Geoscience Reference
In-Depth Information
4.4.1.
The Great Oxygenation Event
All eukaryotic photosynthetic organisms (eukaryotes are organisms
whose cells possess a nucleus; photosynthetic eukaryotes include
plants and algae) use oxygenic photosynthesis (which generates O
2
),
whereas almost all prokaryotic photosynthetic organisms (prokaryotes
are organisms whose cells do not possess a nucleus, e.g. bacteria) use
anoxygenic photosynthesis (which does not generate O
2
). A
single group of prokaryotes uses oxygenic photosynthesis, i.e. the
cyanobacteria,
which are highly abundant in the ocean. Hence
the absence of O
2
in the atmosphere of a planet would not necessarily
mean that this planet is devoid of life, since life there could be
anoxygenic (as was the case on Earth for more than a billion years),
but the presence of O
2
in the atmosphere of this planet would be an
unequivocal indication of the presence of life.
However, O
2
is a poison to living organisms. It is a deadly poison
to strict or obligatory anaerobic organisms, such as certain bacteria
that cannot tolerate the presence of any oxygen. At high
concentrations, O
2
is a violent poison even for aerobic organisms
(who require oxygen for respiration, for example humans) and also for
facultative anaerobic
bacteria (which prefer oxygen to be present, but
can live without it). At low concentrations, O
2
is a slow poison for all
organisms using it. The oxygen that we breathe is essential for our
survival, but it gradually causes our decline since reactive oxygen
species (often called free radicals) are among the causes of ageing.
Oxygenic photosynthesis (simply called photosynthesis from here)
was “invented” by aquatic cyanobacteria three billion years ago.
Before then, only anaerobic organisms existed on Earth and, when O
2
began increasing in the atmosphere, they could only survive in places
without oxygen. Today, we find anaerobic organisms in marine
sediments, in aquatic zones devoid of oxygen and, which may seem
unlikely, sheltered inside the bodies of aerobic organisms. For
example, there are a thousand times more anaerobic bacteria than
aerobic bacteria in the digestive tract of human beings, where the
anaerobic process of fermentation (see section 4.3.2) contributes to
