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I know plants form a critical part of the earth's biosphere, including molecular oxygen production. Molecular oxygen makes up ~20% of our atmosphere. How much of the earth's molecular oxygen in the atmosphere is due to plants? Is there a minimal level of plant life necessary to maintain levels of oxygen so that land mammals can live? Also, are land plants mostly important or are underwater plants a significant source of atmospheric oxygen?

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    $\begingroup$ The answer to the question is none. Earth's oxygen comes from the stars mostly. Now, it is clear that you are asking about Earth's oxygen in a molecular form. I think the question needs a good edit to clarify $\endgroup$ – arkaia Jan 30 '15 at 12:45
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The oxygen in the atmosphere was produced by cyanobacteria during the Great Oyxgenation Event, around 2.3 billion years ago. Cyanobacteria produced & still, produces, to a lesser extent, oxygen by photosynthesis. Plants did not exist during the Great Oxygenation Event, but these day plants replenish atmospheric oxygen by removing carbon dioxide by photosysnthesis, like cyanobacteria. According to David Biello, The Origin of Oxygen in the Earth's Atmosphere the production of oxygen by cyanobacteria during the Great Oxygenation Event took one billion years.

During the Carboniferous period, about 350 million years ago, the amount of oxygen in the atmosphere increased to 30% because the rate of burial of organic matter prevented it from reacting with atmospheric oxygen, History & Significance of Oxygen in the Atmosphere

According to Wallace S. Broecker our atmospheric oxygen supplies are vast:

Simply put, our atmosphere is endowed with such an enormous reserve of this gas that even if we were to burn all our fossil fuel reserves, all our trees, and all the organic matter stored in soils, we would use up only a few percent of the available $\ce{O2}$. No matter how foolishly we treat our environmental heritage, we simply don't have the capacity to put more than a small dent in our $\ce{O2}$ supply.

From Atmospheric oxygen - Broecker

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    $\begingroup$ Broeker also says, "the Earth's plants produce through photosynthesis an amount of O2 equal to that of the atmosphere in about 2,000 years". Of course they're simultaneously absorbing oxygen, that's why it's called a cycle and why removing them wouldn't mean we asphyxiate in 2000 years. But it's sometimes significant what speed the cycle operates relative to the reservoir. It's not like we're still coasting on a 2.3 billion-year-old, or even 350-million-year-old, store. $\endgroup$ – Steve Jessop Jan 30 '15 at 15:15
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    $\begingroup$ How could Cyanobacteria produce that much oxygen? Why hasn't such production continued? $\endgroup$ – Stan Shunpike Jan 30 '15 at 17:56
  • $\begingroup$ I would challenge the idea that the oxygen present in the atmosphere TODAY was produced by cyanobacteria several billion years ago. Today's oxygen is produced by plants, and cycles through in about 4500 years. (Per Wikipedia, anyway: en.wikipedia.org/wiki/Oxygen_cycle#Capacities_and_fluxes ) $\endgroup$ – jamesqf Jan 30 '15 at 19:14
  • $\begingroup$ @jamesqf Suppose you are correct. Then what relevance is the Great Oxygenation Event have for today's atmosphere? In other words, that would mean today's plants would be producing all the oxygen, right? $\endgroup$ – Stan Shunpike Jan 30 '15 at 20:21
  • $\begingroup$ @Stan Shunpike: Yes. Or to be more precise, they are driving the Oxygen Cycle (see the Wikipedia link) that keeps the atmospheric O2 concentration at about 20%. Really simplifying the process, animals breathe O2, which combines with food to produce growth & movement. The combination produces CO2, which gets converted back to O2 and biomass by plants. $\endgroup$ – jamesqf Jan 30 '15 at 21:05
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Yes today’s photosynthesis is occurring but so is the oxidation of the carbon based plant material when they decay and consumed. In order to have a surplus molecular Oxygen quantity the products of photosynthesis cannot undergo oxidation. Our biggest threat is the diminishing of molecular Oxygen because that is what absorbs UV energy and releases it as light before the UV can reach the Earth’s surface and become light and heat where it can cause the most damage with retainment of destructive forces at the earths surface. Remember that petroleum and coal are the products of photosynthesis and we are consuming them at an exponential rate.

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  • $\begingroup$ "Molecular oxygen" is commonly understood to be O₂. The form of oxygen that absorbs UV radiation is ozone, O₃. $\endgroup$ – Gimelist Feb 25 at 12:38
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Depends on your definition of "plants", if you simply mean organisms using chloroplastic photosynthesis which is the most basic definition then the answer is almost all of it. If you want to know how much of the oxygen we breath is from land plants like trees and grasses the answer is roughly 55% annually with 45% being from phytoplankton blooms with trace amounts generated by UV radiation acting on water and certain nitrogen oxides in the upper atmosphere.

It's hard to say what a plant minimum for mammalian survival would look like because the availability of atmospheric oxygen would effect the interactions of that oxygen with the biosphere, hydrosphere, and lithosphere. Also which mammals? Mole-rats need significantly less oxygen per kilogram than humans which need less than mice.

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