Explain light saturation curve.
Answers
There are two simple derivations of the equation that are commonly used to generate the hyperbolic curve. The first assumes photosynthetic rate increases with increasing light intensity until Pmax is reached and continues to photosynthesise at the maximum rate thereafter.
P = Pmax[I] / (KI + [I])
P = photosynthetic rate at a given light intensity
Commonly denoted in units such as (mg C m-3 h-1) or (µg C µg Chl-a-1 h-1)
Pmax = the maximum potential photosynthetic rate per individual
[I] = a given light intensity
Commonly denoted in units such as (µMol photons m-2 s-1 or (Watts m-2 h-1)
KI = half-saturation constant; the light intensity at which the photosynthetic rate proceeds at ½ Pmax
Units reflect those used for [I]
Both Pmax and the initial slope of the curve, ΔP/ΔI, are species-specific, and are influenced by a variety of factors, such as nutrient concentration, temperature and the physiological capabilities of the individual. Light intensity is influenced by latitudinal position and undergo daily and seasonal fluxes which will also affect the overall photosynthetic capacity of the individual. These three parameters are predictable and can be used to predetermine the general PI curve a population should follow.
PI curve Chalker et al 1983.gif
As can be seen in the graph, two species can have different responses to the same incremental changes in light intensity. Population A (in blue) has an initial rate higher than that of Population B (in red) and also exhibits a stronger rate change to increased light intensities at lower irradiance. Therefore, Population A will dominate in an environment with lower light availability. Although Population B has a slower photosynthetic response to increases in light intensity its Pmax is higher than that of Population A. This allows for eventual population dominance at greater light intensities. There are many determining factors influencing population success; using the PI curve to elicit predictions of rate flux to environmental changes is useful for monitoring phytoplankton bloom dynamics and ecosystem stability.
The second equation accounts for the phenomenon of photoinhibition. In the upper few meters of the ocean, phytoplankton may be subjected to irradiance levels that damage the chlorophyll-a pigment inside the cell, subsequently decreasing photosynthetic rate. The response curve depicts photoinhibition as a decrease in photosynthetic rate at light intensities stronger than those necessary for achievement of Pmax.
{\displaystyle P=P_{\max }(1-e^{-\alpha I/P_{\max }})e^{-\beta I/P_{\max }}\,} P=P_{{\max }}(1-e^{{-\alpha I/P_{\max }}})e^{{-\beta I/P_{\max }}}\,
Terms not included in the above equation are:
βI = light intensity at the start of photoinhibition
αI = a given light intensity
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