[HN Gopher] Purple Earth hypothesis
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Purple Earth hypothesis
Author : colinprince
Score : 138 points
Date : 2025-07-24 00:43 UTC (2 days ago)
(HTM) web link (en.wikipedia.org)
(TXT) w3m dump (en.wikipedia.org)
| joshuafuller wrote:
| Jimi was right--Earth was in a purple haze. It just came from
| retinal-based photosynthesizers, not acid.
| O5vYtytb wrote:
| 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.
| morkalork wrote:
| It's not more associated to cannabis? There's many strains
| even named for it.
| zahlman wrote:
| This presumably happened later, as LSD became less popular
| in the underground drug culture.
| worik wrote:
| > LSD became less popular in the underground drug
| culture.
|
| Hmmmm...
|
| Maybe relatively less popular, as the menu of
| recreational drugs is expanded from a very few bad ones
| to a cornucopia of good ones, but still very popular
| zahlman wrote:
| Nowadays I only ever hear people talk about it in the
| context of former users, or general discussion of the
| 70s.
| scythe wrote:
| Less accessible. Illicit LSD production will probably
| never be as widespread as it was before the List of
| Chemicals was introduced. You can develop alternative
| manufacturing methods for most drugs. But a
| hexahydroindolo[4,3-fg]quinoline? Not so easy.
| weinzierl wrote:
| _" 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 wrote:
| 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/)
| darth_avocado wrote:
| 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 wrote:
| In fact, many plants already receive too much sunlight and
| have various mechanisms to limit their exposure.
| adrian_b wrote:
| 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.
| throwawaymaths wrote:
| 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.
| throwawaymaths wrote:
| 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.
| throwawaymaths wrote:
| oops i got them backwards, psII comes from purple.
| sampo wrote:
| > 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_ wrote:
| 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.
| throwawaymaths wrote:
| carbon fixation is a completely separate process. in
| principle you could hook up a sufficiently engineered cell
| to electrodes and do the carbon fixation part in the dark
| by supplying it with juice from the mains.
|
| accordingly there is no particular reason for purple photon
| assimilation to not be attached to carbon fixation...
| though i suppose as the electron energy levels dont quite
| match up it might be a schlep to get purples to make sugar.
| tbrownaw wrote:
| > _" 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?
| slashdev wrote:
| 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 wrote:
| > 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.
| hollerith wrote:
| Solar panels so far don't remove CO2 from the air, though.
| slashdev wrote:
| You can connect them with other equipment to do that, if
| that's your goal. Not very effective though
| svdr wrote:
| The removal is only temporary.
| hollerith wrote:
| Not if the panels were to produce graphite pellets that
| people could bury or dump in ocean trenches.
| dharma1 wrote:
| Can be a couple of hundred years with trees and wood used
| for housing. Long enough to figure things out
| slashdev wrote:
| That's what I'm using as the benchmark, but I was thinking
| more like bio engineering to create an organism that gets
| closer to solar panel efficiency.
|
| Would be potentially very useful for timber or biomass
| production. I doubt people would trust eating it.
| 7734128 wrote:
| A tree grows a leaf slightly more efficiently than we
| create a solar panel though.
| dyauspitr wrote:
| 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.
| throwawaymaths wrote:
| 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.
| zahlman wrote:
| > 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.
| zzo38computer wrote:
| > 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_sorry wrote:
| Doesn't mean they're photosynthesising with all frequencies
| of light though. Probably just pigment.
| scythe wrote:
| I think that retinal might react with porphyrins. The former is
| a reactive aldehyde, the latter is a pyrrole derivative.
| dvh wrote:
| 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 wrote:
| 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_sorry wrote:
| Chlorophyll reflects green light, meaning it doesn't use these
| frequencies.
|
| Who knows, maybe that's why the retinal photosynthesis evolved
| first though.
| joshdavham wrote:
| So the world might've been purple in the past...
|
| That's really neat!
| worik wrote:
| Is this seen in some trees today?
|
| Near me there is a plum tree with purple leaves
|
| Not mentioned in the article...
| anonymous_sorry wrote:
| 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_sorry wrote:
| This page says the theory was first proposed in 2007, but I
| remember being told about it at university around 2003.
| charcircuit wrote:
| 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.
| evrimoztamur wrote:
| 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!
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