Chapter 4 Overview: Nutrient Uptake and Assimilation
Ecology of Phytoplankton
 Chapter 4: Introduction
 Resource requirements for assembling photoautotrophic biomass
 Uptake and assimilation quota per phytoplankton cell
 Elements within a cell: (Chemistry!)
 Protoplasm: 19 elements
Considerable abundance: Hydrogen
Oxygen, and Nitrogen.
Trace abundance: Phosphorous,
Sulfur, Potassium, Sodium, Calcium,
Magnesium and Chlorine.

Law of Supply and Demand? : (Economics!)
 Available
 Soluble
 Diffusible
 Assimilable
 Chapter 4.2: Cell Uptake and Intracellular Transport of Nutrients
 Openings in Cell Membrane
 Diffusion
 Sunlight conversion
 Phytoplankton’s Uphill Battle
 Overcome concentration gradient
 Doubling cell material
 Parent cell must acquire 1 mole of Carbon per 1 mole of Carbon in daughter tissue
 Cellular draw of ~ 151 mmol N and 9.4 mmol P per 1 mole of Carbon
 Chapter 4.2.1: Supply of Nutrient
 Phytoplankton cell limited mechanistically to cell BL
 Governed by Fickian laws of diffusion
 n = am(dC0 /dx) t
 units = mol/m2 sec
 Where n is equal to the number of moles of a solute that will diffuse through an area (a) per unit time
(t), and is dependent on the solute/cell gradient concentration (dC0 /dx)
 Note: m = the coefficient of molecular diffusion of the substance

Further Detail about gradient concentration, enzymes responsible for obtaining C and the Carbon
concentrating mechanism (CCM) see pgs 127-128.
 Munk and Riley’s (1952) consideration of the Boundary Layer (BL)
 Pioneers
 Noted effect of motion
 Swimming, Passive Sinking or Flotation
 All aimed at same evolutionary goal
 Goal: Reproductive Success
 How? Move to location with more favorable concentration gradient, in order to make it easier to
absorb key nutrients at higher rate, possibly speed up assimilation, use nutrients for replication
 For Phytoplankton it’s all about LOCATION, LOCATION, LOCATION! (Real Estate!)
 Chapter4.2.2: Moving Nutrients Into the Cell
 Membrane Transport Systems
 Accept specific target molecules
 Transfer them to sites of deployment
 Expend energy
 Adenine TriPhosphate (Phosphorylation)
 Used in ion gradient maintenance
 Proton motive forces
 Schematic Diagram of Oxidative Phosphorylation

 This reaction releases energy (33 kJ/mol)
Nutrient Uptake
 Involves sequences of Protein-Protein interactions
 Specific target ligands…(Key)
 Peripheral receptor…( Lock)
 Excitation yields transfer response
 Central Reaction: Within complex, catalyze the phosphorylation of substrate!

Operon: repress the expression of other genes which regulate the reaction of nutrient starvation
 Chapter 4.2.3: Empirical Models of Nutrient Uptake (Physics!)
 Dugdale (1967) established that saturable transport capacity and rate of uptake were dependant on one
another
 Displayed that rate of uptake in nutrient starved cells and concentration of nutrients available follow
Michaelis-Menten (MM) enzyme kinetics
 Michealis-Menten Enzyme Kinetics Diagram:


MM assumes substrate>>> enzyme (i.e. enzyme is reaction limiting)

In our case you can increase surrounding areas of nutrient concentrations until, you reach a
maximum point where the uptake of the cell cannot proceed at any faster of a rate

Uptake is analogous to the amount of Enzyme Velocity and Concentration of surrounding nutrients is
analogous to substrate.
Droop and Dugdale (1973 and 1974) accounted for an internal storage factor


VU = [(qmax – q)/(qmax –q0)][VUmax S/(KU +S)]

qmax = replete cell quota

q = instantaneous cell quota

q0 = Droop’s Minimum cell quota

VUmax = saturable cell limit

KU = constant of half saturation

S = resource concentration
Applicable: When supply rates > deployment

Internal storage increases

Uptake rate subsequently slows

Even when surrounding concentrations are high
 Chapter 4.3: Phosphorous: Requirements, Uptake, Deployment in Phytoplankton


