Photosynthesis- Irradiance Curves and

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Photosynthesis- Irradiance Curves and Their
Photoadaptive Responses
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Photophysiological Adaptive Responses to Changes
in QPAR (not UVR)
.. Are evident in whole cell photosynthesis-irradi
ance curves
Ps Pmax . tanh (I/Ik) without photoinhibition
Ps Pmax . tanh (I/Ik) - photoinhibition,
expressed as Exponential or tanh function
Inverse logic can be used to look at changes in
P-I curves, accompanied by other whole cell
measurements conducive to field research, to
deduce what kinds photoadaptive responses are
being evoked over a wide range of phytoplankton
types and ecosytem conditions.
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Comparison of P-I curves for different algae
under different growth conditions
(figure from your readings)

• Which has the highest Pmax?
• Which has the highest alpha?
• Which has the lowest Ik?
• At 50 mEin m-2 s-1, which has the greatest rates
  of Ps?
• At 100 mEin m-2 s-1, which has the greatest
  rates of Ps?
• Above what QPAR does Ceratophylum
• Ps rates exceed those of Hydrilla?

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P-I curve with photoinhibition
Pmax/alpha Ik
Pmax/beta Ib
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4 adaptive strategies observed in P-I curves
Pigment-dependent strategies (most common in
phytoplankton Qpar adaptation)
1. Increasing/ decreasing LHCs in response to
decreasing/increasing Qpar
2. Increasing/ decreasing total no. Ps I and Ps
II rxn ctrs LHCs in response to
decreasing/increasing Qpar
Pigment-independent strategies
3. Changes in Rubsico concentrations (seen in
higher plants, less so in algae)
More often evoked as response to temperature
changes
4. Uncoupling or downregulation of PSI and PS
II from LHCs
typical of diurnal periodicities due to midday
photoinhibition by Qpar and diel periodicities in
photosynthesis due to biological clocks
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Increasing/ decreasing LHCs in response to
decreasing/increasing Qpar
Whole cell Pmax remains the same, while Pmax/Chl
changes
Whole cell alpha changes , why?
Predicts alpha/chl will change but in fact often
remains the same in experiments (see next figs)
Ik changes as inverse of irradiance levels, why?
Can you predict by how much?
Cell pigment concentrations AND pigment ratios
change how? Why?
All these measurements can be made in lab, but
only some in field? Which ones? Consequences??
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Light Adaptation in a model dinoflagellate, based
upon LHC change
Note the LHC change is effective in maintaining
whole cell Pmax over a 10-fold change in
irradiance levels. Keeps growth rates maximal
over same light range. If you had Chl-specific
data only, would you be able to conclude the
same??
Note the relative quantum efficiency for Ps,
e.g. alpha/Chl, remains the same, in
contradiction to predictions if LHC concentration
changed alone. Appears that when LHC
concentrations increase, so do their relative
quantum efficiencies (perhaps a role for nuclear
multigene families). Evidence for this increase
in quantum effiencies seen in action spectra
measurements not shown here.
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Increasing/ decreasing total no. Ps I and Ps II
rxn ctrs LHCs in response to
decreasing/increasing Qpar
Whole cell Pmax changes, while Pmax/Chl remains
the sameopposite from LHC adaptive strategy
Whole cell alpha changes , why?
alpha/chl remains the same
Ik is unchanged, why?
Cell pigment concentrations BUT NOT pigment
ratios change Why?
All these measurements can be made in lab, but
only some in field? Which ones? Consequences??
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Example of dinoflagellate that photoadapts by
chaning number of Ps rxn centers LHCs
NB a single specie appears to displays only 1 of
the 2 possible pigment-dependent adaptations
NB both pigment-dependent strategies are found
within single classes of phytoplankton
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Example of Pigment-Dependent Photoadaptation of
Primary Production
Dinoflagellate transferred from light-saturated
growth conditions to light-limited growth
conditions back again.
a. Growth rate changes on scale of a few days,
not immediately
Question is the response to return to HL a
reversal of the LL response? Why or why not??
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Example of Pigment-Dependent Photoadaptation of
Primary Production (contd)
f) Things that make you go Hmmm If P/R ratio
is not changing, why did cell division rates
slow?? Where is the extra photosynthate being
diverted?? Could this be the beginning of a
winter survival strategy??
Given what you know, which pigment-dependent
HL---gtLL adaptation do you hypothesize is at work?
Given your hypothesis, what do you think will
happen to alpha/Chl and Ik??
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Example of Pigment-Dependent Photoadaptation of
Primary Production (contd)
Question If you wanted to look for just this
photoadaptive response in situ, which
measurements shown in this example would you need
if you wanted to keep things to a minimum?
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3. Changes in Rubsico concentrations also change
P-I curves without changing cell concentrations
of photosynthetic pigments.
Changes in Rubisco will occur for many reasons
but does not appear to be an approach that is
prevalent when light levels change.
Rubisco /or Rubisco activase activity
concentrations are temperature-dependent in many
plants.
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4. Uncoupling or downregulation of PSI and PS
II from LHCs
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Examples of changes in P-I curves as a function
of time of Day, at 4 stations in the Santa
Barbara Channel
These are diatom-dominated communities.
What would be the experimental design(s) to
gather such data in situ??
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Laboratory experiment to see if Periodicity of
Diatoms is controlled by Biological Clock
Here, experimenter is following Pmax/cell and
Pmax/Chl
Experimental design LD cycles first, then shift
to LL conditions and see if periodicity
continues. First day after LL shift does not
count.
Results are difficult to interpret if pigments
shift at same time as the periodicity signal is
independent of pigments. The deviation between
Pmax/cell and Pmax/chl indicates changes in
cellular Chl a content.
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Same LD ---gt LL experiment in Dinoflagellate
cultures.
Pmax/Cell and Pmax/Chl
Pigment concentrations
What would be the experimental design??
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Phytochrome (red light photoreceptors) regulate
aspects of Biological clocks
Red Blue light photoreceptors involved in
photoregulation in red tide dinoflagellates
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Phototaxis Chlamydomonas sense the direction of
light by a single eyespot (red in the fig.).
The position of the eyespot relative to the two
flagella is always the same. 
    If a cell swimming toward the top of the
screen (1) senses light coming in from the right,
this causes an influx of Ca2 into the flagella
(2).  The 2 flagella respond differently to this
increase in Ca2 one flagellum becomes more
active, and the other becomes less active (3). 
This difference in activity causes the cell to
turn toward the light (4).  Cells can be either
positively phototactic (turn toward the light) or
negatively phototactic (turn away from the light).
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