Purple Earth hypothesis

https://en.wikipedia.org/wiki/Purple_Earth_hypothesis

by colinprince • 12 months ago

286 79 12 months ago

en.wikipedia.org

Jimi was right—Earth was in a purple haze. It just came from retinal-based photosynthesizers, not acid.

O5vYtytb• 12 months ago

Fun fact, early hand written lyrics were "purple haze, Jesus saves...". It was a recollection of a dream where he was walking under water. The connection to acid is more so by interpretation of the audience.

morkalork2• 12 months ago

It's not more associated to cannabis? There's many strains even named for it.

weinzierl• 12 months ago

"Retinal-containing cell membranes exhibit a single light absorption peak centered in the energy-rich green-yellow region of the visible spectrum, but transmit and reflect red and blue light, resulting in a magenta color.[5] Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light [..]"

I wonder why no plant evolved to use both and make the more even efficient use of light. These plats would appear dark, maybe almost black. They could live between all the green plants from their scraps so to speak.

"However, the porphyrin-based nature of chlorophyll had created an evolutionary trap[citation needed], dictating that chlorophyllic organisms cannot re-adapt to absorb the energy-rich and now-available green light, and therefore ended up reflecting and presenting a greenish color."

Yes, but why?

pc486• 12 months ago

As far as I understand it, this is a still debated question. One theory is it's about evaporating water: Plausible photomolecular effect leading to water evaporation exceeding the thermal limit (https://www.pnas.org/doi/10.1073/pnas.2312751120).

There are black plants though! And they're studied for the same kind of questions. E.g. The Functional Significance of Black-Pigmented Leaves: Photosynthesis, Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’ (https://pmc.ncbi.nlm.nih.gov/articles/PMC3691134/)

verisimi• 12 months ago

Nigrescens is overdue a 'naming review' with the Culture Ministry.

darth_avocado• 12 months ago

A few different threads based on my limited online research:

1. Absorbing all spectrum of light would provide more energy than the organisms can handle. They need gas to run the engine, and all spectrum would provide jet fuel.

2. Current predominant species of plants evolved from the undergrowth. Original plants would absorb only green, so the undergrowth evolved to absorb the other spectrums because that’s what was left. After a few planet scale extinction events where the sunlight was scarce, being able to absorb a wider spectrum became a successful evolutionary trait and became the predominant one.

3. There are species of fungi that use melanin to absorb radiation for energy source and appear black.

3eb7988a1663• 12 months ago

In fact, many plants already receive too much sunlight and have various mechanisms to limit their exposure.

luqtas• 12 months ago

Absorbing all spectrum of light would provide more energy than the organisms can handle. They need gas to run the engine, and all spectrum would provide jet fuel.

one could ask why not a bigger/powerful organism. and a nice answer for that would be: more energy, more stuff going on, more bad mutations having the chance to exist; impaired evolution ;)

adrian_b2• 12 months ago

While all the phototrophs that are able to split water and produce free oxygen use chlorophyll a, which absorbs only red light and violet light, resulting in a blue-green color, which can be seen as such in some lichens and cyanobacteria, most of them have some accessory pigments, which absorb other parts of the solar spectrum, and then transfer the energy to chlorophyll a.

The green algae, which live only in shallow waters, and the terrestrial plants use as accessory pigment only chlorophyll b, which absorbs a different band of red light than chlorophyll a and also blue light, resulting in a green color.

This is enough for green algae and land plants, because where they live there is abundant light. For land plants the problem is that they have too much light, not too little, with the exception of those which grow under the shadow of trees.

On the other hand, most marine algae use accessory pigments that absorb much more of the solar spectrum, so that the color of chlorophyll is no longer visible and they have overall colors like red, yellow or brown, even very dark brown. This enables such algae to live down to greater depths in the water, where there is less solar light.

So there are a lot of living beings that make very efficient use of light.

Moreover, under water there are many places where practically all light is captured, by multiple layers of algae and bacteria, each layer absorbing some part of the solar spectrum. Even the near infrared light is absorbed by a bottom layer of bacteria, which do not produce oxygen, because the energy of infrared photons is insufficient to split water.

throwawaymaths5• 12 months ago

technically the energy in green is also not enough to split water, which (IIRC) is why PSII must ping pong the photon through multiple collector complexes to achieve an electron with enough energy to crack water.

throwawaymaths5• 12 months ago

IIRC, PSI in the photosynthetic complex comes from purple bacteria, and PSII from green sulfur bacteria, so cyanos (and thus chloroplasts) kind of "already are" "using both". one presumes the option to use both pigments in the harvesting sense has been sampled evolutionarily.

throwawaymaths5• 12 months ago

oops i got them backwards, psII comes from purple.

sampo• 12 months ago

Yes, but why?

