Oxygenic photosynthesis by micro-organisms

ScienceWeek: "Microorganisms are important for many reasons, not the least
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."