Lan\'s Presentation at the Ocean Sciences Meeting 2010

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First, I briefly review of selected recent studies which have improved our understanding of phytoplankton through the concept of optimality. Then, I present my most recent study of the combined effects of temperature and nutrient concentrations on the rates of nutrient uptake by phytoplankton. The point is that our assumptions about the fundamental dependencies affect our interpretation of the patterns observed in field experiments.

Transcript of Lan\'s Presentation at the Ocean Sciences Meeting 2010

  • 1. Optimality-based modeling of phytoplankton: Implications for predictive modeling, interpreting data and designing experimentsS. Lan SmithEBCRP, RIGC, JAMSTEC, Yokohama, Japan Constraints from Fundamental ProcessesNatural SelectionAdaptive Change Apparent KNO3 (mol L-1)Optimally Adapted OrganismsPhysical Environment S. Lan Smith Ocean Sciences Meeting, Feb. 22-26, 2010

2. OptimalityA result of Natural Selection,subject to ConstraintsPlankton are ideal subjects:Constraints from short generation timesFundamental Processes long evolutionary historyTherefore we expect them to Natural Selectionat least approach Optimality, Adaptive Changewhich suggests that this conceptshould be useful for interpreting Optimally Adapted& modeling their behavior.OrganismsPhysical EnvironmentS. Lan Smithp. 2 Ocean Sciences Meeting, Feb. 22-26, 2010 3. Optimality-Based Ideas for Modeling PhytoplanktonSelected Examples Roughly in the Space of Fundamental ProcessesConsidered in Trade-offsLight PhotoacclimationAquisition Wirtz & Pahlow (MEPS, 2010) Pahlow (MEPS, 2005) Armstrong (DSRII, 2006)Armstrong (L&O, 1999)Bruggemann & Kooijman Wirtz (J. Biotech., 2002)(L&O, 2007) Smith & Yamanaka (L&O, 2007)Smith et al. (MEPS, 2009) Nutrient Uptake ResistanceMerico et al. (Ecol. Modelling, 2009) to Predators S. Lan Smith p. 3 Ocean Sciences Meeting, Feb. 22-26, 2010 4. Equations for Rate of Nutrient Uptake Optimal Uptake (Pahlow, MEPS, 2005) Michaelis-Menten EquationV0 SVmax SvOU = vMM = Ks + SA0V0 + 2 V0 S + SA0 Ks is called the This is like a MM equation with:Half-Saturation constant. But it varies with:Nutrient ConcentrationA0Ks = V0 + 2 V0 S A0 SpeciesTemperature This predicts that Ks values (as fit to the MM eqn.) should increase with Affinity & Ks are related: nutrient concentration.VmaxA= Can this explain Ks the observed pattern? A is also called a (Healey. Micrbial Ecol., 1980) S. Lan Smith p. 4 Ocean Sciences Meeting, Feb. 22-26, 2010 5. What do short-term uptake experiments measure?If phytoplankton do not have time to acclimate during expts.,Optimal Uptake (OU) kinetics predicts (Smith et al. MEPS, 2009) forapparent values of Michaelis-Menten constants: Vmax = A0Sa/V0 V0This agrees with observations byKudela & Dugdale (DSRII 47, 2000), 1 + A0Sa/V0but it needs further testing. V0 SaKs =A0 This agrees with the observed pattern for KNO3 from ship-board expts. (Smith et al. 2009). Its all based on a physiological trade-off:Sa is ambient nutrientconcentration, to whichphytoplankton werepre-acclimated beforethe short-term expts. LowNutrientConc.HighNutrientConc. S. Lan Smith p. 5 Ocean Sciences Meeting, Feb. 22-26, 2010 6. Combined Effects of T & Nutrient ConcentrationsGrowth rates increase exponentially with T(Eppley. Fish. Bull. 1972; Bissinger et al. L&O 2008). Max. Uptake Rate, VmaxFor uptake or growth, Vmax is usually assumedto be independent of nutrient concentration:Temperature Michaelis-Menten (MM) kinetics.However, Optimal Upake (OU) kinetics predictsthat Vmax (from short-term expts.) shouldMMincrease hyperbolically with nutrient conc.OU(Smith et al. MEPS 2009). Nutrient Conc. Nutrient Conc.In the ocean, T and Nutrient Conc. are strongly(negatively) correlated.VmaxField expts. observe the combined (net) effects.Assumptions about Uptake Kinetics impactthe interpretation of observations. Temperature S. Lan Smithp. 