1.3 Light & Photosynthesis

Physics of Light

  • Attenuation - reduction in intensity with distance in a medium

    • Input wave decreases in wavelength when traveling through any absorbent medium

    • at 1m only 45% of solar energy that hits the ocean remains, at 10m its only 16% and at 100m is 1% - Source

  • Light absorption - light is absorbed by colored objects or particles

    • detritus, chlorophyl, DOM and water will all absorb light as it passes

  • Scattering - when light encounters particles smaller than a wavelength

    • water & bacteria & suspended particles

  • Reflection & absorption go hand in hand - we see the colors that are reflected, not the ones that are absorbed (white=all, black=none)

  • Irradiance - the flux of radiant energy per unit area

Photosynthesis

Photosynthetically active radiation (PAR)

  • PAR - the amount of light available for photosynthesis

  • 400-700nm wavelength range

  • PAR changes seasonally and varies depending on latitude and time of day, with levels highest during summer at mid-day

  • Measuring PAR:

    • values range from 0 to 3,000 millimoles per square meter, at night PAR = 0

    • measured using a silicon photovoltaic detetor

  • PAR irradiance - radiant flux density, or the radiative energy recieved by unit surface area in unit time carried by photons on the PAR waveband

Variables

  • Kd - vertical attenuation coefficient for downwelling irradiance

    • determines the characteristics of the local light environment

  • Sources - Thomas' dissertation,

Photoinhibition

  • When PAR exceeds the capacity of the light-independent reactions (calvin cycle).

  • Electrons get transferred to oxygen but energy is not transferred photosynthetically

  • Excess energy can damage photosystems

  • Commonly exacerbated by environmental stressors like cold, nutrient limitation, etc.

Chloroplasts & Chlorophyl

  • Chloroplast - organelle within plant cells

    • Stroma - fluid

    • Thylakoid membrane - light reaction & calvin cycle

    • chloroplasts are very similar to cyanobacteria - believe that endosymbiosis led to full integration of chloroplasts

  • Chlorophyl - family of pigments that make plants green, absorb blue and red light, reflect green

    • chl a - all photosynthetic organisms have it, enables light energy to be transformed into chemical energy

    • chl b - algae and trees, increases light absorption and capacity of chl a

    • chl c2 - in some algae, allows chl a to absorb sunlight (reddish-brown in color)

    • chl d - in some cyanobacteria

    • chl f - absorbtion of red light very efficently

Photosystems

Photosystem 2 - oxidizes water and transfers electrons to photosystem 1

  • Photon enters photosystem 2 & is absorbed by light harvesting complex, moving it to P680

  • P680 absorbs red light at 680nm

  • electron is taken from water molecule (oxidized)

  • Photon excites the electron, moving it up in energy state

  • Electron transport chain moves electron from photosystem 2 to 1, making ATP

Photosystem 1 - generates NADPH, uses phosphorylation to generate ATP

  • photon enters photosystem 1, is absorbed by light harvesting complex, moving to P700

  • P700 absorbs light at 700nm

  • electron from photosystem 2 is excited, reacting to form NADP+

  • takes in two electron to make NADPH

  • NADPH & ATP produce a sugar complex, forming a glucose molecule

Fluorescence

  • when photon excites electrons in PSI or PSII:

    • 1 photochemistry of energy creation

    • 2 emits heat

    • 3 emits a photon with a longer wavelength (fluorescence)

  • in fully functional/healthy PSII, the fluorescence is predictable

  • increased fluorescence means that less of the photon energy is going into energy creation and more is being fluoresced

Calvin Cycle (Light independent) - carbon fixation

  • occurs in Stroma

  • Phase 1 - C is fixed by Rubisco

    • Rubisco - enzyme that fixes inorganic carbon to organic

      • if there is too much O2, rubisco will accidentally split O2, creating a toxic byproduct that plants have to deal with

  • Phase 2 - Reduction

  • Phase 3 - Regeneration

Instrumentation

  • Radiometers - light meters, measure UV and visible light, describe shape of light field

  • Satellites - spectral radiometers measure light reflected from the surface layer of the ocean

  • Pulse amplitude modulated (PAM) -

  • Silicon photovoltaic detector - measures PAR

References

Graus & Macintyre 1976

Created a computer model to simulate macromorphology based on light availability that matched the massive - platylike trend we see in nature.

Photoacclimation in shallow corals can also include increased concentrations in photoprotective pigments of the symbiont (xanthophyll, β-carotene) and host (fluorescent proteins), or greater dissipation of absorbed light-energy through non-photochemical quenching (Wall 2020)

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