Phosphorous as an essential requirement for living and functioning

Component of nucleic acid governing protein synthesis

Adenine phosphate transformations to power intracellular transport
Luxury Uptake: Phytoplankton plan for their future! (Investors)

Luxury Uptake in cells may be responsible for a cell containing 8-16 times the minimum quota of P

Theoretically possible to sustain 3-4 cell doublings without any more P uptake
 Chapter 4.3.1: The Sources and Biological Availability of Phosphorous in Natural Waters

P Sources:

Natural Sources

Rainfall

Weathering of Phosphatic minerals and rocks


Due to its low solubility in water in pH neutral conditions, bioavailability in drainage waters is
low
Anthropogenic Sources

Losses of Inorganic Phosphorous to water

Quarrying

Agriculture

Tilling


Waste Treatment
Different Forms

OP(OH)3 Orthophosphoric acid

PO34- (Phosphate)

HPO42- (Hydrogen Phosphate)

H2PO4 (Dihydrogen Phosphate)
 Chapter 4.3.2: Soluble Molybdate-Reactive Phosphorous (MRP) Uptake Kinetics:

Initiated if external P concentrations drop < 20uM

Second ATP driven High affinity system is activated


Between Periplasmic Phosphate-bonding sites
Phosphorous limited uptake rates remain capable of saturating growth rates to non-ideal external
concentration

Faster uptake capacity at low sub-saturated resource levels, yields greater algae affinity for P,
increases algae’s ability to compete for diluted resources
 Chapter 4.3.3: Metabolic-Rate Limitation by Phosphorous

Cyanobacteria are able to maintain full growth down to external concentrations of 100 nmol P/L


Without producing operons, that regulate cell starvation…i.e. the cells are not starving at this
very dilute external concentration
What Are Signs of Starvation?

Presence of regulatory proteins

Abrupt deceleration in rate of protein synthesis

Sharp slowdown in cell growth

Phosphorous availability does not limit phytoplankton activity and growth until the MRP
concentration in surrounding medium falls to the lower limits of analytical detection!

Mixotroph: Photosynthetic organisms capable of supplementing or, perhaps, fulfilling their
requirements for nutrients and carbon by ingestion of organic particles

Mixotrophy is beneficial as a supplemental source of nutrients in small bodies of water, which only
receive nutrients in the form of terrestrial organic matter, but are otherwise are oligotrophic
 Chapter 4.3.4: Capacity Limitation and Potential Phosphorous Yield

Phosphorous can cause severe capacity limitation within large, oligotrophic, deep lakes at high altitudes in
the northern hemisphere
 Chapter 4.4: Nitrogen Requirements, Sources, Uptake and Metabolism in Phytoplankton

N Sources:

Nitrogen is another elements (as well as P), whose relative non-abundance impinges on
phytoplankton ecology

Most likely using an elemental analyzer?! C:N ratios in N replete cells that are capable of attaining
rapid growth is between 6.6 and 8.2

C:N ratios of less than 6 in vegetative cells symptomatic of Carbon deprivation

Higher molecular ratios N:P > 30 are indicative of intracellular phosphorous deficiencies

Lower molecular rations N:P < 10 are indicative of nitrogen deficiencies
 Chapter 4.4.1: The Sources and Availability of Nitrogen to Phytoplankton

N Forms:

Nitrate

Nitrite

Ammonium

DIN: Dissolved Inorganic Nitrogen

DON: Dissolved and Bioavailable Organic Nitrogen

Approximately 80% of the nitrogen in Oceanic Surface Waters is Organic (Anita et al. 1991)
 Chapter 4.4.2: Uptake of DIN by Phytoplankton

DIN uptake occurs at Levels as low as 3-4 mg N/m3

Ammonium uptake is favored because it is both probable and energetically favorable

