Phytoplankton: Nutrients and Growth

Outline

• Growth

• Nutrients

• Limitation

• Physiology

• Kinetics

• Redfield Ratio

• Critical Depth

• Why do we care about phytoplankton growth?

• Biomass – how much phytoplankton at any one time, g C/m2

• Productivity – how fast what is there is growing, g C/m2/year

Microbial Growth

• Mostly involves unicells (single-cells) dividing

• When cells are growing, population numbers increase exponentially

• We can express this with a single parameter we call the growth rate.

Growth Rates in the Ocean

Equation for Growth:

0( )( ) (0) t tB t B e

• B = cell number or biomass concentration (e.g., cells m-3)B(t) = concentration at time tB(0) = initial concentration (concentration at t=0)

• = growth rate (e.g., d-1)

• t = time (e.g., d)

t

B

GrowthStages of Growth – Batch Culture

0

1

2

3

4

5

6

7

0 2 4 6 8

Time (days)

Log

cells

/L

Lag

Log

Growth

Stationary Crash

Nutrients

• LIMITING– Nitrate – Phosphate– Silicate– Iron– Manganese

Nutrients

• LIMITING– Nitrate – Phosphate– Silicate– Iron– Manganese

• NOT LIMITING– Magnesium– Calcium– Potassium– Sodium– Sulphate– Chloride– CO2

• Macronutrients – substances required that make up a few % to 10% of plant (dry weight) N, C, P (for diatoms S)

• Micronutrients – make up less than 1% of dry weight Mg, Z, Co

The Principle Macro-Nutrients for Phytoplankton

Nitrogen

Inorganic (DIN): Nitrate, Nitrite, Ammonium

Organic (DON): Urea, amino acids

NO3- NO2

- NH4+

Phosphorus Inorganic: Ortho-phosphatePO4

-

Silicon Inorganic: Silicic acidSiO3

-

Nitrate Uptake into the Cell

Reduction steps: Reduced forms of nitrogen are ‘preferred’

NO3 NO3 NO2 NH4

Reduction steps

Diffusional Gradient

Presence of concentrated ammonium may inhibit nitrate reductase synthesis

Proteins

Important required and potentially limiting elements:

• Macronutrients: • Nitrogen: NO3-, NO2-, NH4+• Phosphorus: PO43-• Silicon: Si(OH)4• Carbon: CO2, H2CO3, HCO3-, CO32-

• Micronutrients:• Iron: Fe3+• Other trace elements (Zn, Co, Mn, Mo, Cd, Se)

The marine nitrogen cycle

Nutrient Limitation of Production

• Liebig’s Law of the minimum - yield of plant crop is directly proportional to the amount of limiting nutrient present or nutrient with the least amount runs out first. There is one nutrient that limits growth: Add it and growth will be (temporarily) restored.

• Limiting Nutrients in Natural WatersN, P, Fe … ? Si, C, others?

Ways to avoid nutrient limitation:

•Optimization of uptake systems

•Cell size (Surface-to-volume ratio)

•Cell shape

•Storage

•Reduced growth rates

Light and Nutrient Limitation

• If light is available, nutrients are consumed by phytoplankton until a limit is reached.

• Example: spring bloom in temperate waters

North Atlantic: Pronounced spring bloom, often a fall bloom

Nutrient Physiology• Enzymes: Cells: Communities

Nutrient uptake subject to saturation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 1.0 2.0 3.0 4.0

Nutrient Concentration S (e.g., mol l-1)

Upt

ake

Rat

e V

(e.

g.,

pmol

cel

l-1 h

-1)

Nutrient Physiology

• Enzyme – controlled

• Assimilation : involvesUptake (i.e., transport across membrane)Reduction before incorporated into organic molecules

• Rates dependent upon substrate concentration of nutrients

• Nutrient uptake subject to saturation

Michaelis-Menten Kinetics

• V is uptake rate

• Vm is maximum V

• S is substrate concentration

• Ks is the half-saturation constant

ms

SV V

K S

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 1.0 2.0 3.0 4.0

Nutrient Concentration S (e.g., mol l-1)

Upt

ake

Rat

e V

(e.

g.,

pmol

cel

l-1 h

-1)

Vm

Ks

Michaelis-Menten Parameters

• Vm reflects (for example) the total number of enzymes available to do the uptake or reduction reactions

