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?