Maximum DIN uptake rate: 0.6 to 35 uMol N/ mol cell Carbon second

Competent to saturate growth demand
 Chapter 4.4.3: Nitrogen Fixation

Prokaryotes have the exclusive ability to exploit the atmospheric nitrogen gas reservoir

Nitrogen fixation to ammonia is widespread trait in heterotrophic chemolithotrophic bacteria,
photosynthetic bacteria and Cyanobacteria

Biological fixation of N occurred in organisms containing the dinitrogen reductase enzyme


Requires Phosphorous

Catalyzes the reduction of N2 to ammonium via carbohydrate oxidation

Reaction Eqn: 2N2 + 4H+ + 3(CH2O) + 3H2O64NH4+ + 3 CO2

Nitrogen fixation can become rapidly inactivated in the presence of oxygen
Cyanobacteria Have Achieved A Tremendous Evolutionary Achievement, They Possess The Ability To
Operate Under Oxygen Driven Photosynthesis As Well As Anoxic Nitrogen Fixation!

Nitrogen fixation is beneficial only when DIN is truly limiting and where there is energy, phosphorous,
sufficient Iron, Molybdenum and Vanadium
 Chapter 4.5: The Role of Micronutrients

C, O, H, N, P and S major nutrients within cell

Na, K, Ca, Mg, and Cl are present in small quantities at low concentrations within the cell

Used to be known as Trace Elements, now referred to as Micronutrient
 Chapter 4.5.1: The Toxic Metals

Most studies involving micronutrients are performed in lab

Some toxic metals are necessary additions in order to culture media


Some toxic metals are not necessary additions in order to culture media


Iron, Manganese, Zinc, Copper, Molybdenum, and Cobalt
Barium and Vanadium
Advantages of Adding Toxic Metals?

Molybdenum was shown to stimulate carbon fixation and nitrogen uptake rates (Axler et al., 1980)

Specifically involved in the nitrogen metabolism and intercellular transport (Rueter and Peterson,
1987)
 Chapter 4.5.2: Iron

Iron plays a role in photosynthetic carbon and nitrogen reduction

Electron transport and pigment photosynthesis via Fe containing compounds (ferredoxin and
nitrogenase)

Depleted iron is symptomatic when there are reduced levels of cytochrome f (Glover, 1977)

Impairs structural assembly of thylakoid membranes (Guikema and Sherman, 1984)


Iron deficient cells are able to harvest relatively fewer photons in comparison to iron replete cells

Photon energy is used less efficiently

Results in poor synthetic yields of fixed carbon

Lower reductive power

Impaired growth potential

Restrict the direct synthesis of nitrite reductase
Very little Iron is available in pH neutral waters (environmental conditions)

10-7 – 10-5 M Fe

This is due to the low solubility of hydrous ferric oxide

Many Cyanobacteria can acquire and transport Fe via production of extracellular iron-binding
compounds called siderophores, which comprise part of their own high-affinity iron-transport system
(Simpson and Neilands, 1976)

Dilute Fe concentrations at sea


Augmented by direct wet deposition of dust, derived from arid terrestrial origin (Karl, 2002)
Oceanic conditions

Iron concentration is too dilute (~10-10 M) to support further autotrophic biomass

Iron availability is absolutely yield limiting

Nitrogen fixation is not the only process constrained
 Chapter 4.5.3: Organic Micronutrients and Vitamins

Microalgae are known to benefit from organic supplements at low concentrations.

Thiamine, Biotin and Cyanobalamine (vitamin B12)


Essential nutrients for some species
What is their function within a phytoplankton cell?

