Definitions — An organism completely dependent on atmospheric oxygen (O2) for growth is called a(n)…

Introduction / Context:
Accurate terminology for oxygen requirements guides culture setup, infection site prediction, and environmental microbiology. Distinguishing obligate aerobes from facultative or anaerobic types avoids diagnostic errors and informs therapy and bioprocess design.

Given Data / Assumptions:

• The organism requires oxygen to grow and cannot grow anaerobically.
• “Atmospheric oxygen” implies the ~21% O2 present in air.
• Other terms (osmotolerant, acidophile) relate to different stressors.

Concept / Approach:
Obligate aerobes rely on aerobic respiration with oxygen serving as the terminal electron acceptor. Without oxygen, ATP yields drop catastrophically because alternative electron acceptors are not used. This differs from facultative anaerobes (can switch), aerotolerant anaerobes (tolerate O2 but do not use it), and anaerobes (killed by O2). Osmotolerant and acidophile describe osmotic and pH preferences respectively, not oxygen needs.

Step-by-Step Solution:

Verification / Alternative check:
Classic obligate aerobes include Pseudomonas spp. and Mycobacterium spp., which fail to grow in strict anaerobic culture conditions.

Why Other Options Are Wrong:

• Osmotolerant: refers to solute/osmotic stress, not oxygen.
• Acidophile: refers to pH preference.
• Facultative anaerobe: can grow with or without O2, contradicting the stem.

Common Pitfalls:
Equating “can use O2” with “requires O2”; only obligate aerobes require it absolutely.

Final Answer:
obligate aerobe