Re: Where does most of the world's oxygen come from

There is disagreement over which photosynthetic organisms, land plants or 
phytoplankton and algae in the oceans, provide the majority of the earth's 
photosynthesis. Some experts ascribe more than half of current global 
photosynthesis to the oceans. Salisbury and Ross (1985) estimated one third of 
photosynthesis in oceans and two-thirds on land. They presented 1975 estimates 
that rainforests accounted for 22% of global photosynthesis and the oceans 32%. 
We know that oxygen definitely comes from photosynthesis and rain forests do 
produce a significant but decreasing amount because of all the rainforest 
destruction.

Keep in mind that the accuracy of such estimates is not completely known as the 
Woods Hole website states 
"Unfortunately, the global rates of photosynthesis and respiration are neither 
known nor measured well enough to determine annual changes in carbon storage." 
There is a great deal of interest in computer modeling the global carbon-oxygen 
patterns because of global warming/climate change concerns.

Oxygen makes up about 21% or 210,000 ppm of the atmosphere. Atmospheric carbon 
dioxide is now at about 365 ppm, and Salisbury and Ross (1985) state that only 
about 10% of the carbon dioxide in the atmosphere is used each year in 
photosynthesis. If that figure is a good estimate than you can see that a very 
tiny percentage of the earth's total atmospheric oxygen is produced each year 
by photosynthesis, i.e. 36.5 ppm is only 0.017% of total atmospheric oxygen. 
Despite the tiny percentage, annual global photosynthesis is immense. The 
second website cited says annual photosynthesis fixes carbon each year 
equivalent to "a line of railroad cars filled with coal stretching between 
Earth and the Moon 45 times."

Right now, of course, the earth's oxygen level is falling every so slightly as 
the carbon dioxide concentration increases about 1.5 parts per million (ppm) 
each year. The carbon dioxide increase/oxygen decrease is due to deforestation 
and burning of fossil fuels. Thus, we are using some of the atmospheric oxygen 
added by ancient photosynthesis. Fossil fuels, such as coal, oil and natural 
gas represent  carbon fixed by ancient photosynthesis. Fossil fuels can also be 
considered to represent photosynthetic oxygen placed in the atmosphere tens-of-
millions to hundreds-of-millions of years ago. Therefore, you can consider that 
you are breathing some oxygen originally added to the atmosphere by giant 
lycopods, tree ferns and giant horsetails from the Carboniferous era 360 to 286 
million years ago.   


References

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.


Woods Hole Research Center Carbon Cycle


Earth's First-Ever Global Biological Record Documented from Space


Global Terrestrial Ecosytem Carbon Model


The Carboniferous


Carboniferous Forests