Download Photosynthetic Pigments and Light Absorption

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Photosynthetic Pigments and Light Absorption
Introduction
All phytoplankton are autotrophs (i.e., self-feeding), manufacturing their
own food, in their case by the process of photosynthesis- hence are
photoautotrophs and thus also considered primary producers.
PHOTOSYNTHESIS
Biological process of creating high-energy organic material from carbon
dioxide, water and other essential nutrients utilizing the sun's energy
6CO2 + 6H2OC6H12O6 + 6O2
Phytoplankton used to be considered plants, but we now have a more complex
classification. Phytoplankton may be found in almost all Kingdoms.
Evolution of phytoplankton:
• Photosynthesis developed in Purple Photosynthetic Bacteria (PS II) and
Green Sulfur Bacteria (PS I)
• These were presumably phagotrophically engulfed by ancestral
prokaryotes.
• The ancestral prokaryote was in turn engulfed by primitive eukaryotes,
which kept the mitochondria and the chloroplasts – this is called
endosymbiosis (Margulis, 1974).
— Photosynthetic Reactions (the simple version)
-
If we were to design an organism that could utilize the sun’s energy to
produce organic compounds, we would need:
1) An outer wall to contain everything (the cell)
2) A permeable membrane to enclose everything
3) A photochemical (light-sensitive) compound (pigment)
4) A source and sink for electrons (oxidants/reductants)
Photosynthesis is an oxidation-reduction reaction carried out by
photoautotrophs to reduce CO2 through the use of light-energy.
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Oxidation-Reduction Reactions:
3Fe + 2O2  Fe3O4
Fe3O4 + 2C  3Fe + 2CO2
Pigments
(oxidation of iron to form rust)
(reduction of Fe by heating with carbon)
Reduction is the removal of oxygen, the addition of electrons, or the addition of
hydrogen atoms; conversely, oxidation is the addition of oxygen, the removal of
electrons, or the removal of hydrogen atoms.
-In phytoplankton, these oxidation-reduction reactions are catalyzed by the
presence of photosynthetic pigments which are reused, in combination with
water (the electron donor) and CO2 (the electron acceptor)
-All photosynthetic organisms (except for some bacteria) use oxygenic
photosynthesis, meaning they produce oxygen. Almost all have chlorophyll a as
the primary reaction pigment.
— Primary Pigments
- In marine algae, the primary photochemical compound used is Chlorophyll a
• Chlorophyll compounds have a series of pyrrol rings, with a Mg atom at the
center
• Remove the Mg (with weak acid), and you get the degradation product, which
is called a phaeophytin, which still absorbs light, but can’t be used for
photosynthesis (for example, phaeophytin a is the degradation product of
chlorophyll a)
- In addition to Chlorphyll a, there are several similar compounds which are used
by marine algae…some are chlorophyll compounds (Chl a, b, c1, c2, d, divinylchl a, divinyl-chl b)
-There are also other pigments called accessory pigments which are not used
directly for photosynthesis:
•
•
•
Phycobilins have NO metal (why is this important?) and are proteins (watersoluble)
Carotenoid compounds also have no metal, and overlap the chl compounds
in terms of absorption properties
these additional pigments do one of two things:
1) provide more absorption bands in the green
2) protect the phytoplankton from too much energy…chlorophyll can go from
a ground state (no excitation) to a triplet state which is highly
reactive—forms molecular oxygen, which is very dangerous (anti-oxidants
combat this in humans)
• Two examples of protective mechanisms involve xanthophyll cycling:
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— xanthophyll cycle: diadionxanthin  diatoxanthin (dinoflagellates,
diatoms, golden algae, euglenoids, etc.)
— violaxanthinantheraxanthinzeaxanthin (chlorophytes,
phaeophytes, higher plants)
— Each step dissipates excess energy, acting as a photoprotective
mechanism
• There are other compounds, such as Microsporine-Like Amino Acids
(MAAs) that protect against too much UV light
• All of these are examples of photoprotective, or sunscreen-like, pigments
•
•
Each pigment has a specific absorption maximum (wavelength, or color, of
light) that it absorbs best at, and an emission maximum (fluorescence) that it
releases captured energy at…these are referred to as excitation-emission
spectra
We can use the combination of pigments found in any particular algae as a
“fingerprint” or diagnostic to tell us who is there
Some Examples of phytoplankton fingerprints:
Divinyl Chlorophyll a
Peridinin
Fucoxanthin
Phycobilins
Prochlorophytes
Dinoflagellates
Diatoms
Primarily cyanobacteria
IV. Fate of Absorbed Light
• Once light is absorbed, there are three possible fates:
• HEAT—all reactions give off at least some heat, and xanthophylls do this
as a protective mechanism
• FLUORESCENCE—re-radiation at a longer wavelength
• PHOTOCHEMISTRY—the use of the energy to produce oxidationreduction rxns
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Photosynthesis is probably the most efficient transfer of energy that exists on the
earth! It approaches 100% efficiency under optimal conditions, and easily
maintains >50% efficiency. For comparison, most trophic interactions lose 90% of
energy in transfer…man-made solar cells are typically 1-1.5% efficient. This
efficiency is partly due to quantum tunneling, a result of overlapping electron
orbitals in the pigment molecules
• In most algae (excluding the true bacteria), photosynthesis has evolved to
utilize two specialized chlorophyll molecules that do most of the energy
conversion:
• Photosystem II (PSII) absorbs at 680 nm
• Photosystem I (PSI) absorbs at 700 nm
• All the other pigments serve as “funnels” to transfer light energy towards PSII
and PSI…they are called “antenna pigments”
•
Photosystem is composed of functional units, made up of:
PS I
PS II
Associated ‘antenna’ pigments
• Photosynthetic Unit (PSU) is the entire complex of PSII, PSI and antenna
pigments. Typically 2500 Chl a/b molecules per PSU, and typically a 3:1 ratio
between Chl a/b
• Once light is absorbed by the PSU, some fraction is re-released as heat, some
fraction is released at a lower (redder) wavelength as fluorescence, and some
fraction is used for photochemistry—those are the only possible fates, so as one
goes up, the other(s) must go down
• The yield of each reaction is referred to as the Quantum Yield, or how many
photons are required to yield some reaction or byproduct:
Quantum Yield (Φ) = Photons Absorbed / Reaction
• The efficiency of the reaction is called the Quantum Efficiency, and refers to
how close to perfect the reaction is…for example, a high quantum efficiency for
photochemistry would mean that very few captured photons are dissipated as
heat or fluorescence
• Returning to our captured (absorbed) light, each photon that is absorbed has to
be either given off as heat (H), fluorescence (F), or used for photochemistry (P),
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so that the total quantum yield (the total probability that the photon is utilized) is
equal to unity:
ΦH + ΦF + ΦP = 1
• Therefore, if we can measure the yields of any two probabilities, we can
determine the third value.