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The answer to this, I reckon, is that they don't.

They use molecular oxygen (O 2 ) dissolved in the water for respiration, where it acts as a terminal electron acceptor, just as we use molecular oxygen in the air for respiration. We can speak of the water as being oxygenated.

Water is split in photosynthesis, where reducing equivalents from water are used to reduce NADP+ (giving NADPH).

One of the great discoveries of biology, IMO, is that the oxygen formed in green-plant photosynthesis comes from water, not CO 2 .

Tricarboxylic Acid Cycle (Krebs Cycle) Rant

Despite claims to the contrary, most infamously by Racker (1976, pp 28 - 29) and Wieser (1980), but also by Madeira (1988) and Mego (1986) for example, water is not split in the tricarboxylic acid cycle (Krebs Cycle). Banfalvi (1991) also sails pretty close to the wind on this one.

That is, reducing equivalents from water are not passed down the respiratory chain, or in any way used to make ATP, or are in any way a 'source' of free energy. Such claims, IMO, are nonsense.

The definitive answers to the Wieser (1980) paper are given by Atkinson (1981) and Herreros & Garcia-Sancho (1981). Both of these articles are models of clarity, and categorically refute the claims of Wieser (1980). Nevertheless, as shown by the references above, the controversy surfaces periodically.

The only sources of reducing equivalents in the TCA cycle are carbon compounds, and the only electrons passed down the respiratory chain are those 'held' in C-H and C-C bonds (Herreros & Garcia-Sancho, 1981). An ionization is neither an oxidation nor a reduction (see Atkinson, 1981) and neither is a hydration. Adding water to (say) a double bond does not make the compound any more oxidized or reduced. As far as oxygen and electrons are concerned, and to generalize from a biological point of view, what is has it holds - except in photosynthesis.

As you may have guessed, the splitting of the water in the TCA cycle is a pet rant of mine. Thanks for the opportunity of airing my views!

Edit 3

Found this one when searching Google (Brière et al, 2006). In an invited review for the American Journal of Physiology (Cell Physiology) at that!

Finally, TCA cycle should also be considered as a water-splitting process generating oxygen for acetyl-CoA oxidation [they quote Wieser]

So now the TCA cycle is producing oxygen from water. Wonders will never cease!

(end edit)

(end rant)

Edit 2

As rwst and Alan Boyd have drawn attention to, the concentration of dissolved oxygen in water is all important, and varies with (for example) temperature.

In air-saturated buffer at 25oC the concentration of oxygen (O 2 molecules) is about 0.24 mM (0.24 μmoles/ml, or about 0.474 μg-atoms of oxygen per ml). [Chappell (1964)]. This figure decreases with increasing temperature.

Great question, BTW.

References

(Apologies for the incomplete Atkinson and Herreros & Garcia-Sancho references. I have a photocopy of these papers but have been unable to trace the full source. They are both in the 'Letters to the Editor' section of the February 1981 edition of Trends in Biochemical Sciences. They do not appear to be in Pubmed, or anywhere else on-line. Has anyone ever seen these references quoted, or can provide me with a full source?. I'll update if I find anything)

Atkinson, D.E. (1981) TCA Cycle Confusion. Trends in Biochemical Sciences (February 1981 edition; full ref to follow)

Banfalvi,G. (1991) Conversion of Oxidative Energy to Reductive Power in the Citrate Cycle. Biochemical Education, 19 , 24 - 26 [see here] (pdf apparently free to all)

Brière, J.J., Favier, J, Gimenez-Roqueplo, A.P. & Rustin, P (2006) Tricarboxylic acid cycle dysfunction as a cause of human diseases and tumor formation. Am J Physiol Cell Physiol, 291 , C1114-20. [Pubmed] [pdf]

Chappell (1964) The oxidation of citrate, isocitrate and cis-aconitate by isolated mitochondria. Biochem J., 90 , 225-237.[pubmed] [pdf]

Herreros, B. & Garcia-Sancho, J. (1981) TCA Cycle Confusion. Trends in Biochemical Sciences (February 1981 edition; full ref to follow)

Madeira, V.M.C. (1988) Stoichiometry of Reducing Equivalents and Splitting of Water in the Citric Acid Cycle. Biochemical Education 16 , 94 - 96 [pdf] (apparently free to all.)

Mego, J.L. (1986) The Role of Water in Glycolysis Biochemical Education, 14 , 130 - 131. (see here)

Racker, E. (1976) A New Look at Mechanisms in Bioenergetics. Academic Press, New York.

Wieser (1980) Textbook Errrors: The splitting of water by the tricarboxylic acid cycle. Textbook error or textbook omission? Trends in Biochemical Sciences, 5 (Issue 11), 284. [see here]. [pdf], apparently free to all.

Edit 4

I have not been able to locate the exact reference for the incomplete Atkinson and Herreros & Garcia-Sancho references cited above but, in response to this great question, I have put a copy of a very bad scanned version here

Another great reference to this controversy which I should have quoted is the following:

Losada, Manuel . (1978) Energy-Transducing Redox Systems and the Mechanism of Oxidative Phosphorylation. Bioelectrochemistry and Bioenergetics, 5, 296-310 [Science Direct].

The following is the relevant section, for those who do not have access:

A more complete version of the Racker reference quoted above by Losada (1978):

However, pyruvate has only 4 hydrogens to donate to 2 oxygens and with a P:O ratio of 3 should therefore yield only 6 molecules of ATP (Table 1.2). Where are the other 6 hydrogens coming from so that 15 ATP can be formed? The answer to this question gives us what I think is the key to the puzzle why nature has designed the complex acrobatic scheme of the Krebs cycle. Its major purpose I believe is to increase the energy yield by catalyzing cleavage of water.

There are three steps at which water enter the Krebs cycle. - one at the transformation of fumarate to malate an the other two somewhat more indirectly during the utilization of acetyl-CoA and succinyl-CoA. In the course of the Krebs cycle the hydrogens of these water molecules are separated from oxygen and are donated to DPN or to a flavoprotein (e.g., succinate dehydrogenase) and then transported via the oxidation chain of mitochondria as electrons and protons as we shall discuss later.

[Racker, E. (1976). A New Look at Mechanisms of Bioenergetics, pp 5-6, Academic Press].

IMO, the last sentence of this quote is complete rubbish. (For DPN, read NAD. DPN, or diphospo-pyridine-nucleotide (or something close), is an old name for NAD)