including soil and water. This activity is crucial for returning nitrogen to the atmosphere, completing the nitrogen cycle.

The incorrect option is 1: Azotobacter.
Azotobacter is a genus of free-living, nitrogen-fixing bacteria. Instead of denitrification, they are involved in nitrogen fixation, converting atmospheric nitrogen (N2) into ammonia (NH3), which plants can then use. They enrich the soil with usable nitrogen, performing the opposite function of denitrifiers.

The incorrect option is 2: Nitrobacter.
Nitrobacter is a genus of nitrifying bacteria. These bacteria are responsible for the second step of nitrification, where they oxidize nitrites (NO2-) into nitrates (NO3-). This process makes nitrogen available to plants in a different form and is distinct from denitrification, which removes nitrogen compounds from the soil.

The incorrect option is 3: Nitrosomonas.
Nitrosomonas is another genus of nitrifying bacteria. They carry out the first step of nitrification, converting ammonia (NH3) or ammonium (NH4+) into nitrites (NO2-). Like Nitrobacter, their role is in making nitrogen available in the soil, rather than removing it through denitrification.