Factors Affecting
Photosynthesis

Temperature


Light


Eppley (1972)
Sverdrup’s Critical Depth Model
Nutrients
Definition and Units
 Broecker’s Classification Scheme
 Limitations
 Uptake Kinetics

Temperature
•The oceans vary much less than the land
does, both seasonally and daily
•Increased temperature decreases viscosity,
so you sink
•Organisms grow faster, die younger as
temperature increases
•In general, warm water
species are smaller and
have more extensions
Temperature &
Phytoplankton
• Eppley (1972)
plotted species
growth vs. temp.
• Empirically
determined that
all phytoplankton
fit under a curve
Growth Rate
Q10
0
15
30
Temperature (deg C)
Temperature &
Phytoplankton
From Behrenfeld and Falkowski, 1997
Compensation Depth
Positive Net Production
Zc
Positive Net Respiration
Sverdrup’s Critical
Depth Model:
“…there must be a critical depth such that blooming can
occur only if the mixed layer is less than the critical value.”
Assumptions:
• Constant mixing, uniform phytoplankton
• NO Grazers!
• Nutrients are not limiting
• The compensation depth is known
• Production is directly controlled by light
and is linear
Critical Depth
Given the previous assumptions,
the Critical Depth (Zcr) can be
approximated by:
Zcr / (1-e-k•Zcr) = Eo / (Ec x k)
Zcr = Eo / (Ec x k)
Nutrient Distributions
Nutrient Availability



Phytoplankton are most abundant where
there are nutrients
Nutrients are highest near coastal regions
and in upwelling zones
Nutrients and waste products must pass
through the cell membrane
Do Nutrients Really
Diffuse?


However, most phytoplankton cannot rely on
passive diffusion!
Diffusion Mechanisms:
Passive Diffusion (based solely on the
gradient of concentrations)
 Facilitated Diffusion: “channels” allow ions to
move through the cell wall
 Active Uptake: There are
transporters on the cell wall

Uptake Kinetics
Uptake Rate
Passive Diffusion
- Relies on a simple gradient
- Not very efficient
Facilitated Diffusion
- Provides “channels”
Concentration
Active Transport
• Follows Michaelis-Menten Kinetics
• Controlled by # of transporters
And internal enzyme kinetics
Stoichiometry depends on N source and
chemical composition of phytoplankton
Generalized reactions for growth on nitrate and ammonium
1.0 NO-3 + 5.7CO2 + 5.4H 2 O 
→ (C5.7H9.8O2.3N) + 8.5 O2 +1.0 OHP.Q. = 1.49 (O2 evolved / CO2 consumed)
1.0 NH+4 + 5.7CO 2 + 3.4H2 O 
→(C 5.7H9.8O2.3N) + 6.25 O2 +1.0 H+
P.Q. = 1.10
Understand and remember the definition and
significance of the photosynthetic quotient, PQ
Growth on CO2 and the
Macronutrients N and P
It is convenient (and often necessary) to consider the growth
and decomposition of an “average” phytoplankter. Redfield
(Redfield, Ketchum and Richards 1963) showed strong and
profound relationships between dissolved elements that were
consistent with the growth and decomposition of phytoplankton:
C:N:P ~ 106:16:1 - Termed the Redfield Ratios
106 CO 2 + 122 H2 O +16 HNO 3 + H3 PO 4

