Breathe your oxygen Junior - it'll make you grown up big and strong...

March 27, 2009 8:27 PM

All the big organisms on the planet need a healthy supply or oxygen, yet there are a myriad organisms at the other end of the scale that either ignore oxygen or are actively averse to it. How come none of our large scale creatures have come from this anaerobic stock?

[ Obviously, the ones that find oxygen poisonous would find life in the open ocean or on dry land rather tricky. But why did the oxygen suckers manage to make it to a larger scale when those that didn't give a damn about oxygen stayed on the microscopic scales? ]

posted by twine42 to science & nature (16 comments total)

All the big organisms on the planet need a healthy supply or oxygen...

Far from true. The largest living things are Sequioas, and they produce oxygen as a waste product.
posted by Chocolate Pickle at 8:38 PM on March 27

Big is relative.

A group of 47,000 Quaking Aspen (Populus tremuloides) clones (nicknamed "Pando") in the Wasatch Mountains, Utah, USA, is sometimes considered the world's largest organism by mass, covering 43 hectares (110 acres). It is possible that other unknown clonal colonies of trees rival or exceed its size.
posted by furtive at 8:43 PM on March 27

Your question probably can't be answered satisfactorily. Lots of "why" questions about early life pretty much can only be answered, "Because that's what happened."

Once photosynthesis appeared, photosynthetic organisms proceeded to change the environment on the planet so that oxygen haters had to hide in niches, which left them severely disadvantaged in the evolution game.

All the large organisms you're thinking of (animals) are eukaryotes, and they're all descended from early oxygen breathing single-cell eukaryotes.

Why? Because that's what happened. It maybe could have happened some other way, but it didn't.
posted by Chocolate Pickle at 8:43 PM on March 27 [1 favorite has favorites]

Assuming you meant all the large animals on Earth, I think the simplest explanation is that oxidation is a very energetic reaction, oxygen is pretty available on Earth, and lots of energy is required for animal activity.

The chemistry powering anaerobic earthlings is relatively less energetic, and suited to slower growth and less movement.
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posted by General Tonic at 8:53 PM on March 27

Following Chocolate Pickle's "because that's what happened" thought, you might be interested in reading about the idea of contingency in evolution.
posted by frobozz at 8:57 PM on March 27

Plants do need oxygen. They consume both oxygen and carbon dioxide -- they're for two different processes -- and produce oxygen. They produce more oxygen than they consume, hence the popular notion that they don't need oxygen.

Now, to the original question. We combine oxygen and glucose to get ATP, our cellular fuel. Wikipedia's page on anaerobic organisms leads me to believe that alternative anaerobic ways of getting ATP are much less efficient -- so you don't get as much bang for your buck if you're anaerobic. So this question may be sort of like asking, "why don't we see any cars with steam engines on the road?"


(IANABiologist, and IANYB, and this is all just speculation)
posted by qxntpqbbbqxl at 9:12 PM on March 27 [1 favorite has favorites]

Plants aren't anaerobic. They use oxygen to release the energy that they generate through photosynthesis. They may be net producers of oxygen, but I'm not so sure they can actually survive in an oxygen-free environment.

General Tonic has the right idea; aerobic respiration is much, much more efficient than anaerobic respiration (around 15 times more efficient, looking at standard aerobic respiration as compared to fermentation.) You wouldn't really be able to run a large organism anaerobically. Contingency is an important factor in evolution, but I think there are much better reasons for aerobic respiration to have won out.

[on preview, what qxntpqbbbqxl said]
posted by fermion at 9:13 PM on March 27 [1 favorite has favorites]

Incidentally, see this hilarious Yahoo answers thread regarding plants and oxygen.

(evidently, "oxygen is for breathing"... who knew?)
posted by qxntpqbbbqxl at 9:16 PM on March 27

Chocolate Pickle:

The largest living things are Sequioas,

Not even close. The current recordholders are fungal species within genus Armillaria .

and they produce oxygen as a waste product.

Already debunked above.
posted by IAmBroom at 9:42 PM on March 27 [1 favorite has favorites]

Energy you get from metabolism in most organisms is measured in ATP, this is made by using sugars and fats and stuff, we'll just ignore everything but glucose for now. ATP is a chemical that does fancy stuff and transfers phosphates and such, you can find much more if you are interested. For every molecule of glucose, Eukaryotes get 2 ATP for anaerobic metabolism, and 38 for aerobic metabolism.

There are two main pathways of producing ATP - glycolysis and the electron transport chain. Glycolysis is pretty much universal to life on earth and is anaerobic. You get 2 ATP per molecule of glucose in this cycle. The end products differ: pyruvate is modified in an specific way - humans/mammals/some bacteria get lactic acid, yeast get ethanol.

Or you can put the pyruvate into the citric acid cycle/electron transport chain. You do a bunch of cool reactions, and end up with a bunch of H+ ions. In animals and plants, these reactions happen in the mitochondria and then you use the H+ ions to pump a watermill kind of thing which gives you a total of 36-38(!) ATP per glucose in. Bacteria do the same thing with a bit of modifications. Oxygen is the terminal electron acceptor in aerobic metabolism, as it has a really really good capacity for accepting electrons. Pretty much, if you use anything else you get less ATP per glucose. Some anaerobes use something else, some just make do with the 2 ATP.

Most molecules that have this much potential for accepting electrons don't hang round very long, and are potentially damaging (oxygen is too, see the antioxidant health craze). Some theories think that when oxygen first began to be a significant component of the atmosphere it killed out most of the current metabolisms present.

