USA Biology Olympiad (USABO) Practice Exam

What will reduce the denitrifying activity of denitrifying bacteria?

1. Increased temperature

2. Decrease in nitrogen compounds

3. Increase in oxygen concentration

4. Increased moisture levels

The correct answer is: Increase in oxygen concentration

Denitrifying bacteria are crucial for the nitrogen cycle, primarily converting nitrates and nitrites into nitrogen gas, which is released into the atmosphere. This process is anaerobic, meaning it occurs in environments with low oxygen levels. When there is an increase in oxygen concentration, it can inhibit the activity of denitrifying bacteria because these organisms thrive in oxygen-limited conditions. Oxygen can suppress denitrification as denitrifying bacteria will prefer to utilize oxygen for respiration over nitrate, ultimately reducing their ability to carry out denitrification. The other factors mentioned don't have the same inhibitory effects on the denitrification process. For example, increased temperature may enhance microbial activity, although excessively high temperatures could ultimately be detrimental. A decrease in nitrogen compounds would limit the substrates available for denitrification but doesn’t directly inhibit the bacteria's activity. Increased moisture levels typically create favorable conditions for microbial processes, including denitrification, provided that anaerobic conditions are maintained. The increase of oxygen, in contrast, significantly restricts the bacteria's ability to function effectively.