Trees use carbon dioxide and produce oxygen in the presence of sunlight.

But is there any other source?

If yes, are trees the most important source of oxygen, or is there any other source which produce more than trees do?

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    $\begingroup$ Like I mentioned in your previous post— did you google something about it? Did you check about photosynthetic organisms other than land plants? $\endgroup$
    Jun 13 '15 at 14:46
  • $\begingroup$ Yeah, I found that water molecules are split apart after being struck with ultraviolet radiation and oxygen called photolysis and many other chemical reactions which happens in earth atmosphere. I didn't mentioned as it will be quite off topic. $\endgroup$
    – Shashank
    Jun 13 '15 at 15:25
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    $\begingroup$ Why trees? They represent a very small part of the plant population of the planet. They just happen to be big. $\endgroup$
    – terdon
    Jun 13 '15 at 16:23
  • $\begingroup$ @terdon - in terms of biomass trees do account for a lot of vegetation, and yes, because they are big. It's not about numbers, but mass. $\endgroup$
    – AliceD
    Jun 14 '15 at 11:31
  • $\begingroup$ @AliceD in terms of biomass, I'm pretty sure they represent a very small fraction when compared to photosynthesizing algae and phytoplankton as you point out in your answer. Let alone the grasses. Individual trees are big, yes, but there are not that may of them. In any case, 1) I only mentioned the plant population, not biomass and 2) only a very small part of a tree's biomass is actually photosynthesizing so biomass is not really relevant anyway. $\endgroup$
    – terdon
    Jun 14 '15 at 11:35

71% of the earth's surface is taken up by water. Not surprisingly therefore, the seas are an important source of oxygen. National Geographic claims that photosynthesis by phytoplankton (mostly single-celled phototrophs, such as cyanobacteria, green algae and diatoms) account for half of the earth's oxygen production. The other half, they claim, is produced on land by trees, shrubs, grasses, and other plants.

The Ecology Global Network takes it a step further and claims that all marine plants (including phytoplankton) together produce 70 to 80 percent of the oxygen in the atmosphere.

Based on these reports, hence, marine phototrophs account for 50 - 80% of the earth's oxygen production.

With regard to terrestrial oxygen production, NASA reports that 30% of the land is covered by trees, and as much as 45 percent of the carbon stored on land is tied up in forests. So on land, trees are definitely large contributors to oxygen production.

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    $\begingroup$ Not to mention that natural forests are going to be almost entirely oxygen-neutral on average; your typical natural forest will produce as much oxygen as it consumes (the dead trees will eventually decompose, consuming oxygen and releasing carbon dioxide among other things). To make them an oxygen-positive system, you need to prevent the decomposition - for example, in a bog (as in the trees we now mine as coal), or by cutting the trees down and making stuff out of the wood. So it's actually the industrial forests that consume more carbon dioxide on average than they produce. Now. $\endgroup$
    – Luaan
    Jun 15 '15 at 7:16
  • $\begingroup$ It's worth adding that trees are a very recent addition to the planet's oxygen budget. It was the evolution of cyanobacteria in the early oceans that produced all that oxygen in the first place! $\endgroup$ Jun 15 '15 at 12:32

I am not sure which class of organisms have the highest contribution in oxygen production but diatoms do have a significant contribution. The introduction in this paper says that diatoms account for 40% of marine photosynthesis which according to this site is "1/4 of the oxygen we breathe."


Trees are definitely not the only source of oxygen. First, all green plants do photosynthesis, not only trees. Moreover, about half of all photosynthesis on earth is done by microorganisms in the oceans known as phytoplankton.


I thought it was fairly well understood that trees make NO net contribution to the oxygen supply.

As a tree (or any plant) grows, it locks carbon within itself and releases the O from the CO2 into the atmosphere. When that tree dies, it decays by being consumed. All of the C gets recombined with O2 during the decay. Therefore, a quantity of oxygen is lost from the atmosphere and this loss is, by definition, exactly equal the the oxygen produced when it grew.

The best we can say is that trees are constantly locking up a huge quantity of carbon (and therefore allowing an equally huge amount of free oxygen to exist in the atmosphere) but no net gain/loss ever takes place.

Of course, this still doesn't excuse destroying rain forests because that definitely does release CO2 and absorb oxygen in the process.

  • $\begingroup$ Downvotes? Very interesting that no argument was felt necessary before doing so. $\endgroup$
    – Lefty
    Jun 14 '15 at 8:19
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    $\begingroup$ It is true that over the long run, only the geologic carbon cycle makes a net change in the CO2-O2 balance, but that's not what's being discussed here. $\endgroup$
    – Mark
    Jun 14 '15 at 8:53
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    $\begingroup$ @Mark What I'm trying to imply is that the question itself is deeply flawed therefore answers should all be starting with that information and explain why it's wrong before then trying to give a true picture of where the oxygen comes from. $\endgroup$
    – Lefty
    Jun 14 '15 at 10:27

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