In wastewater microbiology, facultative bacteria are organisms that can survive and remain active under which environmental condition(s)?

Introduction / Context:
Understanding the ecological roles of microbes is central to designing biological treatment units and stabilization ponds. Facultative organisms bridge the gap between strict aerobes and strict anaerobes.

Given Data / Assumptions:

• Typical municipal sewage with variable oxygen conditions across unit processes.
• Focus is on oxygen availability, not other nutrients.

Concept / Approach:
Facultative bacteria can switch metabolic pathways depending on oxygen availability. In aerated environments they respire aerobically; in oxygen-deficient zones they shift to nitrate or fermentative pathways as electron-acceptor availability allows. This adaptability is particularly important in facultative lagoons, which have an aerobic upper layer and an anaerobic bottom layer.

Step-by-Step Solution:
Identify the defining characteristic: survival and activity with or without oxygen.Exclude strict categories (only aerobic or only anaerobic).Select the option describing dual tolerance and adaptability.

Verification / Alternative check:
Operational experience shows facultative populations persisting across diurnal DO cycles and depth stratification, sustaining treatment performance despite fluctuating oxygen levels.

Why Other Options Are Wrong:

• Strictly aerobic or strictly anaerobic descriptions contradict the definition of facultative organisms.
• Oxygen is the key variable; sunlight is not the defining requirement for bacteria discussed here.

Common Pitfalls:
Confusing facultative bacteria with anoxic denitrifiers or phototrophs; the term specifically addresses flexibility regarding oxygen presence.

Final Answer:
Both with oxygen and without oxygen (can adapt to either condition).