phytoplankton: nutrients and growth. outline growth nutrients limitation physiology kinetics...
TRANSCRIPT
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Phytoplankton: Nutrients and Growth
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Outline
• Growth
• Nutrients
• Limitation
• Physiology
• Kinetics
• Redfield Ratio
• Critical Depth
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• Why do we care about phytoplankton growth?
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• Biomass – how much phytoplankton at any one time, g C/m2
• Productivity – how fast what is there is growing, g C/m2/year
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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.
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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)
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t
B
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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
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Nutrients
• LIMITING– Nitrate – Phosphate– Silicate– Iron– Manganese
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Nutrients
• LIMITING– Nitrate – Phosphate– Silicate– Iron– Manganese
• NOT LIMITING– Magnesium– Calcium– Potassium– Sodium– Sulphate– Chloride– CO2
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• 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
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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
-
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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
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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)
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The marine nitrogen cycle
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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?
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Ways to avoid nutrient limitation:
•Optimization of uptake systems
•Cell size (Surface-to-volume ratio)
•Cell shape
•Storage
•Reduced growth rates
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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
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Nutrient Physiology• Enzymes: Cells: Communities
Nutrient uptake subject to saturation
0.0
0.2
0.4
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1.0
1.2
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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)
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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
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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
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1.0
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Nutrient Concentration S (e.g., mol l-1)
Upt
ake
Rat
e V
(e.
g.,
pmol
cel
l-1 h
-1)
Vm
Ks
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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
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Michaelis-Menten nutrient uptake kinetics
Optimization of uptake systems
[N]Ks
Vmax
or µmax
upwellingoligotrophic
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• Oligotrophic –↓ [nutrients] ↓ PP
• Eutrophic – ↑ [nutrients] ↑ PP
• Mesotrophic – moderate nutrients and PP
• HNLC – limited by iron
↑ nitrate ↓chlorophyll
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Contrasting Nutrient Kinetics
0
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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
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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
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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
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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
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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
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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
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Nitrate versus phosphate relationship
N:P= 16:1
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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
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Critical Depth and Ocean MixingCritical Depth and Ocean MixingI
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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.
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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)
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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
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REDFIELD STOICHIOMETRY OF LIFEC106:N16:P1
Carbon
Nitrogen
Phosphorus
C:N = 6.6 / C:P = 106 / N:P = 16
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Temperature Effect