Post on 06-Feb-2018
Factors AffectingPhotosynthesis
Temperature Eppley (1972)
Light Sverdrup’s Critical Depth Model
Nutrients Definition and Units
Broecker’s Classification Scheme
Limitations
Uptake Kinetics
Temperature•The oceans vary much less than the landdoes, both seasonally and daily•Increased temperature decreases viscosity,so you sink•Organisms grow faster, die younger astemperature increases
•In general, warm waterspecies are smaller andhave more extensions
Temperature &Phytoplankton
Temperature (deg C)
Gro
wth
Rat
e
0 3015
• Eppley (1972)plotted speciesgrowth vs. temp.
• Empiricallydetermined thatall phytoplanktonfit under a curve
Q10
Temperature &Phytoplankton
From Behrenfeld and Falkowski, 1997
Compensation Depth
Zc
Positive Net Production
Positive Net Respiration
Sverdrup’s CriticalDepth Model:
“…there must be a critical depth such that blooming canoccur 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 lightand is linear
Critical DepthGiven the previous assumptions,the Critical Depth (Zcr) can beapproximated 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 regionsand in upwelling zones
Nutrients and waste products must passthrough the cell membrane
Do Nutrients ReallyDiffuse?
However, most phytoplankton cannot rely onpassive diffusion!
Diffusion Mechanisms: Passive Diffusion (based solely on the
gradient of concentrations)
Facilitated Diffusion: “channels” allow ions tomove through the cell wall
Active Uptake: There are
transporters on the cell wall
Uptake KineticsPassive Diffusion
- Relies on a simple gradient- Not very efficient
Upt
ake
Rate
Facilitated Diffusion- Provides “channels”
Active Transport• Follows Michaelis-Menten Kinetics• Controlled by # of transportersAnd internal enzyme kinetics
Concentration
Stoichiometry depends on N source andchemical composition of phytoplankton
Understand and remember the definition andsignificance of the photosynthetic quotient, PQ
1.0 NO3- + 5.7CO2 + 5.4H 2O → (C5.7H9.8O2.3N) + 8.5 O2 +1.0 OH-
P.Q. = 1.49 (O2 evolved /CO2 consumed)
1.0 NH4+ + 5.7CO2 + 3.4H2O → (C5.7H9.8O2.3N) + 6.25 O2 +1.0 H+
P.Q. = 1.10
Generalized reactions for growth on nitrate and ammonium
It is convenient (and often necessary) to consider the growthand decomposition of an “average” phytoplankter. Redfield(Redfield, Ketchum and Richards 1963) showed strong andprofound relationships between dissolved elements that wereconsistent with the growth and decomposition of phytoplankton:
Growth on CO2 and theMacronutrients N and P
Nitrate and phosphate to proteins, phospholipids, nucleotides, etc.…the implicit PQ is 1.30
€
106 CO 2 +122 H2O +16 HNO3 + H3PO4
→ (CH2O)106 +(NH3)16 +H 3PO4 +138 O2
C:N:P ~ 106:16:1 - Termed the Redfield Ratios
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 partsof the ocean. It is particularly important in Nitrogen Fixation.Copper, Zinc and Nickel have also been implicated ininfluencing the growth of open-ocean phytoplankton. Traceelement interactions are complex, and incompletelyunderstood.
ChloroplastNADPH NADP
Gln + 2-OXG Glu
Glu + NH4+ Glu
NIR
FDX(red)
FDX(ox)
ADP
ATP
GSGOGAT
+ATP
ADP + Pi
NO3- NO3
-
[plasma membrane]
[bul
k flu
id]
[cytosol]
aminoacids
NAD(P)HNAD(P)NR
NO2-
α ketoacids Mitochondrion
TCACycle
+
Adapted from Falkowski and Raven (1997) Aquatic Photosynthesis
N-Metabolism is a Primary Sink For Photo-Reductant
Michaelis-Menten kinetics:
V = Vmax ⋅S
Ks + SV = 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 = Vmax/2
Consistent with underlying mechanism:
S + Ek 1
k –1
k 2E S E + P
S = substrate; E = enzyme; P = product; k = rate constant
Nutrient-uptakekinetics andecological/evolutionaryselection
Phytoplankton isolated from oligotrophic environments havelower Ks values than phytoplankton from eutrophicenvironments (consistent with prediction based on ecologicaltheory)
0.0
0.5
1.0
1.5
2.0
2.5
0 2 4 6 8 10 12
Nutrient Uptake
Spe
cific
Rat
e of
Upt
ake
(d-1)
Nutrient Concentration (µM)
I
IIV
max = 2.25 d-1
Ks = 2.0 µM
Vmax
= 1.5 d-1
Ks = 0.5 µM
Nutrient kinetics for growth (rather than foruptake) are more difficult to determine:experiments involve growth in chemostatculture
Ks < 0.1 µg-at L-1
Droop Kineticsµ = µmax(1 - kq / Q)
µ = growth rate kq = minimum cell quotaQ = current cell quotaQmax = max cell quota
• If an organism has a high degree of “quota flexibility”, itcan vary the ratio kq/Qmax by quite a bit--this allows forluxury uptake
• Redfield Ratios are ONLY approximated when µ/µmaxis close to 1
• Therefore, cell composition can provide an indication ofcell growth status, or limitation
Consequently, chemicalcomposition responds togrowth conditions
0.00
0.02
0.04
0.06
0.08
0.10
N:C
mol
ar ra
tio
0.12
0.0 0.2 0.4 0.6 0.8 1.0µ (d-1)
A
189 µmol m-2 s-1
63 µmol m-2 s-1
N-Limited <——> N-sufficientThe chemicalcomposition ofphytoplankton isvery responsive togrowth conditions.Here, nitrogencontent is lowerwhen growth rate islimited by the supplyof N (carbohydratesare accumulated).
Photoacclimation affects chemical composition
E
L
S
High LightLow Light
after Geider et al. 1996
PL
S
EP
Sizes of arrows are proportional to flux:Sizes of boxes ∝ pool size × growth rate
P = PhotosynthateE = EnzymesS = StorageL = Light Harvesting
Unbalanced growth
High —> LowLow —> High
see Geider et al. 1996
L
S
EP
Pigment synthesis inhibitedSynthesis of enzymes cannot accelerate quicklyPhotosynthate goes to storage
E
L
SP
Pigment synthesis continuesSynthesis of enzymes slows because supply is reducedStored carbon is mobilized into free sugars
Biological and Solubility Pumps
• Geochemists' viewpoint : nitrogen can be "topped up"from the atmosphere by the fixation of N2 gas to NO3;phosphorus has no comparable sources or biologicalpathways, therefore phosphorus limits global production
• Biologists' viewpoint : observational and experimentalwork finds natural assemblages of phytoplankton are morenitrogen-stressed than phosphate-stressed and moreresponsive to nitrate additions rather than phosphorusadditions, therefore nitrogen limits global production
• What about Iron? How about Silica?
What Nutrient Controls the Biological Pump?