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: B = cell number or biomass concentration (e.g., cells m -3 ) B(t) = concentration at time t B(0) = initial concentration (concentration at t=0)  = growth rate (e.g., d -1 ) t = time (e.g., d)

t B

Growth Stages of Growth – Batch Culture 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 NO 3 - NO 2 - NH 4 + PhosphorusInorganic: Ortho-phosphate PO 4 - SiliconInorganic: Silicic acid SiO 3 -

Nitrate Uptake into the Cell Reduction steps: Reduced forms of nitrogen are ‘preferred’ NO 3 NO 2 NH 4 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 Waters N, 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 Nutrient Concentration S (e.g.,  mol l -1 ) Uptake Rate V (e.g., pmol cell -1 h -1 )

Nutrient Physiology Enzyme – controlled Assimilation : involves Uptake (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 V m is maximum V S is substrate concentration K s is the half- saturation constant Nutrient Concentration S (e.g.,  mol l -1 ) Uptake Rate V (e.g., pmol cell -1 h -1 ) VmVm Ks

Michaelis-Menten Parameters V m reflects (for example) the total number of enzymes available to do the uptake or reduction reactions K s 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] KsKs V max or µ max upwelling oligotrophic

Oligotrophic –↓ [nutrients] ↓ PP Eutrophic – ↑ [nutrients] ↑ PP Mesotrophic – moderate nutrients and PP HNLC – limited by iron ↑ nitrate ↓chlorophyll

Contrasting Nutrient Kinetics Nutrient Concentration Uptake or Growth Rate

Nutrient Kinetics in the Community Reflect the ambient nutrient environment Low nutrients = Oligotrophic, tropical waters Max growth rates μ max (generations day -1 ) = 0.1 – 0.2 (Low V m ) Half Saturation constant K s (in μM) = 0.01 – 0.1 low K s High nutrients: Eutrophic coastal, tropical upwelling Max growth rates μ max = 1 – 3 Half Saturation constant K s = High V m, high K s

Nutrient Kinetics in Differing Environments Changes in nutrient kinetics can reflect changes in: Community composition Shift to ‘r’ strategists (i.e. diatoms) dominating population when nutrients become available Organism characteristics Organisms 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 Ratio C : 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 O 2 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 Mixing I

Critical depth and ocean mixing winter If the mixed layer depth is greater than the critical depth, photosynthesis cannot occur. Conversely, when D mix < D CR, positive photosynthesis can occur. When D mix = D CR, it is the onset of the spring bloom in temperate waters.

Critical Depth and Ocean Mixing D cr = (I o /kI c )  (1-e -kDcr ) Good predictor of bloom, all you need to know is: surface irradiance (I o ) extinction coefficient (k) and compensation light intensity (I c ) -measure in lab If -kD cr >>0, then D cr = (I o /kI c )

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 LIFE C 106 :N 16 :P 1 Carbon Nitrogen Phosphorus C:N = 6.6 / C:P = 106 / N:P = 16

Temperature Effect