Sverdrup s Critical Depth Revisited

Transcription

1 Sverdrup s Critical Depth Revisited For Homework 2, question 4, you need some additional information. Sverdrup assumed that the Compensation Depth (Ec) includes respiration from both phytoplankton and EVERYTHING ELSE in the water column, so he used a value substantially higher than the 1% light depth that we use as a rule of thumb. He also assumed that the light was the AVERAGE irradiance at the surface, so it would be (assuming 12 hours of daylight) HALF of the full irradiance (so 50% light instead of 100% light at the surface). Sverdrup s Critical Depth Revisited Therefore, the equation would be: Zcr = Eo / (Ec x k) BUT Eo is 50% rather than 100% (12 hours of darkness), and Ec is assumed to be higher than 1, something like 1-10% of the surface irradiance. Therefore our approximation is closer to: Zcr = (0.5 x Eo) / (Ec x k), where Ec is 5% rather than 1% 1

2 Sverdrup s Critical Depth Revisited It is time we adopted a more critical attitude toward this model instead of continuing to inflict it as a matter of course on students -- Smetacek and Passow, L&O 1990 What s wrong with the model? - Respiration of zooplankton, fish, etc is implicitly included, but we cannot directly estimate that from our light/dark bottles (they don t fit in the bottle) Why are we still using it? - The general idea makes sense You can be mixed deeper than the compensation depth and still have net growth What have we learned so far? 1) We know how to measure light 2) Photosynthesis is controlled by: Temperature Light & Dark reactions Rate of mixing Phytoplankton" Zooplankton" Nutrients" 3) The DARK REACTIONS provide ATP & NADPH to be used as chemical energy for everything else. This brings us to the Nutrient box How do nutrients regulate photosynthesis? 2

3 Nutrient Distributions" Remember that there are plentiful nutrients at depth, but large areas of the ocean with little or no nutrients Primary Production Photosynthesis Energy IN (Sun) (High E molecules) CH 2 O + O 2 (sugar) CO 2 + H 2 O (Low E molecules) Respiration Energy OUT 3

4 Primary Production: building biomass What about lipids, proteins, etc.? Use ATP and NADPH These are the currency for growth Adenosine Triphosphate P (phosphorous) is key Other nutrients? Primary Production Chemical Composition of typical algae 1. The Major Elements % tissue Limiting? Oxygen ~60 No Carbon ~20 No Hydrogen ~10 No 2. The Minor Elements Macro-nutrients Nitrogen 1-5 Often Phosphorous 1-5 Often Micro-nutrients (Na, Cl, Mg, Zn, Si, Co, Fe ) <0.05 Perhaps 4

5 Primary Production So our Equation of Life Photosynthesis 2 + 6H 2O C6H12O6 6 6CO + O Respiration is really more complicated 2 CO 2 + PO 4 + NO 3 + H 2 O à CH 2 O,P,N + O 2 carbon dioxide + phosphate + nitrate + water becomes organic tissue + oxygen Primary Production CO 2 + PO 4 + NO 3 + H 2 O à CH 2 O,P,N + O 2 Redfield Ratio (C:N:P = 106:16:1) Approx. concentration of elements in phytoplankton each in relation to each other; nearly constant ratios C 106 N 16 P 1 (by atoms) C 106 N 16 Si 16 P 1 (diatoms) - Add (Fe,Cu,Mn,Zn) 0.01 (expanded) 5

6 Primary Production Principle of Limiting Factors (aka Liebig s Law: one missing nutrient stunts phytoplankton growth) Macronutrients (N, P) usually the limitation P - comes from rock weathering As PO = 4 (phosphate) Recycled within cells quickly (ATP - ADP - ATP, etc.) NOT a structural component N - plenty in atm (N 2 ), but not available to plants or animals, which need inorganic nitrogen (NO 3-, NO 2-, NH + 4 ) N is most often the main limiting factor for algal growth N-Cycle is a bit more complicated 6

