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food digestion. Moreover, different forms of fermentation have been
domesticated by humans, for the production of cheese, yogurt,
alcoholic drinks, sauerkraut, sausage, bread, products derived from soy
beans (including soy sauce and tofu) and many other food products for
which fermentation helps prolong preservation.
For 2.5 billion years after the appearance of photosynthesis on
Earth, organic matter gradually accumulated in deep marine sediments,
where it was protected from decomposition (oxidation). As O 2 was not
used in the oxidation of this organic matter, this gas accumulated in
the atmosphere until it reached a concentration that was toxic to
anaerobic organisms. They therefore suffered mass extinction, except
those which survived in oxygen-free environments (see previous
paragraph). This is known as the Great Oxygenation Event.
Half a billion years after photosynthesis appeared on Earth, aerobic
organisms began to develop, i.e. organisms that used O 2 for
respiration. Today, aerobic respiration (see section 4.3.2) is used by a
wide variety of living beings, from photosynthetic organisms to
humans. Their respiration, combined with changes in the dynamics of
oceans (whose exact nature is still under debate), progressively
stabilized, half a billion years ago, the oxygen concentration in the
atmosphere to its current value (forms of respiration other than that of
O 2 will be explored in section 4.4.2).
4.4.2. Biogeochemical cycles of oxygen
Figure 4.4 shows the biogeochemical flows of oxygen between
natural reservoirs (below, numbers in italics between brackets refer to
arrows in the figure). Two oxygen cycles are presented here: the first
describes the circulation of this element between the atmosphere, the
biosphere and the lithosphere and the second shows reactions that take
place in the upper atmosphere. Each cycle is composed of a subset of
flows as illustrated in the figure. Several of the oxygen flows
mentioned below were described in section 4.3.2 since the circulation
of oxygen is often coupled with that of carbon.
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