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Photosynthesis rate is also strongly affected by light intensity. The photosynthe-
sis:light (irradiance) curve (P:I curve) shows that photosynthesis increases with
increasing light intensity up to some asymptotic value
P
max
, where the system
becomes light saturated (Figure 5.6). Light is collected by units composed of acces-
sory pigments and a reaction center containing chlorophyll. At low light regime,
photosynthesis increases linearly with light. The initial slope of this part of the P:I
curve is the photosynthesis efficiency factor (
), which means the number of moles
of carbon incorporated per unit of light intensity (quantum yield).
The maximum photosynthesis (
P
max
, or assimilation number) is reached at a
light intensity in which the enzymes involved in photosynthesis cannot act fast
enough to proceed with the excess of light. Several mathematical formulations for
the P:I relationships have been described.
76
One of the most commonly used equa-
tions is the negative exponential with photoinhibition:
α
−
I
P
α
−
βα
I
PP
=−
(
1
e e
)
(5.7)
P
m
m
m
where the normalized photosynthetic rate
P
is a function of incident irradiance
I,
normalized photosynthetic capacity
P
m
, normalized photosynthetic efficiency
α
, and
a dimensionless photo-inhibition parameter (
β
). Respiration is often subtracted from
the right-hand side of the equation.
Both the photosynthesis efficiency factor and the assimilation number depend
on the algae taxon (e.g., dinoflagellates are generally expected to show higher
P
max
than diatoms or green algae) but also vary within one species depending on envi-
ronmental factors such as temperature, nutritional environment, and light regime
Irradiance
FIGURE 5.6
Relationship between photosynthesis and light intensity (irradiance) showing
the photo-inhibition effect of high irradiance. Different lines show P:I curves with different
constants.
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