> For the complete documentation index, see [llms.txt](https://taylor-lindsay.gitbook.io/tl-comps/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://taylor-lindsay.gitbook.io/tl-comps/1.-basics/1.3-light-and-photosynthesis.md). # 1.3 Light & Photosynthesis ## Physics of Light

University of Minnesota Sea Grant - Source

* 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](https://rwu.pressbooks.pub/webboceanography/chapter/6-5-light/) * 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

Byron Inouye

## Photosynthesis #### [Photosynthetically active radiation (PAR)](https://www.fondriest.com/news/photosyntheticradiation.htm#:~:text=Photosynthetically%20Active%20Radiation%20\(PAR\)%20is,the%20summer%20at%20mid%2Dday.) * 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](https://www.youtube.com/watch?v=_hUxKPSNTl0\&ab_channel=CNUBiochemistry) 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](https://www.youtube.com/watch?v=Bb114gD4Ys0\&ab_channel=loblolly90) * 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)