6 Ocean Sciences Meeting, Feb. 22-26, 2010 7. Correlation of T & [NO3] in the Surface OceanNegative Correlation in General(e.g., Silio-Calzada et al. Remote Sens. Environ.112, 2008)Up-welling brings cold, nutrient-rich waterWhile phytoplankton grow, nutrients are depleted & at the same time, water is warmedHere for the data ofHarrison et al (L&O 1996)*Thanks to G. W. Harrison forproviding the complete data set.The regression line wasfitted for log [NO3] vs. log T S. Lan Smith p. 7Ocean Sciences Meeting, Feb. 22-26, 2010 8. Dependence of Uptake Rate, v, on T & Nutrientsfor maximum uptake rate, Vmax as determined by short-term expts.Assuming Multiplicative Effects This widely-applied equation isMichaelis-Menten (MM) from Goldman and Carpenter -Ea/RT [NO ](Limnol. Oceanogr. 19, 1974).v = Vmax e3 Ks + [NO3]Optimal Uptake (OU)v = V0[NO3]aA0/V0 e a-E /RT [NO3] 1 + [NO3]aA0/V0 [NO3]aV0/A0 + [NO3] This ratio determines how Vmax The apparent value of Ksdepends on ambient nutrientdepends on ambient nutrientconcentration, [NO3]a. concentration, beforeIt can be determined separatelysampling for expts.from fits to data: Ks vs. [NO3]a.(Smith et al. MEPS, 2009). S. Lan Smithp. 8Ocean Sciences Meeting, Feb. 22-26, 2010 9. Dependence of Vmax on T & Nutrients for maximum uptake rate, Vmax, as determined by short-term expts, assuming Multiplicative EffectsMichaelis-Menten (MM)Optimal Uptake (OU) -Ea/RT[NO3]aA0/V0 V e-Ea/RT Vmax = V0 eVmax = 01 + [NO3]aA0/V02 parameters were fitted by regression to data sets for Vmax, [NO3]a & T, for each eq., respectively.This ratio was determinedseparately, from fits to V0 potential maximum of Vmaxdata for Ks vs. [NO3] as in Ea Energy of ActivationSmith et al. (MEPS 2009). S. Lan Smithp. 9Ocean Sciences Meeting, Feb. 22-26, 2010 10. data of Harrison et al. (L&O 1996)MM OU T- dependent model 100.0 N &T- dependent model50Fits of Arrhenius T- using fit of T vs. [NO3] 50.0only T- dependent model Chl-Specific Max. NO3 Uptake Rate (nmol h-1(g)-1)dependence, with theQ10 = 1.7Q10 = 3.4 10MM- and OU-based as-5.0 10.05.0sumptions, respectively,for Vmax 1.0 data 1.0 0.5 fits with obs. 0.5Data: Chl-specific max. T & [NO3][NO3] uptake rate.275 285 295275 285295 Inferred Q10 is nearlyT(K)100.0twice as high with the 50 50.0OU-based assumption. 1010.0Residual Square Error:5.0 5.0 MMOU 9.3 8.51.0T- dependent model 1.0 constant Vmax0.5 0.5Fitted values of Ea sig.N &T- dependent modelonly N- dependent modeldiff. from 0 for both.10 1 101 0.1 0.01 0.001[NO3] (mol L-1) S. Lan Smithp. 10Ocean Sciences Meeting, Feb. 22-26, 2010 11. data of Kanda et al. (L&O 1985)MMOU 1.0 1.0Q10 = 1.5Q10 = 2.7Chl-Specific Max. NO3 Uptake Rate (nmol h-1 (g chl)-1 ) Fits of Arrhenius T- depen- dence, with the MM- and 0.5 0.5OU-based assumptions, re- spectively, for Vmax Data: Chl-specific max. N &T- dependent model 0.1 0.1 T- dependent model [NO3] uptake rate.using fit of T vs. [NO3] using fit of [NO3] vs. T only T- dependent modelInferred Q10 is nearly285 290295 300285 290295300T(K) twice as high with the OU- 1.0 1.0 based assumption. Residual Square Error:0.5 0.5 MMOU0.820.34 Fitted values of Ea sig. diff. data 0.1fits with obs. 0.1 N &T- dependent model from 0 for both. T & [NO3] using fit of T vs. [NO3] only N- dependent model 0.10.01 0.001 0.1 0.01 0.001[NO3] (mol L-1) S. Lan Smith p. 11 Ocean Sciences Meeting, Feb. 22-26, 2010 12. ConclusionsOptimality-based ideas imply different Interpretations of Observations. Specifically for Combined Effects of T & Concentration on UptakeEstimated Q10s are 2X greater assuming OU vs. MM kinetics.Caveat: The observed Vmax were Chl-specific Chl:N ratios tend to be greater under nutrient-rich conditions, which should under-estimate N-specific rates at high N (low T)Therefore my analysis probably over-estimates Q10sfor both MM- and OU- kineticsYet even with biomass-specific Vmax, OU would yield higher Q10sbecause of the strong negative correlation of T & [NO3}Significant Uncertainties remain about T-dependence & uptake kinetics We need more controlled experiments & field observations of biomass-specific ratesS. Lan Smith p. 12 Ocean Sciences Meeting, Feb. 22-26, 2010