Phytoplankton Growth, Nutrients, Phytoplankton Growth, Nutrients, and Temperatureand Temperature

Introduction to Biological OceanographyIntroduction to Biological Oceanography20042004

John Cullen (Storm-Stayed)John Cullen (Storm-Stayed)

Required Reading:

McCarthy, J. J. (1981). The kinetics of nutrient utilization. In: Platt, T. (ed) Physiological Bases of Phytoplankton Ecology. p. 83-102.

What we should have learned so farWhat we should have learned so far

marine.rutgers.edu/opp/

Coscinodiscus waelesiiPhytopia CD-ROMBigelow Laboratory

Phytoplankton provide food energy for marine food webs and strongly influence chemical cycles in the seaPhytoplankton provide food energy for marine food webs and strongly influence chemical cycles in the sea

The measurement of light tells us much about The measurement of light tells us much about the ocean, including distributions of the ocean, including distributions of phytoplankton and influences on their growthphytoplankton and influences on their growth

The measurement of light tells us much about The measurement of light tells us much about the ocean, including distributions of the ocean, including distributions of phytoplankton and influences on their growthphytoplankton and influences on their growth

marine.rutgers.edu/opp/

The major The major causescauses of variations in primary of variations in primary productivity are related to light and productivity are related to light and nutrientsnutrients

The major The major causescauses of variations in primary of variations in primary productivity are related to light and productivity are related to light and nutrientsnutrients

marine.rutgers.edu/opp/

Because phytoplankton need light for photosynthesis and nutrients to support growth Because phytoplankton need light for photosynthesis and nutrients to support growth

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Irradiance (µmol m-2 s-1)

netgross

respiration

carbohydrates

nucleid acids

http

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http://www.agen.ufl.edu/~chyn/age2062/lect/lect_02/

Lipids

Protein

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Nutrient Concentration (µM)

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IIV

max = 2.25 d-1

Ks = 2.0 µM

Vmax

= 1.5 d-1

Ks = 0.5 µM

Photosynthesis

The Growth and Chemical Composition ofPhytoplankton is a Major Driver of

Ocean Chemistry

NutrientsNutrients Decomposition Decomposition NutrientsNutrients Decomposition Decomposition

Light + Nutrients Growth ConsumptionLight + Nutrients Growth Consumption

Bottom

106 CO 2 122 H2O +16 HNO3 H3PO4 [(CH2O)106 +(NH3 )16 +H3PO4 ] +138 O2

[(CH2O)106 +(NH3)16 +H 3PO4 ] +138 O 2 106 CO2 122 H 2O +16 HNO3 H3PO4

Chemical Composition of Phytoplankton(protein is a major constituent)

Like the form of nutrient for growth, the chemical composition of phytoplankton can vary

Stoichiometry depends on N source and chemical composition of phytoplankton

Understand and remember the definition and significance of the photosynthetic quotient, PQ

1.0 NO3- + 5.7CO2 5.4H 2O (C

5.7H9.8O2.3N) + 8.5 O2 +1.0 OH-

P.Q. 1.49 (O2 evolved / CO2 consumed)

1.0 NH4+ + 5.7CO 2 3.4H2O (C5.7H9.8O2.3N) + 6.25 O2 +1.0 H+

P.Q. 1.10

Generalized reactions for growth on nitrate and ammonium

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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One of our jobs is to describe how light, nutrients, and temperature influence the photosynthesis, growth, and chemical composition of phytoplankton. Quite a job!

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Temperature

Temperature Effects in the Ocean

Eppley1972

Nutrients and Growth

• Growth of phytoplankton depletes nutrients consistent with their chemical composition

• Growth cannot continue when nutrients run out• When one nutrient is depleted first, unbalanced

growth can proceed• We need to know how growth conditions and

nutrient limitation affect chemical composition and growth rates of phytoplankton

Effects of Nutrient Concentration:Michaelis-Menten Kinetics

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Nutrient Concentration (µM)

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IIV

max = 2.25 d-1

Ks = 2.0 µM

Vmax

= 1.5 d-1

Ks = 0.5 µM

V Vmax S

K s S

Michaelis-Menten kinetics:

V Vmax S

K s 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 + E

k 1

k –1

k 2

E S E + P

S = substrate; E = enzyme; P = product; k = rate constant

Nutrient-uptake kinetics and ecological/evolutionaryselection

It was subsequently demonstrated that phytoplankton isolated from oligotrophic environments had lower Ks values than phytoplankton from eutrophic environments (consistent with prediction based on ecological theory)

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Nutrient Concentration (µM)

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IIV

max = 2.25 d-1

Ks = 2.0 µM

Vmax

= 1.5 d-1

Ks = 0.5 µM

However:

Nutrient uptake experiments are generally performed under unnatural conditions.

