Geoscience Reference
In-Depth Information
We cannot dismiss that aerobic photosynthesis, the type which
principally occurs today, could have appeared very quickly after
anaerobic photosynthesis and that it could have rapidly become the
principal contributor to the global accumulation of oxygen.
1.3.4.
A moderate increase in oxygenation (between 2.5 and 0.5
billion years ago)
After the first rapid accumulation of oxygen, around 2.5 billion
years ago (beginning of the Proterozoic Period), it is very likely that
the terrestrial environment continued to accumulate oxygen, albeit
more slowly. Reconstructions [BER 04] suggest levels of O
2
in the
atmosphere were in the order of only 10 - 15% 500 million years ago
(either 0.1 and 0.15 bar of
p
O
2
for a hypothesis with an atmospheric
pressure of 1 bar). This signifies a very slow augmentation average, in
the order of 4% volume increase per billion years.
The cause of the deceleration that followed the initial rapid
accumulation of O
2
is based on evolutionary adaptations that permitted
an increase of oxygen recycling from organic matter, thus consuming
more of the oxygen produced by photosynthesis. The first of these
adaptations was aerobic respiration, which is only, in fact, the
biological version of the oxidation of organic matter by oxygen as
described above. This adaptation could only appear after anaerobic
photosynthesis, under the selective pressure of an environment at least
temporarily oxygenated, and could only spread with the first global
accumulation of oxygen.
Aerobic respiration has a better energy yield than anaerobic
respiration. As such, it provides the cell the advantage of
counteracting the toxic effects of oxygen. In this case, it is possible
that the preferential oxidation of certain molecules (external fuel or
synthesized reserves of the ATP type) enabled the preservation of
essential molecules of the cellular machinery (DNA, RNA, proteins).
From the moment aerobic respiration was selected for, other
evolutionary refinements could develop, particularly in the ocean.