→ (CH2 O)106 + (NH 3 )16 + H 3PO 4 +138 O 2
Nitrate and phosphate to proteins, phospholipids, nucleotides, etc.
…the implicit PQ is 1.30
Micronutrients (Trace Elements)
e.g.,
Cu, Zn, Ni, Co, Fe, Mo, Mn, B, Na, Cl
Generally, these are required to act as cofactors in enzymes
(Ferredoxin [Fe], Flavodoxin [Mn], Carbonic Anhydrase [Zn])
Iron is well recognized as being in short supply over large parts
of the ocean. It is particularly important in Nitrogen Fixation.
Copper, Zinc and Nickel have also been implicated in
influencing the growth of open-ocean phytoplankton. Trace
element interactions are complex, and incompletely
understood.
N-Metabolism is a Primary Sink For Photo-Reductant
Chloroplast
NADPH
ADP
ATP
[bulk fluid]
NO3-
NO3ADP + Pi
NA
Gln + 2-OXG
GS
ATP Glu +
D(
P)
H
NR
NADP
GOGAT
NH4+
Glu
+
Glu
FDX(red)
NA
D(
P)
NIR
FDX(ox)
+
α ketoacids
Mitochondrion
NO2amino
acids
TCA
Cycle
[cytosol]
[plasma membrane]
Adapted from Falkowski and Raven (1997) Aquatic Photosynthesis
Michaelis-Menten kinetics:
S
V = Vmax ⋅
Ks + S
V = uptake rate (e.g., N taken up per unit particulate N per unit time); d -1
Vmax = maximum uptake rate
Ks = Substrate concentration at which V = V max/2
Consistent with underlying mechanism:
k1
ES
S+E
k2
E+P
k –1
S = substrate; E = enzyme; P = product; k = rate constant
Specific Rate of Uptake (d )
-1
Nutrient-uptake
kinetics and
ecological/evolutionary
selection
Nutrient Uptake
2.5
-1
Vmax = 2.25 d
2.0
II
K = 2.0 µM
s
1.5
I
1.0
-1
Vmax = 1.5 d
K = 0.5 µM
0.5
s
0.0
0
2
4
6
8
10
12
Nutrient Concentration (µM)
Phytoplankton isolated from oligotrophic environments have
lower Ks values than phytoplankton from eutrophic
environments (consistent with prediction based on ecological
theory)
Nutrient kinetics for growth (rather than for
uptake) are more difficult to determine:
experiments involve growth in chemostat
culture
Ks < 0.1 µg-at L-1
Droop Kinetics
µ = µmax(1 - kq / Q)
µ = growth rate
kq = minimum cell quota
Q = current cell quota
Qmax = max cell quota
• If an organism has a high degree of “quota flexibility”, it
can vary the ratio kq/Qmax by quite a bit--this allows for
luxury uptake
• Redfield Ratios are ONLY approximated when µ/µmax
is close to 1
• Therefore, cell composition can provide an indication of
cell growth status, or limitation
Consequently, chemical
composition responds to
growth conditions
N-Limited
<——> N-sufficient
0.12
N:C molar ratio
0.10
A
0.08
0.06
0.04
189 µmol m-2 s-1
0.02
0.00
0.0
-2 -1
63 µmol m s
0.2
0.4
0.6
-1
µ (d )
0.8
1.0
The chemical
composition of
phytoplankton is
very responsive to
growth conditions.
Here, nitrogen
content is lower
when growth rate is
limited by the supply
of N (carbohydrates
are accumulated).
Photoacclimation affects chemical composition
High Light
Low Light
L
L
P
E
P
S
P = Photosynthate
E = Enzymes
S = Storage
L = Light Harvesting
S
E
Sizes of arrows are proportional to flux:
Sizes of boxes ∝ pool size × growth rate
after Geider et al. 1996
Unbalanced growth
High —> Low
Low —> High
L
L
E
P
P
S
Pigment synthesis inhibited
Synthesis of enzymes cannot
accelerate quickly
Photosynthate goes to storage
see Geider et al. 1996
S
E
Pigment synthesis continues
Synthesis of enzymes slows
because supply is reduced
Stored carbon is mobilized into
free sugars
Biological and Solubility Pumps
What Nutrient Controls the Biological Pump?
• Geochemists' viewpoint : nitrogen can be "topped up"
from the atmosphere by the fixation of N2 gas to NO3;
phosphorus has no comparable sources or biological
pathways, therefore phosphorus limits global production
• Biologists' viewpoint : observational and experimental
work finds natural assemblages of phytoplankton are more
nitrogen-stressed than phosphate-stressed and more
responsive to nitrate additions rather than phosphorus
additions, therefore nitrogen limits global production
• What about Iron? How about Silica?