Wastewater microbiology terminology: Bacteria that can survive and metabolize with or without free dissolved oxygen are known as

Introduction / Context:
Biological treatment processes rely on microbial communities with differing oxygen requirements. Clarifying the terms aerobic, anaerobic, and facultative is crucial for understanding stabilization ponds, activated sludge, trickling filters, and septic processes.

Given Data / Assumptions:

• “With or without free oxygen” means bacteria can function under both oxic and anoxic/anaerobic conditions.
• Focus is on metabolic flexibility rather than a strict requirement.

Concept / Approach:
Aerobic bacteria require oxygen as an electron acceptor. Anaerobic bacteria cannot tolerate free oxygen and use other acceptors (such as CO2, nitrates, sulphates). Facultative bacteria adapt to the environment: in the presence of oxygen they respire aerobically; without oxygen they switch to anaerobic pathways. This property makes facultative organisms valuable in pond systems where oxygen availability varies diurnally and by depth.

Step-by-Step Solution:
Identify the defining phrase “with or without oxygen”.Match to microbial class with metabolic flexibility: facultative bacteria.Select Option C accordingly.

Verification / Alternative check:
Design guidelines for facultative stabilization ponds explicitly rely on such organisms to stabilize BOD in both surface aerobic and deeper anoxic layers, corroborating the term usage.

Why Other Options Are Wrong:

• Aerobic: require oxygen; do not fit “without oxygen”.
• Anaerobic: cannot tolerate oxygen; do not fit “with oxygen”.
• Obligate bacteria: term indicates strict requirement (obligate aerobes or anaerobes), not flexible.
• Photosynthetic bacteria: classification by energy source, not by oxygen tolerance.

Common Pitfalls:

• Confusing “facultative” with “optional presence”; it specifically refers to metabolic capability in both conditions.
• Assuming facultative bacteria will perform identically under both regimes; kinetics differ.

Final Answer:
Facultative bacteria