Scientific writing style is not always very good at highlighting the unknowns. "We don't know this" doesn't make very convincingly looking text, so people tend to avoid admitting it up front.

But you are, of course, correct to ask.

Like another comments said, this is an open question.

One theory is, that while the algae floating in water were absorbing broad spectrum, the algae growing attached at the bottom of the water evolved to chlorophyll to capture whatever was left at the edges of the spectrum. And then later land-based plants would have evolved from the water plants that were already attaching themselves to the bottom. But then why are also the current ocean-floating algae green now?

http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/imgbio/pl...

Another theory is that a perfectly-absorbing leaf would somehow absorb too much energy and get overheated, and that it was better to absorb only part of the available light.

None of these theories are fully convincing, so the question remains open.

enopod_• 12 months ago

According to the article, at least todays retinal-based photosynthesis is anoxygenic and does not invole carbon fixation. At night, these cells metabolism stops. Chlorophyllic photosynthesis with attached carbon fixation allows the cell to build up starch during the day, which it breathes under the use of oxygen at night, so the cell remains active during the night. Looks like a big evolutionary advantage to me. Also, light is not the limiting factor for plant growth, it‘s usually water or nutrient availability.

tbrownaw• 12 months ago

"We don't know this" doesn't make very convincingly looking text, so people tend to avoid admitting it up front.

Saying definitively that we don't know something (1) requires an investment of time to verify that lack of knowledge, and (2) can become incorrect at any time.

If you want to do something with the answer but find that it doesn't exist, sure make a note of that to request that someone could maybe try to find out. But if it's just a curiosity rather than directly relevant, why bother?

Etherlord87• 12 months ago

How about this: it takes energy to build a photo-synthesis machine. A machine like this won't consume all photons but only some range of electromagnetic spectrum. This is because you need to have such structure to allow light to pass through the outer part of the body of the organism, but be absorbed at a very specific place inside. This is why multi-junction solar cells exist, to use more of the solar spectrum. It would cost a plant more energy to build such a multi-layered chloroplast, so perhaps having just a single layer using a single range of the spectrum is the most optimal (at least as a local maximum).

flockonus2• 12 months ago

Another theory is that a perfectly-absorbing leaf would somehow absorb too much energy and get overheated

If having both pigments means the plant would be close to black, overheating is an absolutely valid hypothesis imo, plants just like animals have optimal temperature metabolism and often getting too hot is deadly, while under optimal temperature is tolerable.

slashdev5• 12 months ago

Evolution has had billions of years to improve on photosynthesis, but there still seems to be a lot left in the table.

Could we engineer a more efficient photosynthesis?

jmb99• 12 months ago

Could we engineer a more efficient photosynthesis?

Yes! They’re called solar panels, and our best ones are about 4x more efficient than the most efficient photosynthesis processes in nature, afaik.

tomrod• 12 months ago

Evolution ... improve[d] on photosynthesis

Evolution simply describes the macro process of death and birth as affected by the environment, no improvement planned. It could well be that a certain set of traits survive purely by chance or slightly improved odds in one set of circumstances that are detrimental later.

dyauspitr• 12 months ago

It might be better to be exclude IR and UV so they don’t have to spend a lot of resources on cooling and anti mutagenic devices.

throwawaymaths5• 12 months ago

no, that doesn't make sense because the cells are being irradiated at those wavelengths anyways. Absorption in uv would if anything, shade the cell from uv induced damage.

zahlman3• 12 months ago

I wonder why no plant evolved to use both and make the more even efficient use of light. These plats would appear dark, maybe almost black.

Many varieties of seaweed would seem to meet the description. Although I'm not sure that any of them are naturally anything like black without processing. Certainly some of them are brown, though.

mr_toad• 12 months ago

Brown kelp uses fucoxanthin to absorb green light. But they’re no longer considered to be plants.

zzo38computer• 12 months ago

I wonder why no plant evolved to use both and make the more even efficient use of light. These plats would appear dark, maybe almost black.

I have seen some black plants around where I live.

anonymous_sorry4• 12 months ago

Doesn't mean they're photosynthesising with all frequencies of light though. Probably just pigment.

scythe2• 12 months ago

I think that retinal might react with porphyrins. The former is a reactive aldehyde, the latter is a pyrrole derivative.

__MatrixMan__• 12 months ago

I asked this same question last week and got some good answers: https://news.ycombinator.com/item?id=44630224

Three things that stood out to me:

- Terrestrial plants evolved from green algae. There are other colors of photosynthetic algae. The advantage of absorbing red and blue may have been about avoiding competition with nearby red algae (which absorbs green), and about how blue light penetrates more deeply.

- The trick is not capturing as much energy as possible, but rather capturing and routing energy to the reaction centers such that it neither overshoots nor undershoots the energy necessary for the reaction. This works better with two absorption peaks instead of one (more here: https://www.quantamagazine.org/why-are-plants-green-to-reduc...). The absorption peaks of chlorophyll are both adequately distant from one another, and also more or less centered on the visible (i.e. most energetic) spectrum emitted by the sun.

