Photosynthesis: w00t!Sherry PalaciosToday’s Objectives- Review ending of “How do we measure photosynthesis”- Complete lecture on “Abiotic/biotic controls ofphotosynthesis”- Go over Behrenfeld et al. 2006 paper “Climate-driven trendsin contemporary ocean productivity”- Introduce uptake kineticsReview: Photosynthesis is the conversion of light andinorganic elements to organic compounds This is a RATE, while chlorophyll is BIOMASS We can measure the rate many ways--oxygenevolution and carbon uptake are the mostcommon Light Reactions describe the Z-scheme Dark Reactions describe the conversion ofinorganic C, N, P, etc. to organic material (don’trequire light)Photosynthesisversus IrradianceCurvesIrradiancePhotosynthesisalphaPmaxbetaEkThe rate of photosynthesisup to the Ek point iscontrolled by the lightreactions--after that, itslows down because thedark reactions are limitingPhotosynthesisversus IrradianceCurvesIrradiancePhotosynthesisalphaPmaxbetaEkBeta is caused by so muchlight that the cell starts toget damaged--can’t get ridof the energy asfluorescence or “work”, soheat builds upPhotosynthesisversus IrradianceCurvesIrradiancePhotosynthesisLow, Medium,and High LightAdapted cells….Phytoplankton adapt to the light environment by changingthe amount and type of pigment per cell, which changesthe light reactions (the dark reactions are controlledprimarily by cell size, nutrients, and temperature)Key Points on P vs. ECurves… provide a mathematical description of how torepresent phytoplankton growth--useful forour NPZ model the slope is controlled by the light reactions,while Pmax is controlled by the dark reactions tells us what the algae (or plants) are adaptedto--low or high light, slow or fast growthToday’s Lecture covers some of the most importantthemes in Biological OceanographyKeep an eye out for these terms:Eppley & Q10PBOptcompensation depth & critical depthmacronutrients & micronutrientsRedfield RatioMichaelis-Menten kineticsDroop kineticsLuxury uptakeBalanced vs. un-balanced growthFactors AffectingPhotosynthesis Temperature Eppley (1972) Light Sverdrup’s Critical Depth Model Nutrients Definition and Units Broecker’s Classification Scheme Limitations Uptake KineticsTemperature!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 extensionsTemperature &PhytoplanktonTemperature (deg C)Growth Rate03015• Eppley (1972)plotted speciesgrowth vs. temp.• Empiricallydetermined thatall phytoplanktonfit under a curveQ10Temperature &PhytoplanktonCan we usetemperature effectson photosynthesisto estimateprimary productivityfrom satellites?PBopt & TemperaturePBopt & TemperatureLight!Compensation DepthZcPositive Net ProductionPositive Net RespirationSverdrup’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 linearCritical 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)Nutrients!Nutrient DistributionsNutrient Availability Phytoplankton are most abundant wherethere are nutrients Nutrients are highest near coastal regionsand in upwelling zones Nutrients and waste products must passthrough the cell membraneDo Nutrients ReallyDiffuse? However, most phytoplankton cannot rely onpassive diffusion! Diffusion Mechanisms: Passive Diffusion (based solely on thegradient of concentrations) Facilitated Diffusion: “channels” allow ions tomove through the cell wall Active Uptake: There aretransporters on the cell wallUptake KineticsPassive Diffusion- Relies on a simple gradient- Not very efficientUptake RateFacilitated Diffusion- Provides “channels”Active Transport• Follows Michaelis-Menten Kinetics• Controlled by # of transportersAnd internal enzyme kinetics ConcentrationWho takes up nutrients thefastest?Stoichiometry depends on N source andchemical composition of phytoplanktonUnderstand and remember the definition andsignificance of the photosynthetic quotient, PQ1.0 NO3-+ 5.7CO2+ 5.4H2O ! " ! (C5.7H9.8O2.3N) + 8.5 O2+1.0 OH-P.Q. = 1.49 (O2evolved / CO2consumed)1.0 NH4++ 5.7CO2+ 3.4H2O ! " ! (C5.7H9.8O2.3N) + 6.25 O2+1.0 H+P.Q. = 1.10Generalized reactions for growth on nitrate and ammoniumIt 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 PNitrate and phosphate to proteins, phospholipids, nucleotides, etc.…the implicit PQ is 1.30! 106 CO2+ 122 H2O +16 HNO3+ H3PO4" # " (CH2O)106+(NH3)16+H3PO4+138 O2C:N:P ~ 106:16:1 - Termed the Redfield RatiosMicronutrients (Trace Elements)e.g.,Cu, Zn, Ni, Co, Fe, Mo, Mn, B, Na, ClGenerally, 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.Michaelis-Menten kinetics:V = Vmax!SKs+ SV = uptake rate (e.g., N taken up per unit particulate N per unit time); d-1Vmax = maximum uptake rateKs = Substrate concentration at which V = Vmax/2Consistent with underlying mechanism:S + Ek1k–1k2E SE + PS = substrate; E = enzyme; P = product; k = rate constantNutrient-uptakekinetics andecological/evolutionaryselectionPhytoplankton isolated from oligotrophic environments havelower Ks values than phytoplankton from eutrophicenvironments (consistent with prediction based on ecologicaltheory)0.00.51.01.52.02.50 2 4 6 8 10 12Nutrient UptakeSpecific Rate of Uptake (d-1)Nutrient Concentration (!M)IIIVmax = 2.25 d-1Ks = 2.0 !MVmax = 1.5 d-1Ks = 0.5 !M Nutrient kinetics