Geoscience Reference
In-Depth Information
because the energy supply of light from the Sun was less random than
that of organic fuel in the first ocean. Some anaerobic photosynthetic
cells still exist today in a few rare anoxic environments where light
manages to penetrate. They live, for example, in the subsurface of
certain fine deposited sediments subject to marsh tides [SEI 93].
To simplify the discussion, below, we will only speak about
oxygenic photosynthesis (which produces oxygen) and leave aside the
more anecdotal case of photosynthesis which does not produce
oxygen, even though it could have played an important role at the very
beginning of life on Earth.
The simplified equation for oxygenic photosynthesis is:
H
2
O
+
CO
2
+
light → CH
2
O + O
2
[1.1]
This formula is the same for photosynthetic anaerobic life forms
(which can only live in the absence, or near absence, of oxygen,
essentially bacteria) and aerobic life forms (bacteria, algae, terrestrial
plant life). In effect, oxygen is certainly a product of this formula, but
this does not imply that O
2
is locally concentrated to the point where
photosynthetic cells are necessarily aerobic. The adaptation of these
cells, aerobic or anaerobic, can differ according to the confinement
and the concentration of oxygen that results, but the balance
in carbon
and oxygen from oxygenic photosynthesis is always the same.
The CH
2
O is evidently only an approximation of the average
proportion of carbon, hydrogen and oxygen of the organic molecules
of the cell. Moreover, this formula neglects the role of the other
necessary elements (nitrogen, phosphorus, iron, etc.).
The key trait of this balance is that the production of organic matter
by photosynthesis is accompanied by a production of oxygen (O
2
).
The anaerobic photosynthesis in the ocean was the first source of O
2
in the terrestrial environment since the first ocean was an anoxic
environment. Still, the existence of this source was certainly a
necessary, though not sufficient, condition for the oxygenation of the
environment. This condition, still true today, can only be produced by
