Microbiology in water treatment Bacteria that require free (dissolved) oxygen for survival and metabolism are called:

Introduction / Context:
Microbial classification by oxygen requirement guides process selection in wastewater and water treatment (e.g., activated sludge versus anaerobic digestion). Knowing which organisms require oxygen is fundamental to designing aeration and biological steps.

Given Data / Assumptions:

• “Free oxygen” refers to dissolved oxygen present in water.
• Typical environmental engineering definitions apply.

Concept / Approach:

• Aerobic bacteria: require dissolved oxygen for respiration.
• Anaerobic bacteria: thrive in absence of free oxygen; oxygen may be toxic.
• Facultative bacteria: can switch between aerobic and anaerobic metabolism.

Therefore the name for organisms that require free oxygen is “Aerobic bacteria”.

Step-by-Step Solution:
Match oxygen requirement in the stem to the correct category → aerobic.Exclude anaerobic (no oxygen) and facultative (can use either condition).

Verification / Alternative check:
Standard microbiology texts define aerobic respiration as oxygen-dependent, distinguishing it from anaerobic pathways (e.g., nitrate/sulfate reduction, fermentation).

Why Other Options Are Wrong:

• Anaerobic: opposite of what is described.
• Facultative: not strictly requiring oxygen.
• “None of these”: incorrect because “Aerobic bacteria” fits exactly.

Common Pitfalls:
Confusing “facultative” with “aerobic”; facultative organisms survive without oxygen as well.

Final Answer:
Aerobic bacteria