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This is only tangentially related, but I have always wondered why chlorophyll absorbs blue and red, but reflects green--green being sunlight's brightest compone

This is only tangentially related, but I have always wondered why chlorophyll absorbs blue and red, but reflects green--green being sunlight's brightest component. It's almost as if there was some evolutionary pressure towards being very visible in sunlight which is more important than evolving ways to collect as much sun energy as possible. When I guess at this I end up with something along the lines of reflected green being used as a signal to a neighboring plant: "I'm already here, grow in some other direction instead." There is some evidence that plants do this (https://en.wikipedia.org/wiki/Crown_shyness https://en.wikipedia.org/wiki/Crown_shyness, https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep11604091 https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep1160...) but it's not clear that the need to do so is so strong that it would overshadow the drive to collect as much energy as possible. Or perhaps there's something to do with the physics of absorbing light to drive a chemical reaction that makes it better to absorb at red and blue while passing on green (450nm and 680nm are not harmonics--so if this is the case it's more complex than which sorts of standing waves would fit in some chemical gap or other).

amelius 1y ago
Red is nature's warning signal, and blue was already taken by the sky, so the only option left was green. Just kidding of course, it is an interesting question.
- amelius 1y ago
  Maybe it has something to do with keeping the albedo of the planet at a higher value.
pinkmuffinere 1y ago
This is fascinating, I’d never realized there is this seeming-paradox! Thanks for mentioning it
privatelypublic 1y ago
Have you looked into band-gaps? Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho. Theres probably something more efficient at converting light into simple sugars.
cheschire 1y ago
Maybe it was in response to an extinction level event that filtered sunlight for a long time, removing green but allowing primarily only blue or red.
- selcuka 1y ago
  It still doesn't explain the need to reflect green, though. They could have evolved to be black and absorb all energy.
  - AngryData 1y ago
    Well I think the question then becomes can they use that much energy without cooking themselves alive? Maybe the entire spectrum is simply too much incoming energy to dissipate effectively and blue and red absorption are either more efficient or is easier to accomplish than green.
  - privatelypublic 1y ago
    One, you're adding intent to a process that has no intent. It's not reflected- it's just not used (and thus absorbed). If the plant has more energy that it needs already to produce offspring... what pressures exist to produce more?
    - selcuka 1y ago
      It is reflected. That's why plants are green. You seem to have missed the context of my reply: > an extinction level event that filtered sunlight for a long time, removing green The lack of green sunlight would not necessarily result in a mutation that reflects green. There is no evolutionary pressure for reflecting/not using something that doesn't exist. In fact, it would be adding intent to a process that has no intent.
o11c 1y ago
It could also be to prevent overstimulation; "maximize energy" is not really the goal. A lot of plants can die from too much Sun unless their other inputs are just right (plenty of water, etc.).
NoImmatureAdHom 1y ago
Here's a recent take: https://www.quantamagazine.org/why-are-plants-green-to-reduce-the-noise-in-photosynthesis-20200730/ https://www.quantamagazine.org/why-are-plants-green-to-reduc... TLDR: Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. Making use of green would cause variance to be large enough that the gains would not offset the losses. So, avoid green and have lower variance --> higher energy capture on average.
- fstarship 1y ago
  This video talks about it as well, it includes an interview with the author https://m.youtube.com/watch?v=TgGoW5AIKEY https://m.youtube.com/watch?v=TgGoW5AIKEY
- amelius 1y ago
  > Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. That would be easy to test, I suppose. In fact, perhaps we're already doing so by letting plants live in our offices with 60Hz flicker, and perhaps higher frequency flicker caused by LEDs and PWMs. In short, I'm not buying this theory just yet.
cornstalks 1y ago
> green being sunlight's brightest component. It actually peaks between magenta and blue: https://sunwindsolar.com/blog/solar-radiation-spectrum/ https://sunwindsolar.com/blog/solar-radiation-spectrum/ Green is only bright to us because of our cone sensitivities overlapping.
- scythe 1y ago
  That graph shows sunlight in the upper atmosphere, but at sea level, the 400-450 nanometer blues are partially scattered out. The peak we see on the ground is broader, and centered more around 450-550 nanometers, a range that tends more towards teal or "Miami green". Wikipedia shows both spectra: https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe... I think this is also why the sky appears to be a deeper, darker blue at higher altitudes.
  - cornstalks 1y ago
    The chart I linked shows both upper atmosphere and sea level irradiance, for what it's worth. Though it's possible the chart simply isn't the most accurate. I only did casual research. Doing more research, looking at [1] (the data source for that Wikipedia image), it looks like the peak is between 480 nm (cyan) and 540 nm (lime green) for "global tilt" and 480 nm to 580 nm (yellow) for "direct + circumsolar" (I have no idea what the difference is between "global tilt" and "direct + circumsolar"). Interesting. It seems like the chart in my original comment isn't as precise as I believed. [1]: https://www.nrel.gov/grid/solar-resource/spectra-am1.5 https://www.nrel.gov/grid/solar-resource/spectra-am1.5
- bradrn 1y ago
  > It actually peaks between magenta and blue No, it actually peaks wherever you want it to peak, depending on how you plot it: https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception https://www.oceanopticsbook.info/view/light-and-radiometry/l...
  - thaumasiotes 1y ago
gettingoverit 1y ago
> something to do with the physics of absorbing light to drive a chemical reaction Exactly that. Blue does two steps of the process, while red does only one. There's a cost for synthesizing all that machinery, so absorbing green would just be not worth it. > 450nm and 680nm are not harmonics In fact they're in 3:2 ratio with 1% margin. But they don't have to be. Take a look at fluorescence: it converts one wavelength to another, and they don't have to be multiples of each other. Once photon gets absorbed onto a chemical, the electronic structure of the molecule decides what will happen to it.
