of which is their responsibility, direct or indirect, for the
production of nearly all of the oxygen we breathe. Oxygen is
produced during photosynthesis by a reaction that can be written
as CO(sub2) H(sub2)O --> CH(sub2)O O(sub2). Here,
'CH(sub2)O' is a geochemist's shorthand for more complex forms
of organic matter. Most photosynthesis on land is carried out by
higher plants, not microorganisms; but terrestrial
photosynthesis has little effect on atmospheric oxygen because
it is nearly balanced by the reverse processes of respiration
and decay. By contrast, marine photosynthesis is a net source of
oxygen because a small fraction (approximately 0.1%) of the
organic matter synthesized in the oceans is buried in sediments.
This small leak in the marine organic carbon cycle is
responsible for most of our atmospheric oxygen."