7 Liebig s Law, Blackman s Law Liebig s Law of the Minimum: plant (or phytoplankton) accumulation of biomass is limited by the most limiting element. If you add that, you get more yield (biomass). Example: add iron, get a bloom. Iron is Liebig-Limiting, but once you add it, something else eventually runs out. Liebig s Law, Blackman s Law Blackman s Law of Limiting Factors: even though some element may ultimately limit biomass, you can also change growth rate. The rate of growth is controlled by the slowest factor. Example: increase light, you can increase growth UNTIL you reach the maximum rate of chemical reactions (such as RUBISCO). You can increase the growth rate so long as you haven t run out of something (Liebig limitation) 7

8 Back to Nutrients Elements required for biological growth are considered nutrients. Nutrients can be classified based on availability: 1) Bio-limiting (can control algal growth/biomass). N, P, Si, Fe. 2) Bio-Unilimiting (always abundant). Na, Cl, K, Mg, etc. 3) Bio-Intermediate (can behave like a nutrient). Cd, Zn 4) Scavenged (particle-reactive) Pb, Th (we will return to these!) Bio-Limiting Concentrations are drawn down in surface waters by biology, replenished at depth by regeneration 8

9 Bio-Intermediate Profile looks nutrient-like, but generally not drawn down completely in the surface waters because not used by all organisms Bio-Unlimiting Used biologically, but always in excess in the oceans (profile is more influenced by salinity) 9

10 Scavenged More complicated the pink is dissolved, while the red and blue are associated with particles. When particles are present, they rapidly remove scavenging-type elements Nutrient Availability" Phytoplankton are most abundant where there are nutrients Nutrients are highest near coastal regions and in upwelling zones Nutrients and waste products must pass through the cell membrane 10

11 Do Nutrients Really Diffuse?" However, most phytoplankton cannot rely on passive diffusion! Diffusion Mechanisms: Passive Diffusion (based solely on the gradient of concentrations) Facilitated Diffusion: channels allow ions to move through the cell wall Active Uptake: There are transporters on the cell wall Uptake Kinetics" Passive Diffusion - Relies on a simple gradient - Not very efficient Uptake Rate Facilitated Diffusion - Provides channels Active Transport Follows Michaelis-Menten Kinetics Controlled by # of transporters And internal enzyme kinetics Concentration 11

12 Michaelis-Menten kinetics: V = V max S K s + S V = 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 + E k 1 E S k 2 E + P k 1 S = substrate; E = enzyme; P = product; k = rate constant Michaelis-Menten versus PE curves" Photosynthesis and nutrient kinetics curves look similar because they are governed by the same process: Initial slope is dependent on amount of pigments (light) or cell transporters (nutrients) It slows down (curves) because the dark reactions can t process fast enough Light has a beta portion because too much light burns the cell--there s no equivalent for nutrients (nutrients don t burn the cell, but they CAN poison the cell) You can change a PE curve by changing the pigments (more pigments = more efficient); same is true for nutrients, but it s cell size (more nutrient transporters) instead of pigments 12

13 Who Takes Up Nutrients Fastest? Prochlorococcus (~0.8 µm diameter) Large Diatom (~200 µm diameter) For a given cell size, the rate of uptake is controlled by the number of transporters you can fit on the surface relative to the volume Who Takes Up Nutrients Fastest? Small cells have a LARGE surface area to volume ratio. This results in a steep initial slope (alpha) Large cells have a SMALL surface area to volume ratio. They are less efficient at taking up low concentrations of nutrients, but have more storage capacity (higher Vmax) Therefore, as a general rule, small cells outcompete large cells at low nutrient concentrations, and vice versa 13

14 If small cells have lower Ks, but large cells can store more, how do we represent that mathematically? The Michaelis-Menten equation does NOT ALLOW for storage! Droop proposed that you could fix Michaelis-Menten kinetics by including terms that allow large cells to take up nutrients and save them for later. 14

