ASLO Aquatic Science Meeting, Nice, France 29 January, 2009
Optimal uptake kinetics: physiological acclimation explainsthe pattern of nitrate uptake by phytoplankton in the ocean
S. Lan Smith, Yasuhiro YamanakaEcosystem Change Research Program, FRCGC (JAMSTEC), Japan
Markus Pahlow & Andreas OschliesLeibniz Institute of Marine Sciences at Kiel Univ. (IFM-GEOMAR), Germany
S. Lan Smith, FRCGC ECRP
1V(S) = [ (As S)−1 + (Vmax)−1 ]
More generalReduces to MM as a special case
Affinity-based Equation (Aksnes & Egge, MEPS, 1991)
Optimal Uptake (OU) Equation (Pahlow, MEPS, 2005)
Uptake Sites more sites => Greater Affinity, A (lower Ks)Internal Enzymes more enzymes => Greater Vmax
Both are mostly protein& containlots of N.
US
Michaelis-Menten (MM) Equation
Uptake Rate, U(S) = [ Ks + S ]Vmax S
Rate Expressions for Nutrient Uptake
fA = fractional allocation of internal N: A = A0 fA Vmax= V0 (1 − fA)
Acclimation
NutrientIons
Ion Channels= Uptake Sites
Cell
Internal Enzymes
Low Nutrient Conc. High Nutrient Conc.
S. Lan Smith, FRCGC ECRP
OU Equation for a single (limiting) nutrient assuming instantaneous acclimation (Pahlow, MEPS, 2005):
VOU = V0 SL
(V0 ) + 2(V0 SL)0.5+ SL A0 A0
for Limiting nutrient, L with conc. SL Pahlow's Optimal Uptake Equation came from combining these two equations:
VLim = 1
fA = 1
[(1−fA)V0, L ]−1 + [fAA0, LSL]
−1 (A0, LSL)1/2 + 1 V0, L
Affinity-based uptake equation (Aknes & Egge, MEPS, 1991), asmodified by Pahlow (MEPS, 2005) so that V0 and A0 vary with N allocation.
Optimal acclimationequation (Pahlow, 2005).He assumed instantaneous acclimation.
Optimal Uptake Kinetics: Combining two equations
S. Lan Smith, FRCGC ECRP
for Limiting nutrient, L with concentration SL Pahlow's Optimal Uptake Equation came from combining these two equations:
VLim = 1
fA = 1
[(1−fA)V0, L ]−1 + [fAA0, LSL]
−1 (A0, LSL)1/2 + 1 V0, L
Here "SL" is the concentration in thebottles for incubation experiments.
If phytoplankton do NOT have time to acclimate,The "S" in this equationis NOT the instantaneousconcentration, SL. Let Sa be the concentrationto which phytoplankton have acclimated,BEFORE sampling andbefore the expts. to measure uptake rate.
SL
expts. ship
Sa
Ocean
sample water,add nutrients
Short-term Approximation for Optimal Uptake Kinetics
S. Lan Smith, FRCGC, JAMSTEC April, 2008
Sa = concentration in seawater (before sampling).
This short-term approximation predictshow Vmax and Ks (fit to the MM eqn.) should vary with nutrient conc., Sa
It applies over timescales shorter than the time required to acclimate.
S is the concentration in the bottles during incubation expts.
The OU Equation (Pahlow, MEPS, 2005)can be re-arranged to:
VOU =
( A0 Sa)0.5
V0 S V0 1 + (
A0 Sa)0.5 V0 (
V0 Sa)0.5 + S A0 Thus,
Ks = ( V0 Sa)0.5
A0
Ks values come from fitting the MM equation to experimental data. Modern expts. usually last < 2-3 hours, to minimize changes in concentration.
Short-term OU Approximation predicts changes in Ks
S. Lan Smith, FRCGC, JAMSTEC 6 November, 2008
Kapp
(µm
ol L
-1}
[NO3] (µmol L-1]
0.0
0.5
1.0
1.5
0 1 2 3 4
KN = 0.7
OU
short-term OU
Michaelis-Menten (MM) Equation:
VMM = Vmax S Κs + S where S is nutrient concentration
OU Equation for a single nutrient (Pahlow, MEPS, 2005):
VOU = V0 S (V0 )
+ 2(V0 S)0.5 + S A0 A0
Acclimated (long-term) response
Uptake Rate Equations
The New The Classic
Short-term OU approximation
VOU =
( A0 Sa)0.5
V0 S V0 1 + (
A0 Sa)0.5 V0 (
V0 Sa)0.5 + S A0
Apparent Half Saturation "Constants" Predicted by OU kinetics
S. Lan Smith, FRCGC, JAMSTEC April, 2008
log NO3
log KNO3
-2.0 -0.5 0.5-1.5
-0.5
0.5
1.5
log Ks = − 0.089 + 0.62 log NO3
log Ks = − 0.152 + 0.50 log NO3
Data (x) from marine field studies as compiled by Collos (2005)
Least-squares fits to the data:
n = 38 data pts.
Collos et al. (J. Phycol., 2005) examined the relationship between half-saturation constant for nitrate uptake and nitrate concentration, using data complied from many field studies. They found a linear relationshipbetween the log-transformed data. log KNO3 = a + b log [NO3]
which is equivalent to: KNO3 = a [NO3] b
Optimal Uptake kinetics predicts: KNO3 = a [NO3] 0.50
b is not significantly different from 0.50(ANOVA, p < 0.20)The data is cosistent with OU kinetics.