In addition, think of the time the taxa (multicelled animals/plants vs bacteria/archea) have been around. We are one very small branch of the total life on earth, and as such are a much smaller sample of its metabolic diversity. If we don't have the variation and a pressure, we can't evolve to use something else.
posted by scodger at 10:06 PM on March 27 [4 favorites has favorites]

Clearing up something above: aerobic and anaerobic usually refer to respiration. Plans to not breathe oxygen, they make it. True, they use the oxygen that they make, but then they release the rest into the environment. This oxygen production was likely an instance of coevolution with the oxygen breathers who gave so much back to the plants (ecosystems 101!). In conclusion though: plants are anaerobic.

Also: Fungi are the biggest organisms. How big? Impossible to say for sure, but 1,665 football fields is a good estimate (Note: despite this link being pretty good, mushrooms do not have 36,000 sexes. That does not make sense).

You are of course referring to animals though. Oxygen breathers need oxygen for one main purpose: to make ATP. You'll probably remember from high school biology all that stuff about mitochondria and electron transport chains? Well that's pretty much it. Oxygen is an electron receptor at the end of the electron transport chain.

Anaerobic organisms aren't seen all that much (aside from plantae) because we as humans tend to stick to where we can breathe. They're still going strong though. It's possible that if they had evolved quicker than we did, they'd be the ones writing this question to AskMe. They seem to have fallen behind in chasing the red queen (basically, all organisms must always be evolving otherwise they'll fall behind and go extinct... or at least they won't be as secure as they used to be). As it stands though, ATP generation and use (that's where all our energy comes from) is a really great system with some pretty complicated machinery involved. That's why us oxygen breathers are so in charge.

Why did we get all the cool ATP stuff and anaerobic species seem to be missing out? Maybe that's a question best filed away with other related questions, like "Why are all of life's amino acids L-amino acids?". It's a crazy world we live in!
posted by battlebison at 10:22 PM on March 27

Clearing up something above: aerobic and anaerobic usually refer to respiration. Plans to not breathe oxygen, they make it. True, they use the oxygen that they make, but then they release the rest into the environment. This oxygen production was likely an instance of coevolution with the oxygen breathers who gave so much back to the plants (ecosystems 101!). In conclusion though: plants are anaerobic.

I am not a scientist, but I did stay awake in Earth Science class. That is wrong*. Plants use oxygen. Plants respirate. Plants obtain energy through aerobic respiration.

(Link)

*No offense, and of course, I could be mistaken somehow.
posted by Nonce at 11:17 PM on March 27

Or there's the evolutionary angle. If there were more oxygen consumers than oxygen creators then the oxygen generators would die out. etc, etc. So clearly oxygen isn't being consumed by too many creatures or we'd all asphyxiate.* One evolutionary way to look at it is early (simple, small) creatures shat oxygen, by chance. Later evolutionary material (more complex, larger) creatures learned how to live off of that waste.

The next evolutionary generation will live off of TV broadcasts and styrofoam.

*All things being equal and not bringing humanity's mucking with the equations into it.
posted by Ookseer at 2:10 AM on March 28

@Nonce: Sorry, I should have clarified that plants respire (since they make ATP), but the oxygen they breathe is going out. Because of a concentration gradient, CO2 enters through the stoma and O2 leaves. After doing some poking around though, I am mistaken, plants are generally considered aerobic because of their internal use of oxygen, not because of a reliance on external sources. Sorry about that!
posted by battlebison at 9:36 AM on March 28

(Note: despite this link being pretty good, mushrooms do not have 36,000 sexes. That does not make sense).

Nonce, neither do flying furry mammals, lizards that squirt blood out of their eyes, octopuses that can mimic dozens of other animals and unlock complicated mechanisms, parasites that can make snails go insane & pulsate with color, etc.

Nonetheless, all are true.

In the case of the fungi (mushrooms are only the fruiting bodies of the organism, BTW), this is accomplished with multiple sex-determining genes. Instead of an X-Y gene, there are three such genes in one fungi I'm aware of, and instead of only two choices (X or Y), many are possible. Example:

Schizophyllum commune:
specificities
...thus rendering 9*32*9*9 = 22,328 sexes.

That is, Fergie the Fungus is of gender Aα1+Aβ17+Bα-9+Bβ2. She* can mate with any Schyzophyllum commune except another Aα1+Aβ17+Bα-9+Bβ2 (because that would be unnatural, and violate God's laws, and be an abomination unto the Lord).

Why? Simple. If Fergie spews forth her spores (in a church-sanctified, committed relationship) into the wind, and they land on another mushroom's spores, the odds are 22327 to 1 that the two fungi will have a beautiful, bouncing, baby fungus. But if her own spores clump together, which is far more likely given how many of her spores are in the immediate area, nothing will happen. This ensures the advantage of sexual reproduction.

* English only has 3 gendered pronouns; luckily, Aα1+Aβ17+Bα-9+Bβ2 fungi self-identify as females.
posted by IAmBroom at 2:08 PM on March 31

Oops, some text got deleted:

Schizophyllum commune has the following sex genes:
Aα gene: 9 different specificities
Aβ: 32 specificities
Bα: 9 specificities
Bβ: 9 specificities
...thus rendering 9*32*9*9 = 22,328 sexes.
posted by IAmBroom at 2:10 PM on March 31