Phytoplankton: Nutrients and Growth

1. Phytoplankton: Nutrients and Growth

2. Outline • Growth • Nutrients • Limitation • Physiology • Kinetics • Redfield Ratio • Critical Depth

3. Why do we care about phytoplankton growth?

4. Biomass – how much phytoplankton at any one time, g C/m2 • Productivity – how fast what is there is growing, g C/m2/year

5. 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.

6. Growth Rates in the Ocean Equation for Growth: • B = cell number or biomass concentration (e.g., cells m-3)B(t) = concentration at time tB(0) = initial concentration (concentration at t=0) • m = growth rate (e.g., d-1) • t = time (e.g., d)

7. B t

8. Growth Stages of Growth – Batch Culture Stationary Crash Log Growth Log cells/L Lag Time (days)

9. LIMITING Nitrate Phosphate Silicate Iron Manganese Nutrients

10. Nutrients • LIMITING • Nitrate • Phosphate • Silicate • Iron • Manganese • NOT LIMITING • Magnesium • Calcium • Potassium • Sodium • Sulphate • Chloride • CO2

11. 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

12. Inorganic (DIN): Nitrate, Nitrite, Ammonium NO3- NO2- NH4+ Nitrogen Organic (DON): Urea, amino acids Phosphorus Inorganic: Ortho-phosphate PO4- Silicon Inorganic: Silicic acid SiO3- The Principle Macro-Nutrients for Phytoplankton

13. Nitrate Uptake into the Cell Reduction steps NO3 NO3 NO2 NH4 Proteins Diffusional Gradient Reduction steps: Reduced forms of nitrogen are ‘preferred’ Presence of concentrated ammonium may inhibit nitrate reductase synthesis

14. 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)

15. The marine nitrogen cycle

16. 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?

17. Ways to avoid nutrient limitation: • Optimization of uptake systems • Cell size (Surface-to-volume ratio) • Cell shape • Storage • Reduced growth rates

18. 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

19. Uptake Rate V (e.g., pmol cell-1 h-1) Nutrient Concentration S (e.g., mmol l-1) Nutrient Physiology • Enzymes: Cells: CommunitiesNutrient uptake subject to saturation

20. 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

21. Uptake Rate V (e.g., pmol cell-1 h-1) Nutrient Concentration S (e.g., mmol l-1) Michaelis-Menten Kinetics Vm • V is uptake rate • Vm is maximum V • S is substrate concentration • Ks is the half-saturation constant Ks

22. 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

23. Michaelis-Menten nutrient uptake kinetics Optimization of uptake systems Vmax or µmax Ks [N] oligotrophic upwelling

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

25. Contrasting Nutrient Kinetics Uptake or Growth Rate Nutrient Concentration

26. 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

27. 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

28. 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

29. C : N : P Redfield Ratio 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) Distribution of Macro-Nutrients Elemental distributions within phytoplankton are relatively constant throughout the World Ocean. 106 C : 16 N : 1 P : 270 O

30. 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

31. Nitrate versus phosphate relationship N:P= 16:1

32. 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

33. Critical Depth and Ocean Mixing I

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

35. Critical Depth and Ocean Mixing Dcr = (Io/kIc)(1-e-kDcr ) If -kDcr >>0, then Dcr = (Io/kIc) • Good predictor of bloom, all you need to know is: • surface irradiance (Io) • extinction coefficient (k) • and compensation light intensity (Ic) -measure in lab

36. 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

38. REDFIELD STOICHIOMETRY OF LIFEC106:N16:P1 Carbon Nitrogen Phosphorus C:N = 6.6 / C:P = 106 / N:P = 16

39. Temperature Effect