• Ks reflects (for example) the affinity of the enzyme for the substrate, or the surface to volume ratio of the cell

Michaelis-Menten nutrient uptake kinetics

Optimization of uptake systems

[N]Ks

Vmax

or µmax

upwellingoligotrophic

• Oligotrophic –↓ [nutrients] ↓ PP

• Eutrophic – ↑ [nutrients] ↑ PP

• Mesotrophic – moderate nutrients and PP

• HNLC – limited by iron

↑ nitrate ↓chlorophyll

Contrasting Nutrient Kinetics

0

0.2

0.4

0.6

0.8

1

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Oligotrophic

Eutrophic

Nutrient Concentration

Up

take

or

Gro

wth

Rat

e

Nutrient Kinetics in the Community• Reflect the ambient nutrient environment

• Low nutrients = Oligotrophic, tropical watersMax growth rates μ max (generations day -1) = 0.1 – 0.2 (Low Vm)

Half Saturation constant Ks (in μM) = 0.01 – 0.1

low Ks

• High nutrients: Eutrophic coastal, tropical upwelling

Max growth rates μ max = 1 – 3

Half Saturation constant Ks = 2 - 10

High Vm, high Ks

Nutrient Kinetics in Differing Environments

• Changes in nutrient kinetics can reflect changes in:

• Community compositionShift to ‘r’ strategists (i.e. diatoms) dominating population when nutrients become available

• Organism characteristicsOrganisms adapt to lower nutrients by changing size, number, or characteristics of nutrient assimilation enzymes

Stoichiometry of Growth

• Elemental composition of the planktonic community – A.C. Redfield

106 C : 16 N : 1 P

• This reflects how elements are taken from the water column during primary production

Distribution of Macro-Nutrients

Elemental distributions within phytoplankton are relatively constant throughout the World Ocean.

Redfield RatioC : N : P

106 : 16 : 1

Carry out to other elements (e.g., Si)

C : N : Si : P

106 : 16 : 16 : 1

(i.e., for diatoms, N : Si is about 1)

106 C : 16 N : 1 P : 270 O

Redfield Ratio

Utility: If you know 1 elemental uptake rate, others can be estimated because the constant relationship.

Important Assumption (usually not met): Balanced Growth (all elements taken up at same rate at same time - not realistic).

Factors affecting Redfield:•Timing•Cell condition•Growth rate•Nutrient availability

Nitrate versus phosphate relationship

N:P= 16:1

Applications of the Redfield Ratio

• Health of the organismal community: if growth is less than optimal, C:X goes up.

• AOU: Apparent Oxygen Utilization:Deficit in O2 compared to saturation … indicates how much biomass increased over a long period of time.

• Modeling: In computer models of the carbon cycle, you trace one element (i.e. nitrogen) and assume how carbon goes based on the ratio

Critical Depth and Ocean MixingCritical Depth and Ocean MixingI

Critical depth and ocean mixingCritical depth and ocean mixingtemp.

z

temp.

z

critical

comp.

spring summer

comp.

critical

mixed layer deeper than critical depth- net loss

growth

winter

If the mixed layer depth is If the mixed layer depth is greatergreater than the critical depth, than the critical depth, photosynthesis cannot occur. Conversely, when Dphotosynthesis cannot occur. Conversely, when Dmixmix< D< DCRCR, positive , positive

photosynthesis can occur. When Dphotosynthesis can occur. When Dmixmix= D= DCRCR, it is the onset of the , it is the onset of the

spring bloom in temperate waters.spring bloom in temperate waters.

Critical Depth and Ocean MixingCritical Depth and Ocean Mixing

Dcr = (Io/kIc)(1-e-kDcr )

Good predictor of bloom, all you need to know is:•surface irradiance (Io)•extinction coefficient (k)•and compensation light intensity (Ic) -measure in lab

If -kDcr >>0, then Dcr = (Io/kIc)

Given that the photosynthetic machinery is so conserved among plants and algae in the sea, then why is diversity so high?

Moreover, given the special adaptations for light and nutrient acquisition in the sea, why do you still see high diversity at any single point in time and space?

Expect competitive exclusion: G. Evelyn Hutchinson’s

Paradox of the Plankton

REDFIELD STOICHIOMETRY OF LIFEC106:N16:P1

Carbon

Nitrogen

Phosphorus

C:N = 6.6 / C:P = 106 / N:P = 16

Temperature Effect