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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%

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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

Phytoplankton"

Nutrients"

Zooplankton"

How do nutrients regulate photosynthesis?

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

3) The DARK REACTIONS provide ATP & NADPH to be used as chemical energy for everything else. This brings us to the Nutrient box

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Nutrient Distributions"

Remember that there are plentiful nutrients at depth, but large areas of the ocean with little or no nutrients

Primary Production

(Low E molecules)

(High E molecules)

CO2 + H2O

CH2O + O2 Energy IN (Sun)

Energy OUT

Photosynthesis Respiration

(sugar)

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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

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Primary Production So our “Equation of Life”

CO2 + PO4 + NO3 + H2O à CH2O,P,N + O2

carbon dioxide + phosphate + nitrate + water becomes organic tissue + oxygen

2612622 666 OOHCOHCO +→+

is really more complicated…

Photosynthesis

Respiration

Primary Production

Redfield Ratio (C:N:P = 106:16:1) –  Approx. concentration of elements in phytoplankton –

each in relation to each other; nearly constant ratios

C106N16P1 (by atoms)

C106N16Si16P1 (diatoms)

- Add (Fe,Cu,Mn,Zn)0.01 (expanded)

CO2 + PO4 + NO3 + H2O à CH2O,P,N + O2

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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 PO4= (phosphate)

•  Recycled within cells quickly (ATP - ADP - ATP, etc.)

•  NOT a structural component

–  N - plenty in atm (N2), but not available to plants or

animals, which need inorganic nitrogen (NO3-, NO2

-, NH4+ )

•  N is most often the main limiting factor for algal growth

•  N-Cycle is a bit more complicated…

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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)

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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

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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)

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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

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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

Upt

ake

Rate

Facilitated Diffusion - Provides “channels”

Active Transport • Follows Michaelis-Menten Kinetics • Controlled by # of transporters And internal enzyme kinetics

Concentration

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Michaelis-Menten kinetics:

V = Vmax ⋅S

Ks + SV = 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 + Ek 1

k –1

k 2

E S E + P

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

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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

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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….

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ChloroplastNADPH NADP

Gln + 2-OXG Glu

Glu + NH4+ Glu

NIR

FDX(red)

FDX(ox)

ADP

ATPGS GOGAT +ATP

ADP + Pi

NO3- NO3

-

[plasma membrane]

[bul

k flu

id]

[cytosol]

aminoacids

NAD(P)HNAD(P)NR

NO2-

α ketoacids Mitochondrion

TCACycle

+

Adapted from Falkowski and Raven (1997) Aquatic Photosynthesis

N-Metabolism is a Primary Sink For Photo-Reductant

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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:"

Growth on CO2 and the Macronutrients N and P"

Nitrate and phosphate to proteins, phospholipids, nucleotides, etc. …the implicit PQ is 1.30

€

106 CO 2 +122 H2O +16 HNO3 + H3PO4

" → " (CH2O)106 +(NH3)16 +H 3PO4 +138 O2

C:N:P ~ 106:16:1 - Termed the Redfield Ratios

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. "

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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

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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.

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• 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?

What Nutrient Controls the Biological Pump?

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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 high-biomass 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

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An example from a global model. See http://darwinproject.mit.edu/?p=445 “diazotrophs” are nitrogen fixers. They require massive quantities of iron, but are not N limited.

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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)

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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.

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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)

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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.

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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….