Factors Affecting Photosynthesis!

Transcription

1 Factors Affecting Photosynthesis! Temperature Eppley (1972) Light Sverdrup s Critical Depth Model Nutrients Limitations Uptake Kinetics

2 Temperature! The oceans vary much less than the land does, both seasonally and daily Increased temperature decreases viscosity, so you sink Organisms grow faster, die younger as temperature increases In general, warm water species are smaller and have more extensions

3 Temperature & Phytoplankton! Q10 Eppley (1972) plotted species growth vs. temp. Empirically determined that all phytoplankton fit under a curve Growth Rate Temperature (deg C)

4 Temperature & Phytoplankton!

5 Eppley Equation:!!! Vmax = maximum growth rate!! T = Temperature!! Vmax = 0.85 x e^(0.063xt)!! This means that with nothing but a thermometer you can predict the maximum growth rate for any algae!!! Note: this was updated by Bissinger et al. 2008, L&O 53: , but the general concept is still valid!!

6 B P opt & Temperature!

7 Photosynthesis & Temperature! Remember: in the laboratory, we can measure photosynthesis versus irradiance (PvsE) and calculate Ek, Pmax, and alpha." " If we take this idea and go to the field, a vertical profile of photosynthesis in the ocean is essentially a PvsE curve, where you are measuring photosynthesis versus depth, and irradiance changes with depth. "

8 Photosynthesis & Temperature! When we mesure in the field, we call P max, maximum photosynthesis, P opt to distinguish it from a lab experiment.! " For both lab and field experiments we often divide P by biomass (chlorophyll) so that the value is not changing simply because we have more or less biomass. This is P B max (lab) and PB opt (field).!

9 B P opt & Temperature!

10 B P opt & Temperature!

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12 Mini-Summary!! Biological rates (including photosynthesis) generally increase with increasing temperature!! We can model photosynthesis versus irradiance in field data by plotting PB versus optical depth (light levels)!! P B increases with increasing opt temperature, until the temperature gets too high!

13 Compensation Depth! Positive Net Production Z c Positive Net Respiration

14 Sverdrup s Critical Depth Model:! there must be a critical depth such that blooming can occur 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 light and is linear

15 Critical Depth! Given the previous assumptions, the Critical Depth (Z cr ) can be approximated by: Z cr / (1-e -k Zcr ) = E o / (E c x k) Z cr = E o / (E c x k) This says that so long as the average depth of the mixed layer is shallower than the compensation depth, you get a bloom

16 Nutrient Distributions!

17 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

18 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

19 Uptake Kinetics! Passive Diffusion - Relies on a simple gradient - Not very efficient Uptake Rate Concentration Facilitated Diffusion - Provides channels Active Transport Follows Michaelis-Menten Kinetics Controlled by # of transporters And internal enzyme kinetics

20 Michaelis-Menten kinetics: S V = V max 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

21 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

22 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

23 N-Metabolism is a Primary Sink For Photo-Reductant Chloroplast NADPH NADP ATP NO 3 - NO 3 - ADP ATP Gln + 2-OXG GOGAT GS Glu + NH + 4 FDX (red) Glu + Glu [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

24 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

25 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. "

26 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?

27 Summary! 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