Thiamine : Co-factor in the decarboxylation of pyruvic acid

Biotin: Co-factor in the carboxylation and transcarboxylation reaction that occur during
photosynthesis

Vitamin B12 : Mediates reactions involving intramolecular recombinations of C-C bonds

These substances are photosensitive

The demand and supply of organic trace substances seem not to exert any strong ecological outcome
on the competitive potential of plankton in the wild
 Chapter 4.6: Major Ions

Ca, Mg, Na, K and Cl consist of major ions in lake and sea water

No less important than P or N

Ecological role is unclear and not well understood
 Chapter 4.6.1: Cations


Among freshwaters, calcium is the dominant ion

Form : CaCO3 (Calcium Carbonate)

Bicarbonate (CO32-) anions in freshwater electrochemically balance the cations that cause calcium
water hardness

Bicarbonate is derived from the salt of a weak acid (does not completely dissociate in water), allows
the strong alkaline ions (Ca+) to press the pH above neutrality (basic)

This is counteracted by the presence of free CO2 in solution

Free CO2 is formed by the dissociation of the bicarbonate anion

Buffers the water at a mildly alkaline level

Provides a source of bioavailable DIC
Magnesium is second most abundant

Effect of sodium and potassium are rarely considered to have much influence on algal composition

Except for impacting ionic strength and ionic osmosis across cell membrane

Conclusion: Regulation of phytoplankton growth by Na or K is unlikely in nature
 Chapter 4.6.2: Anions

Bicarbonate and the other major ions (chloride and sulfate) do not appear to limit algal production
 Chapter 4.7: Silicon: Requirements, Uptake, Deployment in Phytoplankton

Requirements for trace amounts of silicon

Involved in protein synthesis

Used in scale production and skeletal reinforcements

[Si] Freshwaters >> [Si] Saltwater

Uptake and intracellular transport of H4SiO4 proceeds by way of a membrane-bound carrier


Also conforms to Michealis-Menten kinetics
Species differences in size, shape and vacuole size determine the amount of silicon needed to complete
the new cell, varies in relation to the mass of the cytoplasm
 Take Home Point: Phytoplankton Can Be Considered The Proverbial ‘Jack Of All Trades’ Because Of Its
Unique Ability To Do So Many Things

In Its Lifetime A Phytoplankton Cell Will Dabble In The Principles Of:

Chemistry = Resource Requirements For Assembling Photoautotrophic Biomass/ 19 Elements Within
A Cell

Economics = Law Of Supply And Demand, Are Required Nutrients Available, Soluble, Diffusable, And
Assimilable

Real Estate = Location, Location, Location, A Phytoplankton Cell Will Move To Location With More
Favorable Concentration Gradient, In Order To Make It Easier To Absorb Key Nutrients At Higher
Rate, Possibly Speed Up Assimilation, Use Nutrients For Replication

Investment = Phytoplankton Cells Invest Excess Nutrients In What Is Known As A Luxury Uptake In
Cells Which Is Responsible For A Cell Containing 8-16 Times The Minimum Quota Of P. Theoretically
Possible To Sustain 3-4 Cell Doublings Without Any More P Uptake

Physics = Many Processes Within Phytoplankton Follow Michealis-Menten Enzyme Kinetics Such As,
Nutrient Uptake And H4sio4 Uptake And Intracellular Transport By Way Of A Membrane-Bound
Carrier
References:
Plant Cell Diagram: http://cyberlab.lh1.ku.ac.th/elearn/faculty/fisher/fi15/web/images/tmp/cp2.jpg
Phytoplankton Picture: http://www.imr.no/english/__data/page/6419/phytoplankton.jpg
Oxidative Phosphorylation Diagram:
http://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/atpstruct.gif
Michealis-Menten Kinetics Diagram: http://www.graphpad.com/curvefit/1969e190.gif
Weathering Rocks Picture:
http://www.csir.co.za/rhp/state_of_rivers/state_of_umngeni_02/photos/midland_mount_800.jpg
Jamesville Quarry Picture: http://myhome.sunyocc.edu/~harrism//jvillequarry/full%20size/IMG_9625.jpg
Image of O-Phosphoric Acid: http://en.wikipedia.org/wiki/Orthophosphoric_acid
Image of Phosphate: http://en.wikipedia.org/wiki/Phosphate
Image of Hydrogen Phosphate: http://en.wikipedia.org/wiki/Phosphate
Image of Nitrate Molecule: http://en.wikipedia.org/wiki/Nitrate
Image of Nitrite Molecule: http://en.wikipedia.org/wiki/Nitrite