Explore this cemetery for graves, information and tombstones for names in Sterling Cemetery in Sterling, Kansas, a Find A Grave Cemetery.
It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis. The rest is produced via photosynthesis on land by plants. Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO 2 / O 2 balance since the early Precambrian Eon.
One of those tests was to determine the percentage of O2 in the atmosphere. Too much is just as dangerous as too little. Gary Elmer is correct in stating that the O2 content at sea level is typically 20.9 percent. However, some chemical process can produce free oxygen, increasing the content.