deleted 1y ago
[deleted]
adrian_b 1y ago
Chlorophyll a, which is the pigment that actually uses solar energy to split water, absorbs red light and violet light. Thus its color is blue-green, as it can be seen in some lichens that have only symbiotic cyanobacteria. This is most likely a historical accident, with no special meaning. Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a. The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a. Thus the addition of chlorophyll b increases considerably the amount of captured energy. The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black. Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.
- __MatrixMan__ 1y ago
  I don't think the mystery goes away when you consider the other photosynthetic pigments. chlorophyll-a, chlorophyll-b, lutein, B-carotene, zeaxanthin, lycopene... they're all active between 450 nm and 550 nm. And then chlorophyll-a and chlorophyll-b have secondary activity between 650 nm and 700 nm. The the lack of photosynthetic activity between 550 and 650 is still suspicious. I've learned from other commenters here that my assumptions about the gap corresponding with peak solar energy weren't on solid ground, but there is a gap. Perhaps a different way to frame the question is: why do the chlorophyll pigments have two peaks, while the others appear to have only one? Perhaps they have an evolutionary past which involves absorbtion from a star besides sol?
  - adrian_b 1y ago
    Most organic pigments have 2 or more absorption bands, but this is not always apparent for humans, because the second band is located in near ultraviolet or in near infrared. Like I have said, in marine algae you can find a variety of colors, as already seen in their names, e.g. red algae, yellow algae, brown algae. Most of them have strong absorption for green light, due to the auxiliary pigments that they happen to use. Only the green algae and their descendants, the land plants, do not absorb green light and as a consequence they are competitive with the other algae only in places with abundant light, i.e. in very shallow waters or on dry land, where they can get all the energy that they can use without damage, so they do not need better coverage of the solar spectrum. The energy of the captured photons does not matter much, because even the red photons have enough energy. The captured photons are not used for "photosynthesis", which is a misnomer whose origin lies in a time when the mechanism of "photosynthesis" was not known. In oxygenic phototrophs, a part of the captured energy is used to oxidize some manganese atoms so strongly that they can oxidize the oxygen from water, converting it into free dioxygen. The hydrogen from water is bound into a reduced organic substance (NADPH), which will be used later as a reducing agent (without needing light) to make carbohydrates from carbon dioxide. The rest of the energy of the captured photons is used to pump ions through the membrane of the chloroplasts. The energy stored in an ion gradient will be used later to power all organic syntheses. The ionic pumps could work even with infrared photons, as they do in phototrophic bacteria that live under algae, so they have modified chlorophylls whose red absorption band is shifted into infrared, away from the red band that is removed from solar light by the algae sitting above. However, those bacteria cannot split water, the energy of infrared photons is too low for that (splitting water needs around 1.25 eV, while red photons have over 1.5 eV).
vikramkr 1y ago
There's also a chance that the primary photosynthesiizers on each happened to be purple for a while (purple earth) and the ancestors of plants absorbed red/blue and ignored green because they were getting leftovers. Also, even now, iirc the limiting step in oxygenic photosynthesis is by far rubisco's incorporation of CO2, so there's no immediately obvious fitness function that would be optimized by just increasing the efficiency of light harvesting.
dr_orpheus 1y ago
There was a proposed theory on this the spread of absorption created more stability in the power generation of plants over different conditions. This was supposed to be a more important factor than being able to absorb the peak and highest energy. https://www.science.org/doi/10.1126/science.aba6630 https://www.science.org/doi/10.1126/science.aba6630
- __MatrixMan__ 1y ago
  I think this paper is what I was looking for, thanks. I may have to reread it after becoming more familiar with the chemical/physical nature of: > Photoexcitation energy is rapidly transferred through an antenna network before reaching the reaction center I didn't know that. With this in mind, perhaps a better formulation of my question is not: > Why are plants green? But instead: > Why are green photosynthetic pigments more common than others? Based on my read of this paper, the answer to that would be that a pigment which absorbed only a single narrow band of light would be prone to being either over or under powered most of the time. Absorbing red and blue, but not green, provides more opportunities to deliver constant power at the reaction center despite varying light conditions.
zeckalpha 1y ago
I didn't see any mention of this so thought I would add: This sounds like https://en.wikipedia.org/wiki/Retinal https://en.wikipedia.org/wiki/Retinal and https://en.wikipedia.org/wiki/Purple_Earth_hypothesis https://en.wikipedia.org/wiki/Purple_Earth_hypothesis. Going through history, there have been times where the Earth has had oxygen spikes https://en.wikipedia.org/wiki/Geological_history_of_oxygen https://en.wikipedia.org/wiki/Geological_history_of_oxygen (Examples https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event or https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_event https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_eve...) Cool image showing how this process is unstable: https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/File:Black-band_ironstone_(aka).jpg https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/F... You might be interested in the different photosynthesis cycles: https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Alarm_photosynthesis https://en.wikipedia.org/wiki/Alarm_photosynthesis - this one was only discovered in 2016! Research into these may have profound impact on climate change.