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16 N-Metabolism is a Primary Sink For Photo-Reductant Chloroplast NADPH NADP ATP NO 3 - NO 3 - ADP ATP Gln + 2-OXG Glu GS GOGAT + Glu + NH + 4 Glu FDX (red) [bulk fluid] ADP + Pi NAD(P)H NR NAD(P) NO 2 - NIR FDX (ox) amino acids + α ketoacids Mitochondrion TCA Cycle [plasma membrane] [cytosol] Adapted from Falkowski and Raven (1997) Aquatic Photosynthesis 16

17 Growth on CO 2 and the Macronutrients N and P" It is convenient (and often necessary) to consider the growth and decomposition of an average phytoplankter. Redfield (Redfield, Ketchum and Richards 1963) showed strong and profound relationships between dissolved elements that were consistent with the growth and decomposition of phytoplankton:" C:N:P ~ 106:16:1 - Termed the Redfield Ratios 106 CO H 2 O +16 HNO 3 + H 3 PO 4 " (CH 2 O) 106 +(NH 3 ) 16 +H 3 PO O 2 Nitrate and phosphate to proteins, phospholipids, nucleotides, etc. the implicit PQ is 1.30 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 parts of the ocean. It is particularly important in Nitrogen Fixation. Copper, Zinc and Nickel have also been implicated in influencing the growth of open-ocean phytoplankton. Trace element interactions are complex, and incompletely understood. " 17

18 Summary so far: Cells growing optimally have Redfield Ratio proportions (on average) Small cells take up nutrients faster than large cells, but can t store excess nutrients. Uptake can be described by Michaelis-Menten kinetics, but we need to add cell Quotas to account for storage (Droop kinetics) Nutrient uptake is coupled to photosynthesis because ATP/ NADPH are used the cell is constantly balancing the formation of storage compounds, growth, etc. Nutrients & Photosynthesis" Active uptake requires ATP and NADPH Therefore, some of the energy from PSII and PSI goes to nutrients, NOT to the Calvin-Benson Cycle The Photosynthetic Quotient (PQ) describes how much extra photosynthesis is required: PQ=1.3 means that for every 100 units of energy going to carbon fixation, 30 units (30%) goes to nutrients, primarily N 18

19 The secret of photosynthesis A phytoplankton cell is like a potato--it s full of starch, oils, and other energy storing compounds that let the cell survive when it s not in sunlight. 19

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21 What Nutrient Controls the Biological Pump? Geochemists' viewpoint : nitrogen can be "topped up" from the atmosphere by the fixation of N2 gas to NO3; phosphorus has no comparable sources or biological pathways, therefore phosphorus limits global production Biologists' viewpoint : observational and experimental work finds natural assemblages of phytoplankton are more nitrogen-stressed than phosphate-stressed and more responsive to nitrate additions rather than phosphorus additions, therefore nitrogen limits global production What about Iron? How about Silica? What Nutrient Controls the Biological Pump? Geochemists' viewpoint : nitrogen can be "topped up" from the atmosphere by the fixation of N2 gas to NO3; phosphorus has no comparable sources or biological pathways, therefore phosphorus limits global production Biologists' viewpoint : observational and experimental work finds natural assemblages of phytoplankton are more nitrogen-stressed than phosphate-stressed and more responsive to nitrate additions rather than phosphorus additions, therefore nitrogen limits global production What about Iron? How about Silica? 21