- The idea that the green-yellow region of the visible spectrum is most energetic is, if not a misconception, at least more complex that it seems (https://www.oceanopticsbook.info/view/light-and-radiometry/l...).

schobi• 12 months ago

I agree with the overall discussion, but in the last link (Curtis Mobley Ocean Optics Book) the argument is broken.

Mobley plotted two graphs of the solar irradiation and a black body fit. The only difference is supposed to be the x axis with a plot over frequency or wavelength. This is a non-linear mapping and a different shape is expected. But the result is that the peak irradiation level is at either 501nm or 882nm (when converted back). That can't be right. The labeling of the x axis does not change the maximum.

What he meant to do was to plot either the solar irradiation in W m^-2 nm^-1 or as the number of photons. With lower energy photons (towards the red) the same irradiation level will consist of more photons. This shifts the maximum number of photons towards higher wavelengths. 880nm sounds plausible.

ryandamm• 12 months ago

This is actually even more puzzling than it seems, because the green part of the spectrum is the most energetic.

I've read elsewhere that photosynthesis is partially limited by dealing with free radicals, and at peak light flux, many plants would be damaged by the monoatomic oxygen species that light-capturing complexes would create. Hence pigments that reflect some green light.

Article:

https://www.quantamagazine.org/why-are-plants-green-to-reduc...

dr_dshiv• 12 months ago

Most of the green reflectance is an illusion because the human retina amplifies green light as much as 25× more than red (think of a green 5mw laser vs red).

On the other hand, leaves do absorb green light: nearly 75% is absorbed, in contrast to 85% of non-green light. See https://www.researchgate.net/figure/Reflectance-spectra-of-g...

sssilver• 12 months ago

Would osmin basil[1] fit the bill?

[1] https://www.google.com/search?client=safari&sa=X&sca_esv=923...

dvh• 12 months ago

Do you think it is just coincidence that chlorophyll is green and sun has peak luminosity in green frequencies? Or did chlorophyll win just because of that?

OgsyedIE• 12 months ago

In the rest of the niches in the entire domain of life it is the case that many different strategies were tried simultaneously, usually with a sole predominating outcome.

anonymous_sorry4• 12 months ago

Chlorophyll reflects green light, meaning it doesn't use these frequencies.

Who knows, maybe that's why the retinal photosynthesis evolved first though.

deadbabe• 12 months ago

Maybe other frequencies “taste” better.

And that’s why many humans don’t like eating their greens, because even plants don’t like greens.

joshdavham• 12 months ago

So the world might've been purple in the past...

That's really neat!

worik2• 12 months ago

Is this seen in some trees today?

Near me there is a plum tree with purple leaves

Not mentioned in the article...

anonymous_sorry4• 12 months ago

No that's just a pigment. They still contain chlorophyll.

Often you'll find leaves in full sun are redder, because they need less chlorophyll to operate at full efficiency. Leaves more in shade may be darker, as they require more chlorophyll (meaning light is absorbed across most of the visual spectrum by the pigment and chlorophyll together)

anonymous_sorry4• 12 months ago

This page says the theory was first proposed in 2007, but I remember being told about it at university around 2003.

charcircuit• 12 months ago

Something similar happened with me for another theory's article. I knew it existed before the article said. But I only had a primary source to prove it. Since it's not a secondary source I couldn't fix the article, so I put it on the talk page. Now the talk page has been wiped and the article is still wrong about the origin.

ipsum2• 12 months ago

Happens frequently. Usually the Wikipedia article is written by the guy who wants to claim credit for an existing design or idea, so he zealously guards the page against the truth. It's just disappointing.

wpollock• 12 months ago

I remember vaguely reading about this in the 1970s, maybe Scientific American? Cannot find that article now though.

evrimoztamur• 12 months ago

Considering assembly theory (https://en.m.wikipedia.org/wiki/Assembly_theory) for a possible explanation. The OP does state retinal is simpler, but it's significantly more basic and is organic.

On the other hand: Chlorophyll(s) all have a single magnesium caught at the center of a chlorin 'net'. It seems significantly harder to manufacture!

wonderwonder• 12 months ago

We really don't give enough credit to the people that maintain Wikipedia and do the work required to make these articles. Its a shame that a lot of political / social influence has started to creep into some of the articles but on the whole its really an incredible benefit to humanity.

gitremote• 12 months ago

PBS Eons has an episode on this:

When the Earth Was Purple https://m.youtube.com/watch?v=IIA-k_bBcL0

(This is great science channel. PBS should have continued to receive federal funding.)

Isamu• 12 months ago

Someone should add citations to the Evidence section.

Purple Earth hypothesis

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