amelius 1y ago

Red is nature's warning signal, and blue was already taken by the sky, so the only option left was green. Just kidding of course, it is an interesting question.

amelius 1y ago
Maybe it has something to do with keeping the albedo of the planet at a higher value.

pinkmuffinere 1y ago

This is fascinating, I’d never realized there is this seeming-paradox! Thanks for mentioning it

privatelypublic 1y ago

Have you looked into band-gaps? Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho. Theres probably something more efficient at converting light into simple sugars.

cheschire 1y ago

Maybe it was in response to an extinction level event that filtered sunlight for a long time, removing green but allowing primarily only blue or red.

selcuka 1y ago
It still doesn't explain the need to reflect green, though. They could have evolved to be black and absorb all energy.
- AngryData 1y ago
  Well I think the question then becomes can they use that much energy without cooking themselves alive? Maybe the entire spectrum is simply too much incoming energy to dissipate effectively and blue and red absorption are either more efficient or is easier to accomplish than green.
- privatelypublic 1y ago
  One, you're adding intent to a process that has no intent. It's not reflected- it's just not used (and thus absorbed). If the plant has more energy that it needs already to produce offspring... what pressures exist to produce more?
  - selcuka 1y ago
    It is reflected. That's why plants are green. You seem to have missed the context of my reply: > an extinction level event that filtered sunlight for a long time, removing green The lack of green sunlight would not necessarily result in a mutation that reflects green. There is no evolutionary pressure for reflecting/not using something that doesn't exist. In fact, it would be adding intent to a process that has no intent.

o11c 1y ago

It could also be to prevent overstimulation; "maximize energy" is not really the goal. A lot of plants can die from too much Sun unless their other inputs are just right (plenty of water, etc.).