22 ON AVERAGE, the flux of material from the surface to depth, the ratio of nutrients at depth, and the biomass of the oceans are in Redfield proportions. Since only cells growing rapidly are near Redfield proportions, this implies that most of the ocean is at near optimal growth rates but the open ocean is low biomass. This makes sense if the open ocean is dominated by small cells (not much biomass, but very efficient at acquiring low levels of nutrients), while the highbiomass areas are dominated by larger cells. So the open oceans are not biological deserts but instead are growing at near maximal rates! They are just not accumulating biomass. Limiting Nutrients In theory, ANY element (nutrient) could be limiting. However, Redfield ratios suggest that it would be C, N, P, and maybe Si: Redfield Ratio = 106:16:16:1 C:N:Si:P In MOST of the ocean, it s considered to be either N or Fe In SOME regions, other nutrients can be limiting such as P RARELY, some trace compound may become limiting, such as Zn, Cu, or even Vitamin B 22

23 4/21/13 An example from a global model. See diazotrophs are nitrogen fixers. They require massive quantities of iron, but are not N limited. 23

24 Summary of Photosynthesis and Nutrients " Primary production is limited in the ocean by temperature, light, and nutrients We assume that phytoplankton strive to maintain balanced growth, meaning they keep the same proportions of C, N, P, Fe, etc. We can convert the different rates based on simple rules: Redfield Ratio, Photosynthetic Quotients, Quantum Yields, etc, and we can use simplified models such as change in chlorophyll or data from satellites to estimate productivity Over long time periods (millions of years), P is ultimately limiting, but over short time periods, it is usually N or Fe Growth Rates We have discussed several aspects of growth but have not defined it. Start with the concept that the fundamental biological unit is the organism (1 fish, 1 whale, 1 phytoplankton cell) For multicellular organisms, growth can mean increase in mass of the individual, or increase of individuals (reproduction) 24

25 Growth Rates For phytoplankton, we generally assume cell size does not change very much. So growth is increase in cell number. But counting cells is difficult! So we often use proxies such as chlorophyll. Redfield tells us that for healthy cells, the chemical composition is essentially constant. Growth Rates We THEREFORE ASSUME that we can measure ANY property of the cells (or population) and convert to growth rate! Photosynthesis is equivalent to growth. Nutrient uptake is equivalent to growth. Change in biomass is equivalent to growth We just convert by using Redfield, PQ, etc. 25

26 Growth Rates Growth rates are given units of reciprocal time (d -1 ). For phytoplankton (binary fission), doubling time is given as growth rate (µ) divided by the natural log of 2, and is given as time (hours, days, etc). Nutrient uptake, carbon assimilation, etc. can also be expressed as reciprocal time (velocity, V, d -1 ) and is assumed to be equivalent to growth rate if the cells are in balanced growth. Growth Rates Phytoplankton (and bacteria) generally grow in PHASES. When we talk about Growth Rate we usually mean exponential growth. You can calculate that as the slope of log(biomass) versus time. Remember Redfield, so we can use any metric of biomass (CHL, carbon, nitrogen, cells, etc) 26

27 Growth Rates Each organism has a unique growth curve, in response to light, nutrients, temperature. In this case, Species B grows better at low nutrients, and Species A grows better at high nutrients. For some nutrient concentrations, they grow about equally well (and would co-exist). Growth Rates Review: for single-celled organisms, growth is increase in cell number. For multicellular organisms, also includes changes in biomass and reproduction. Redfield tells us that growth can be tracked using ANY metric we want, but the only truly correct one is the individual (cell, organism). Assuming Redfield is correct, Vmax (nutrients) is the same as Pmax (photosynthesis) is the same as µ (growth) if everything is in balance. New term: a growth (µ) versus nutrient graph is called a Monod equation, but is identical to Michaelis-Menten (or Droop) kinetics. 27

28 Productivity Review AUTOTROPHS are responsible for the base of the food chain 99.9% of the productivity is driven by sunlight Chlorophyll is BIOMASS, Primary Productivity is the RATE and is equivalent to GROWTH. Sunlight is high and nutrients are low at the surface Primary Production requires light and nutrients--as with all biological reactions, it runs faster when temperatures are higher (heat equals faster chemistry) SO: we would expect high primary production where there s high light, warm temperatures, and lots of nutrients. 28