MAP Technical Reports Series No. 106 UNEP

3.2.2 Metabolic processes
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3.2.2.1 Primary production and biomass formation
Primary production is defined as the process by which organic matter is produced from
dissolved inorganic substances. This process requires energy that either comes from radiation
(light), or from inorganic chemical reactions. Considering the magnitude of utilization of light as
energy source by organisms (plants containing chlorophyll), the process involving chemical
reactions is of minor importance, and can be ignored in the context of the objectives of the
present report. Chemodynamic autotrophic processes are the only ones to maintain deep sea
hydrothermal-vent communities.
Photoautotrophic processes are secluded to occur in the euphotic zone, which extends
to the depth at which light intensity is reduced to about 1% of the normal surface irradiation in
the wave length range of between 400 and 700 nm. Light attenuation in water is an exponential
function measured by the so-called attenuation coefficient, §. Accordingly, the depth of the
euphotic zone, z eu, equals 4.6/§. In the most transparent oceanic offshore waters z eu may extend
to more than 200 m. Normally, z eu is much less, and in turbid inshore waters may be only a few
meters, and even less. Light attenuation in water is higher, not only due to mineral turbidity and
dissolved organic substances, such as humic substances, but also due to the presence of biota,
particularly algae. The cross-sectional attenuation coefficient relative to chlorophyll present in
planktonic algae is in the order of 0.01 to 0.02 m 2 /mg Chl a. These relationships, in connection
with the extent and frequency by which periods of density stratification alternate with cycles of
deep water mixing, have a bearing on the degree single bodies of waters are affected by the
process of eutrophication.
Photosynthesis, i.e. the formation of sugars and polysaccharides from CO 2 (carbon
dioxide) and water, is the initial process in primary production. This reaction is expressed in
formal stoichiometry as:
(+ energy)
106 HCO 3 - + 106 H2O ===> 106 C 6H 12O 6 + 106 O 2 + 106 OH - Eq.(1)
(sugar)
Photosynthetic quotient PQ = mol O 2 / mol CO 2 = 106/106 = 1
Accordingly, the incorporation of 106 molecules of carbon dioxide is accompanied by an
equal number of 106 oxygen molecules 3 liberated; i.e. the photosynthetic quotient PQ equals 1.
This oxygen accumulates in the water, and if the process is very intense, as in eutrophic waters
with algal blooms, waters become supersaturated in oxygen, and pH increases.
Phytoplankton biomass formation. In reality the reaction described in equation (1) is
much more complex, and proceeds over various component reactions. Chlorophyll plays the
role of light energy captor and transmitter, and phosphorus bonds (e.g, the ADP-ATP system
acts as transient energy repository that in connection with the NADP-NADPH redox system fuels
important reactions involved in cellular metabolism and the synthesis of proteins and other cell
components). This explains the key role of phosphorus in eutrophication.
3 For consistency with subsequent arguments the molar figure 106 has been used; any
integer, also 1, could be used for the purpose.