NoImmatureAdHom 1y ago

Here's a recent take: https://www.quantamagazine.org/why-are-plants-green-to-reduce-the-noise-in-photosynthesis-20200730/ https://www.quantamagazine.org/why-are-plants-green-to-reduc... TLDR: Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. Making use of green would cause variance to be large enough that the gains would not offset the losses. So, avoid green and have lower variance --> higher energy capture on average.

fstarship 1y ago
This video talks about it as well, it includes an interview with the author https://m.youtube.com/watch?v=TgGoW5AIKEY https://m.youtube.com/watch?v=TgGoW5AIKEY
amelius 1y ago
> Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. That would be easy to test, I suppose. In fact, perhaps we're already doing so by letting plants live in our offices with 60Hz flicker, and perhaps higher frequency flicker caused by LEDs and PWMs. In short, I'm not buying this theory just yet.

cornstalks 1y ago

> green being sunlight's brightest component. It actually peaks between magenta and blue: https://sunwindsolar.com/blog/solar-radiation-spectrum/ https://sunwindsolar.com/blog/solar-radiation-spectrum/ Green is only bright to us because of our cone sensitivities overlapping.

scythe 1y ago
That graph shows sunlight in the upper atmosphere, but at sea level, the 400-450 nanometer blues are partially scattered out. The peak we see on the ground is broader, and centered more around 450-550 nanometers, a range that tends more towards teal or "Miami green". Wikipedia shows both spectra: https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe... I think this is also why the sky appears to be a deeper, darker blue at higher altitudes.
- cornstalks 1y ago
  The chart I linked shows both upper atmosphere and sea level irradiance, for what it's worth. Though it's possible the chart simply isn't the most accurate. I only did casual research. Doing more research, looking at [1] (the data source for that Wikipedia image), it looks like the peak is between 480 nm (cyan) and 540 nm (lime green) for "global tilt" and 480 nm to 580 nm (yellow) for "direct + circumsolar" (I have no idea what the difference is between "global tilt" and "direct + circumsolar"). Interesting. It seems like the chart in my original comment isn't as precise as I believed. [1]: https://www.nrel.gov/grid/solar-resource/spectra-am1.5 https://www.nrel.gov/grid/solar-resource/spectra-am1.5
bradrn 1y ago
> It actually peaks between magenta and blue No, it actually peaks wherever you want it to peak, depending on how you plot it: https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception https://www.oceanopticsbook.info/view/light-and-radiometry/l...
- thaumasiotes 1y ago

gettingoverit 1y ago

> something to do with the physics of absorbing light to drive a chemical reaction Exactly that. Blue does two steps of the process, while red does only one. There's a cost for synthesizing all that machinery, so absorbing green would just be not worth it. > 450nm and 680nm are not harmonics In fact they're in 3:2 ratio with 1% margin. But they don't have to be. Take a look at fluorescence: it converts one wavelength to another, and they don't have to be multiples of each other. Once photon gets absorbed onto a chemical, the electronic structure of the molecule decides what will happen to it.

deleted 1y ago

[deleted]

adrian_b 1y ago

Chlorophyll a, which is the pigment that actually uses solar energy to split water, absorbs red light and violet light. Thus its color is blue-green, as it can be seen in some lichens that have only symbiotic cyanobacteria. This is most likely a historical accident, with no special meaning. Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a. The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a. Thus the addition of chlorophyll b increases considerably the amount of captured energy. The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black. Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.

