Environmental microbiology — bacteria and oxygen requirement: Bacteria that can survive and metabolize with or without free dissolved oxygen are called

Introduction / Context:
Understanding bacterial metabolism relative to oxygen is essential for designing biological treatment processes (activated sludge, trickling filters, anaerobic digesters) and for interpreting water-quality indicators.

Given Data / Assumptions:

• The question concerns survival and metabolism both in the presence and absence of free oxygen.
• Standard microbiological classifications apply.

Concept / Approach:
Facultative bacteria can shift metabolic pathways depending on oxygen availability, using aerobic respiration when oxygen is present and switching to fermentation or anaerobic respiration when it is absent. This flexibility underpins many wastewater processes where zones alternate between oxic and anoxic conditions to remove carbon and nitrogen.

Step-by-Step Solution:

Verification / Alternative check:
In secondary treatment, facultative organisms dominate mixed liquor; stabilization ponds may also rely on facultative action depending on depth and sunlight.

Why Other Options Are Wrong:
Aerobic require oxygen; anaerobic cannot tolerate oxygen; microaerophiles need very low oxygen; strict aerobes always need oxygen.

Common Pitfalls:
Equating facultative with “tolerant” but non-metabolizing; misapplying to protozoa or algae instead of bacteria.

Final Answer:
Facultative bacteria