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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? $\endgroup$
    – WYSIWYG
    Jun 13, 2015 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, 2015 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, 2015 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, 2015 at 11:31
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    $\begingroup$ Unfortunately, this, as a relatively highly upvoted question, doesn't show research effort whatsoever. It seriously doesn't take more than a few Googlings to find out what 'aquatic' organisms can photosynthesize. $\endgroup$
    – M.A.R.
    Jun 14, 2015 at 19:18

3 Answers 3

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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, 2015 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, 2015 at 12:32
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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."

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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.

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