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I am well aware of traditional anaerobic respiration (lactic acid or alcohol produced - no Krebs cycle) and traditional aerobic respiration (O2 is used at the end of the Citric acid cycle).

I am wondering how to classify respiration that uses the Citric acid cycle, but consumes a nitrite/ate or sulfite/ate instead of O2. Is that anaerobic or aerobic?

NADH and Nitrite

I could only find this image on Google image search catch but it said it was from http://lecturer.ukdw.ac.id/dhira/Metabolism/RespAnaer.html

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4 Answers 4

up vote 7 down vote accepted

Your question is based on a bit of confusion about electron acceptors that is very common and that drives microbiologists crazy. Energy (as ATP) is generated when electrons are moved from an electron donor to an electron acceptor. In respiration, the electron acceptor is inorganic -- oxygen for aerobic respiration or an inorganic molecule such as sulfate or nitrate for anaerobic respiration. Use of an organic molecule such as pyruvate as an electron acceptor is properly called fermentation. Both anaerobic respiration and fermentation are termed anaerobic because they don't use oxygen. Any introductory microbiology textbook will cover these concepts in detail, because they are important to understanding the metabolic diversity of bacteria. Brock Biology of Microorganisms is a good text.

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A friend of mine checked 8 different introductory biology textbooks: 6 of them described respiration as using oxygen. However 2 of them did not specifically state oxygen as the terminal electron acceptor, which I am convinced is technically correct. Nitrogen as a terminal electron acceptor is an exception and not the rule, which is why it is so often overlooked in textbooks. –  JoeHobbit Feb 15 '13 at 5:31

Anaerobic respiration is a respiration where the final electron acceptor is different than oxygen. The final acceptor can be a less oxidizing than oxygen, like sulfate (SO42-), nitrate (NO3-), or sulfur (S). For example bacteria that use sulfate are obligate anaerobs.

The Krebs cycle cannot take place in the absence of oxygen, although oxygen is not directly involved in the cycle. The oxygen is required for the electron transport chain, which oxidized NADH and FADH2 back to NAD+ and FAD+, which are involved in the four reduction reactions of the cycle.

Thus, aerobic respiration includes both Krebs cycle and final electron acceptor of oxygen, whereas anaerobic respiration does not.

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I thought that NADH and FADH2 could be recycled using Nitrites - I edited in a diagram. –  JoeHobbit Jan 23 '12 at 22:46
I still don't think that denitrifying bacteria will utilize Krebs cycle. Why would they do it anyways if no energy whatsoever comes from it? –  Gergana Vandova Jan 23 '12 at 22:55
One GTP (ATP) still comes out of one turn of the Krebs cycle. Although inefficient, there is still some sort of gain. –  jp89 Jan 23 '12 at 23:10
@jp89 All energy produced in the Krebs cycles comes from the electron transport chain, i. e. energy cannot be produced without oxygen. –  Gergana Vandova Jan 23 '12 at 23:19
"Most microbes respire (use an electron transport chain), although oxygen is not the only terminal electron acceptor that may be used. " - en.wikipedia.org/wiki/Microbial_metabolism , see also; nitrificationnetwork.org/Introduction.php (Ctrl+F, electron transport chain) –  JoeHobbit Jan 24 '12 at 1:29

Agree that anaerobic respiration involves the electron transport chain and alternative terminal electron acceptors such as nitrate. However, respiring cells (whether respiring aerobically or anaerobically) can run the Krebs cycle. As long as there is a terminal electron acceptor, the NADH/FADH2 generated by the Krebs cycle can be oxidized back to NAD+ or FAD.

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Welcome to biology.SE. Your answer, while interesting, does not address the question asked: "I am wondering how to classify respiration that uses the Citric acid cycle, but consumes a nitrite/ate or sulfite/ate instead of O2. Is that anaerobic or aerobic?" –  terdon Oct 14 '12 at 12:03
The status of this question and its answers is completely messed up. This post has the correct answer - use of anything other than oxygen as a terminal electron acceptor is anerobic respiration. Cells using anerobic respiration will (normally) have a Krebs cycle. The answer from @Gergana Vandova is incorrect, despite its upvote score. –  Alan Boyd Dec 7 '12 at 10:55

While aerobic organisms during respiration use oxygen as a terminal electron acceptor, anaerobic organisms use other electron acceptors. These inorganic compounds have a lower reduction potential than oxygen, meaning that respiration is less efficient in these organisms and leads to slower growth rates than aerobes. Many facultative anaerobes can use either oxygen or alternative terminal electron acceptors for respiration depending on the environmental conditions.

Most respiring anaerobes are heterotrophs, although some do live autotrophically. All of the processes described below are dissimilative, meaning that they are used during energy production and not to provide nutrients for the cell (assimilative). Assimilative pathways for many forms of anaerobic respiration are also known:http://en.m.wikipedia.org/wiki/Sulfur_oxidizing_bacteria

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