__MatrixMan__ 1y ago
I don't think the mystery goes away when you consider the other photosynthetic pigments. chlorophyll-a, chlorophyll-b, lutein, B-carotene, zeaxanthin, lycopene... they're all active between 450 nm and 550 nm. And then chlorophyll-a and chlorophyll-b have secondary activity between 650 nm and 700 nm. The the lack of photosynthetic activity between 550 and 650 is still suspicious. I've learned from other commenters here that my assumptions about the gap corresponding with peak solar energy weren't on solid ground, but there is a gap. Perhaps a different way to frame the question is: why do the chlorophyll pigments have two peaks, while the others appear to have only one? Perhaps they have an evolutionary past which involves absorbtion from a star besides sol?
- adrian_b 1y ago
  Most organic pigments have 2 or more absorption bands, but this is not always apparent for humans, because the second band is located in near ultraviolet or in near infrared. Like I have said, in marine algae you can find a variety of colors, as already seen in their names, e.g. red algae, yellow algae, brown algae. Most of them have strong absorption for green light, due to the auxiliary pigments that they happen to use. Only the green algae and their descendants, the land plants, do not absorb green light and as a consequence they are competitive with the other algae only in places with abundant light, i.e. in very shallow waters or on dry land, where they can get all the energy that they can use without damage, so they do not need better coverage of the solar spectrum. The energy of the captured photons does not matter much, because even the red photons have enough energy. The captured photons are not used for "photosynthesis", which is a misnomer whose origin lies in a time when the mechanism of "photosynthesis" was not known. In oxygenic phototrophs, a part of the captured energy is used to oxidize some manganese atoms so strongly that they can oxidize the oxygen from water, converting it into free dioxygen. The hydrogen from water is bound into a reduced organic substance (NADPH), which will be used later as a reducing agent (without needing light) to make carbohydrates from carbon dioxide. The rest of the energy of the captured photons is used to pump ions through the membrane of the chloroplasts. The energy stored in an ion gradient will be used later to power all organic syntheses. The ionic pumps could work even with infrared photons, as they do in phototrophic bacteria that live under algae, so they have modified chlorophylls whose red absorption band is shifted into infrared, away from the red band that is removed from solar light by the algae sitting above. However, those bacteria cannot split water, the energy of infrared photons is too low for that (splitting water needs around 1.25 eV, while red photons have over 1.5 eV).

vikramkr 1y ago

There's also a chance that the primary photosynthesiizers on each happened to be purple for a while (purple earth) and the ancestors of plants absorbed red/blue and ignored green because they were getting leftovers. Also, even now, iirc the limiting step in oxygenic photosynthesis is by far rubisco's incorporation of CO2, so there's no immediately obvious fitness function that would be optimized by just increasing the efficiency of light harvesting.

dr_orpheus 1y ago

There was a proposed theory on this the spread of absorption created more stability in the power generation of plants over different conditions. This was supposed to be a more important factor than being able to absorb the peak and highest energy. https://www.science.org/doi/10.1126/science.aba6630 https://www.science.org/doi/10.1126/science.aba6630

__MatrixMan__ 1y ago
I think this paper is what I was looking for, thanks. I may have to reread it after becoming more familiar with the chemical/physical nature of: > Photoexcitation energy is rapidly transferred through an antenna network before reaching the reaction center I didn't know that. With this in mind, perhaps a better formulation of my question is not: > Why are plants green? But instead: > Why are green photosynthetic pigments more common than others? Based on my read of this paper, the answer to that would be that a pigment which absorbed only a single narrow band of light would be prone to being either over or under powered most of the time. Absorbing red and blue, but not green, provides more opportunities to deliver constant power at the reaction center despite varying light conditions.

zeckalpha 1y ago

I didn't see any mention of this so thought I would add: This sounds like https://en.wikipedia.org/wiki/Retinal https://en.wikipedia.org/wiki/Retinal and https://en.wikipedia.org/wiki/Purple_Earth_hypothesis https://en.wikipedia.org/wiki/Purple_Earth_hypothesis. Going through history, there have been times where the Earth has had oxygen spikes https://en.wikipedia.org/wiki/Geological_history_of_oxygen https://en.wikipedia.org/wiki/Geological_history_of_oxygen (Examples https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event or https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_event https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_eve...) Cool image showing how this process is unstable: https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/File:Black-band_ironstone_(aka).jpg https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/F... You might be interested in the different photosynthesis cycles: https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Alarm_photosynthesis https://en.wikipedia.org/wiki/Alarm_photosynthesis - this one was only discovered in 2016! Research into these may have profound impact on climate change.