Significant relationship(b != 0) at level p < 10-9
r2 = 0.562
Dependence of KNO3 on Nitrate Concentration in Field Studies
S. Lan Smith, FRCGC, JAMSTEC April, 2008
log NO3
log KNO3
-2.5 -1.0 0.0
-3
-2
-1
0
log Ks = − 0.17 + 0.62 log NO3
log Ks = − 0.36 + 0.50 log NO3
Data (x) compiled from 2 studies
Least-squares fits to the data:
n = 61 data pts.
But the data in Collos (2005) came frommany studies, some of which used different methods to determine KNO3. The data at right are from two studies: Harrison et al (Limnol. Oceanogr., 1996) N. Atlantic McCarthy et al (Deep Sea Res. II, 1999) Arabian Sea
which used similar methods. This provides an independent test, over wide regions of the ocean,and over wide ranges of concentration.
Significant relationship (b != 0) at level p < 10-10 r2 = 0.54 for the general equationr2 = 0.52 for the eqn. with b = 0.50
b is not significantly different from 0.50(ANOVA, p < 0.119 )The data is cosistent with OU kinetics.
An Independent Test Using Data from Different Field Studies
S. Lan Smith, FRCGC, JAMSTEC April, 2008
The mix of species varies with location & with nutrient concentrations
"Grey Area" = What we don't understand
Why does the nitrate uptake response of the community of phytoplankton agree with the OUequation ?
Is it just luck ?For two independent compilations?
Field Experiments Measure the Response of Assemblages
S. Lan Smith, FRCGC ECRP
Large Species
NutrientIons
Ion Channels= Uptake Sites Small Species
For changing nutrientconcentration
low high
The same trade-off.
Speciation: Different Organisms evolve to dominate in different environments.
Same Cell
NutrientIons
Ion Channels= Uptake Sites
Internal Enzymes
CellFor changing nutrientconcentration
low high A trade-off: different uses for the same resource.
Acclimation: Each Organism changes in response to its environment.
The same trade-off could apply to species evolving for different environments.
Why should the same equation describe so many different species?
S. Lan Smith, FRCGC, JAMSTEC April, 2008
Traditional in terms of MM kinetics
Differences in Ks & Vmax reflectintrinsic differences between species.
Different species have evolved different strategies to aquire nutrients and grow.
Different environments favordifferent strategies. e.g., high nutrients vs. low nutrients w/ pulses
Implies low predictability: We cannot predict patterns in these intrinsic properties.
New in terms of OU kinetics
All phytoplankton acclimate to nutrient concentrations in thesame way, subject to the same physiological trade-off.
Yes, there are differences, but the overall pattern can mostly be explained by the similarities.
Implies high(er) predictability they all behave similarly
Different Interpretations of Phytoplankton's Response to Nutrients
S. Lan Smith, FRCGC, JAMSTEC 6 November, 2008
Collaboration with Prof. A.Oschiles & Dr. M Pahlow (Kiel Univ., Germany)
OU kinetics (The SPONGE from Smith & Yamanaka, Limnol. Oceanogr. 52, 2007)substituted into a marine ecosystem model: Univ. of Victoria Earth System Climate Model (Schmittner et al, GBC 19, 2005; Schmittner et al., GBC 22, 2008) Two nutrients (N & P), coarse resolution OGCM (1.8 x 3.6 degrees) Long simulations: years 1865-2000Significant Differences: OU version vs. Original (Michaelis-Menten kinetics).
Relative Difference: Integrated P. Prod. (OU - MM) %
Apparent half-saturation constant, Kapp
KN = 0.7 mmol L-1 in original MM version
What difference does OU kinetics make in Global Models?
S. Lan Smith, FRCGC, JAMSTEC ECRP
Top: Relative (%) Difference (OU - MM) in Annual PP for Year 2000.
more PP in subtropical gyres less in upwelling regions
Bottom: Absolute Difference (OU - MM) in zonally averaged PP for selected years
difference increasing over time
Difference:OU − MM 2100 2000 1770
Differences in Zonnally Averaged Primary Production
ASLO Aquatic Science Meeting, Nice, France 29 January, 2009
Optimal Uptake kinetics describes the observed variations in "half-saturation constants" fit to the MM equation for field expts.
The Optimality-based explanation:All phytoplankton acclimate to the ambient nutrient concentrationby altering their physiology, subject to the same trade-off. This implies greater predictability than MM kinetics. This acclimation requires more time than typical incubationexperiments used to measure uptake (2-3 h). MM parameters from short-term expts. do not accurately represent the long-term response of phytoplankton.
In a global Earth System Climate Model, OU kinetics yields significantly different dynamics than MM kinetics.
OU kinetics is a superior alternative to MM kinetics for modeling.
S. Lan Smith, FRCGC, JAMSTEC April, 2008
Data (x) from field expts. in the N. Atlantic Harrison et al. (Limnol. Oceanogr. 41, 1996)
Line is the best-fit of the Short-term OUApproximation for Vmax.
log [NO3]
log
V max
1 2 3 4
-0.5
0.0
0.5
1.0
1.5 Vmax varies more than predicted bythe Short-term OU approximation.
We expect this because of:
Inhibition of Nitrate Uptake by NH4 reducing rates at low nitrate where [NH4] is often higher
Possible temperature dependence (lower rates at Nitrate, where temp. should be low)
Vmax varies even more than Ks
