Denitrification characteristics: Which statement best describes who carries out denitrification in nature and treatment systems?

Introduction / Context:
Denitrification is the stepwise reduction of nitrate/nitrite to gaseous nitrogen forms under low-oxygen conditions. Knowing the physiological types of organisms involved is key for designing bioreactors and managing soils.

Given Data / Assumptions:

• The options include claims about typical physiology (facultative anaerobes) and genus-level predominance.
• Multiple genera can denitrify; no single genus dominates all environments.

Concept / Approach:
Most denitrifiers are facultative anaerobes that switch from oxygen to nitrate/nitrite as terminal electron acceptors when oxygen is scarce. Common genera include Pseudomonas, Paracoccus, Alcaligenes, and some Bacillus. While Pseudomonas is frequently cited, “predominantly Bacillus” overgeneralizes. Therefore the most correct, broadly applicable statement is that denitrification is usually carried out by facultative anaerobes.

Step-by-Step Solution:
Recall physiology: facultative anaerobes perform respiration with nitrate in low oxygen. Note that many genera participate; no single genus dominates universally. Evaluate options: (a) is broadly true; (b) is context-dependent; (c) is overstated. Select (a) as the most accurate generalization.

Verification / Alternative check:
Wastewater and soil microbiology references describe denitrification consortia with diverse facultative organisms; process control targets anoxic conditions and carbon supply, not specific genera.

Why Other Options Are Wrong:

• Predominantly Pseudomonas: Often important but not universally predominant.
• Predominantly Bacillus: Some Bacillus denitrify, but they are not typically singled out as predominant.
• All of the above: Fails because (c) is not generally true.
• Only strict anaerobes: Incorrect; most denitrifiers are facultative.

Common Pitfalls:
Over-emphasizing one genus based on a specific study; environmental context matters.

Final Answer:
Denitrification is carried out usually by facultative anaerobes.