Procedure for measuring nitrate uptake kinetics: a culture is grown on nitrite (easy to measure) until the point of depletion, then subsamples are supplemented with different concentrations of nitrate; the initial rate of uptake is then determined and described as a function of initial concentration.

The complication arises because the phytoplankton are in unbalanced growth, adjusting physiologically to changing conditions as the experiment is performed.

(In the field, nitrate and ammonium assimilation is measured with 15N tracers)

Nutrient kinetics for growth (rather than for uptake) are more difficult to determine:

experiments involve growth in chemostat culture

Ks < 0.1 µg-at L-1

The chemostat work produced another type of nutritional pattern that was easier to measure: Cell Quota

from Droop, in McCarthy, 1981Algal growth could be described as a function of internal stores of a limiting nutrient.

Consequently, chemical composition responds to growth conditions

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63 µmol m-2 s-1

N-Limited <——> N-sufficientThe chemical composition of phytoplankton is very responsive to growth conditions. Here, nitrogen content is lower when growth rate is limited by the supply of N (carbohydrates are accumulated).

A consequence of variable cell quota (e.g., N cell-1) is that even if nutrient uptake per cell (nmol N cell-1 h-1) is constant as a function of nutrient limitation, the maximum specific rate of nutrient uptake (Vm; µg-at N (µg-at cell N)-1 h-1) will increase with nitrogen limitation.

from McCarthy, 1981

Two reasons for “luxury uptake”

Reduced Cell Quotaat lower growth rates

Enhanced uptake percell under nutrientlimitation

see Morel, F. M. M. 1987. Kinetics of nutrient uptake and growth in phytoplankton. J. Phycol. 22: 1037-1050.

Kinetics of uptake vs for growth are not the same

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Nutrient Concentration (µM)

I

IIV

max = 2.25 d-1

Ks = 2.0 µM

Vmax

= 1.5 d-1

Ks = 0.5 µM

Uptake Growth

Ks for growth < 0.1 µg-at L-1

Photoacclimation affects chemical composition

E

L

S

High LightLow Light

after Geider et al. 1996

PL

S

EP

Sizes of arrows are proportional to flux:Sizes of boxes pool size growth rate

P = PhotosynthateE = EnzymesS = StorageL = Light Harvesting

Photoacclimation and P vs E

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Chemical composition responds to growth conditions

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N-Limited <——> N-sufficientThe chemical composition of phytoplankton is very responsive to growth conditions. Here, nitrogen content is lower when growth rate is limited by the supply of N (carbohydrates are accumulated).

Chemical composition responds to growth conditions

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N-Limited <——> N-sufficientCarbon content is also higher when irradiance is higher.

How does chemical composition change?

Unbalanced growth

High —> Low

Low —> High

see Geider et al. 1996

L

S

EP

Pigment synthesis inhibitedSynthesis of enzymes cannot accelerate quicklyPhotosynthate goes to storage

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L

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Pigment synthesis continuesSynthesis of enzymes slows because supply is reducedStored carbon is mobilized into free sugars

Unbalanced Growth

When nitrogen ran out (day 6), photosynthesis continued, but C was stored as starch. Growth was unbalanced, and much different than “Redfield”. When N was supplied, starch was used, protein was synthesized, and Redfield was restored.

When we measure growth in the field, we do not generally know if balanced growth is occurring.

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ExponentialGrowth

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Chemical composition responds to growth conditions

A central tendency is toward Redfield:

C:N = 6.6 by atoms

C:Chl of about 50

Higher light, N or P limitation:

C:Chl goes up

Further reading: Geider, R.J. (1987). Light and temperature dependence of the carbon to chlorophyll a ratio in microalgae and cyanobacteria: implications for physiology and growth of phytoplankton. New Phytol. 106:1-34.

Chemical composition responds to growth conditions

Lower temperature is like higher light

N limitation:

C:N goes up

P limitation:

C:P goes up

Further reading: Goldman, J.C. (1980). Physiological processes, nutrient availability, and concept of relative growth rate in marine phytoplankton ecology. In: Falkowski P.G., (ed.) Primary Productivity in the Sea. Plenum, New York, pp. 179-194.

SummaryPhytoplankton are microscopic organisms that provide food for life in the sea.

They do this by growing (cell division). This requires

Light

CO2

major nutrients (N, P, and Si for some), and

micronutrients (including Fe)

The growth process is fueled by

Photosynthesis and Nutrient Assimilation

SummaryPhytoplankton cells are composed of

Protein (cellular structure and enzymes: contains N)

Carbohydrate (energy storage)

Lipids (energy storage, membranes)

…and other stuff

The relative proportions of these constituents change between taxa and with physiological state or nutrient limitation. That alters the stoichiometry of nutrient assimilation and growth. This stoichiometry strongly influences biogeochemical cycles in the sea.