Geoscience Reference
In-Depth Information
elements other than carbon. An indirect impact could be that of
variations in the acidity of the ocean, which make the development of
certain species of primary producers (phytoplankton) of calcium tests
more difficult. Nevertheless, the quantitative effect has not been proved
yet and it is very possible that some species without tests could rapidly
occupy the nutritive niche left vacant by the disadvantaged species.
We will now return to the instance of regulation of other major
nutritive elements for photosynthesis.
On average, the current organic matter resulting from
photosynthesis (primary production, essentially carried out by
phytoplankton) has a composition of major elements in the following
atomic ratios: C
106
H
263
O
110
N
16
P. Of course, the composition of
different species of phytoplankton can vary slightly from this mean,
but it remains within a narrow range of variations.
The average composition above likely evolved in the Earth's
distant past, in parallel with the oxygenation which modified the
chemical forms of available nutrients, and with the biological
evolution which adapted the phytoplanktonic metabolism to new
conditions. The current average relative element composition of
photosynthetic organic marine matter is, however, known precisely for
carbon, nitrogen and phosphorus (C
106
N
16
P).
This composition means that each time photosynthesis consumes
106 atoms of carbon (via the unlimited use of CO
2
), it must also find
263 atoms of hydrogen (via the unlimited use of H
2
O in aquatic
mediums), 110 atoms of oxygen (via the unlimited use of H
2
O), 16
atoms of nitrogen (mainly via the eventually limited use of NO
3
-
or
N
2
) and 1 atom of phosphorus (via the eventually limited use of
PO
4
3-
).
In the set of major nutritive elements (C, H, O, N, P), it is therefore
essentially nitrogen and phosphorus that limit oceanic photosynthesis.
The atomic ratios C/N = 106/16 and N/P = 16 are called the Redfield
ratios, from the name of the author who brought them to light for the
first time in the 1950s [RED 58]. When the minor nutritive elements
are sufficient, which is most often the case, photosynthesis is limited
