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While I was studying Cellular Respiration: Harvesting Chemical Energy, I found this question:

An organism is discovered that consumes a considerable amount of sugar, yet does not gain much weight when denied air. Curiously, the consumption of sugar increases as air is removed from the organism's environment, but the organism seems to thrive even in the absence of air. When returned to normal air, the organism does fine. Which of the following best describes the organism?

A) It must use a molecule other than oxygen to accept electrons from the electron transport chain.

B) It is a normal eukaryotic organism.

C) The organism obviously lacks the citric acid cycle and electron transport chain.

D) It is an anaerobic organism.

E) It is a facultative anaerobe.

Answer: E

I wonder why the answer is E not D. How can I know that it is a facultative anaerobe? Do anaerobic organisms die in air?

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    $\begingroup$ Strict anaerobic organisms cannot proliferate when air is present. Facultative on the other hand can. $\endgroup$ – Chris May 14 '17 at 20:29
  • $\begingroup$ But he didn't say anything about proliferate $\endgroup$ – Mahmoud Usama Fawzy May 14 '17 at 20:31
  • $\begingroup$ I think "does fine" implies proliferation, but I agree the wording could have been clearer. $\endgroup$ – Victor Chubukov May 14 '17 at 23:42
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There are organisms with anaerobic metabolism that tolerate oxygen: aerotolerant anaerobes. "Anaerobic" refers to the metabolic system. You cannot be sure of oxygen tolerance based solely on metabolic system.

Factors influencing the tolerance of anaerobic bacteria to oxygen

Here is why this is a facultative anaerobe.

This organism does not gain weight when denied air. One concludes its metabolism is less efficient.

Curiously, the consumption of sugar increases as air is removed from the organismʹs enviroment

Anaerobic metabolism uses sugar less efficiently than aerobic metabolism in organisms that can use both methods. This means oxidative metabolism is the preferred method.

So: this is a facultative anaerobe because it can get by using anaerobic metabolism, but two pieces of information show that for the organism, anaerobic mode is less efficient than aerobic metabolism. Brewers yeast is such an organism.

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    $\begingroup$ My answer is correct, since i only talk about obligate anaerobes. Just as described in the book in the link. $\endgroup$ – Adrian May 19 '17 at 21:03
  • $\begingroup$ I edited your answer for the reasons specified in the edit history. If you disagree with this change, please feel free to rollback. $\endgroup$ – canadianer May 19 '17 at 22:17
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Yes, (obligate) anaerobes die in the presence of oxygen (at atmospheric levels). They do not have the detoxifying enzymes that convert toxic metabolic products such as hydroxyl racials or superoxide anions into nontoxic H2O or O2. These enzymes present in aerobic and faculative anaerobic organisms are superoxide dismutase, peroxidase, among others.

Reference:

a Hentges DJ. Anaerobes: General Characteristics. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 17. Available from: https://www.ncbi.nlm.nih.gov/books/NBK7638/

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