Purple Earth hypothesis
en.wikipedia.org
en.wikipedia.org
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?
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/)
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.
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 ;)
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.
The energy required to split water is around 1.25 eV, while the energy of red photons is already around 1.5 V and the energy of green photons is well above 2 eV.
However, it is true that plants need 2 red photons (or of higher energy), not 1, to generate both free oxygen and a reduced organic substance, NADPH, which is used later to reduce carbon dioxide into carbohydrates. The reason is that the captured solar energy is used not only for these redox reactions, but also for pumping ions across the chloroplast membrane. The energy stored in the ion gradient will be used later to power the chemical syntheses, which need both a reducing agent and additional energy.
The ping-pong is done with electrons, not with photons. There are 2 photosystems, which absorb separately photons, using their energy to transport electrons against the potential gradient. The electrons pass through both photosystems in series, achieving a potential difference between the endpoints that is greater than what is required for splitting water.
The potential difference over each photosystem is significantly smaller than corresponding to the energy of the absorbed photon, because only a fraction of the energy is used for electron transport against a potential gradient, while the rest is used for ion transport against a ion concentration gradient.
Photosystem II contains manganese ions that are oxidized so strongly that they can oxidize the oxygen from water, converting it into free dioxygen. Photosystem I is able to make a strong reducing agent, to which the hydrogen remaining from water is bound.
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.
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.
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?
And (3) doesn’t necessarily make you lose credibility but too many believe it does and partially it can.
Those who believe science is truth are innocently choosing an answer without full knowledge. That is the definition of religion and faith.
True science is a search for truth like philosophy and theology. I think you can get too caught up in any one of these three.
If you have hard core faith, that’s awesome, but put your faith into that which won’t fail you.
In science, there is clear delineation between things we know to be true (laws), things that appear to be true based on our current knowledge (theories) and things that may or may not be true but seem to be true based on limited data/limited research (hypotheses).
I don't think the three (I only see two, though, but don't want to nitpick) approaches are really comparable.
Newton's First Law (Law of Inertia) – assumes objects remain at rest or in uniform motion unless acted upon by an external force; inaccurate in non-inertial (accelerating) frames without fictitious forces.
Newton's Second Law – assumes constant mass and low velocities; inaccurate at relativistic speeds where mass increases with velocity.
Newton's Third Law – inaccurate in electromagnetic, quantum, or relativistic contexts.
Newton's Law of Universal Gravitation – assumes instantaneous action at a distance and weak fields; replaced by General Relativity in strong fields or at high velocities.
Boyle's Law – inaccurate at high pressures, low temperatures, or with real gases, which deviate from ideal behavior.
Charles's Law – assumes ideal gas behavior; deviations occur at high pressures or low temperatures.
Ohm's Law – assumes constant resistance; inaccurate for non-ohmic materials like diodes or at high voltages where resistance varies.
2nd Law of Thermodynamics – inaccurate with fluctuations at microscopic scales, temporarily decreasing entropy.
Kepler's Laws – inaccurate when considering relativistic effects.
Laws of Motion of Fluids (e.g., Bernoulli's Law) – assumes incompressible, non-viscous flow; inaccurate for real fluids with viscosity or compressibility effects.
Dalton's Law of Partial Pressures – assumes gases do not interact; inaccurate at high pressures or with strong intermolecular forces.
Fick's Laws of Diffusion - inaccurate in complex systems with reactions or non-constant diffusion coefficients.
Laws of Electromagnetic Induction (Faraday's Law) – assumes ideal conditions; real systems have hysteresis and other non-ideal effects.
Laws of Radioactive Decay – exponential decay is inaccurate as does not fully regard environment or decay chain complexities.
The search for truth, if we are still arguing that, where is it? I don't understand how anybody can argue that the search is the same across theology and science. In a sense it is the search for the same thing, but in so many ways it is not. The commenter I was replying to is confounding the difference, and this comment, seems to ask me to point out what mistakes theology made, and I know of plenty.
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.
Similarly, at the cell level, we have just the one photosynthesis process in plants and that isn't going to take an entirely different path to work on different wavelengths. The evolutionary investment is in chlorophyll with a magnesium element at the heart of it. Imaginably you would need an entirety different molecule with some other magic atom in the middles of the usual organic chemistry and that isn't going to happen given the sunk cost in the chlorophyll way of doing things. There is a greater chance that we will get six hands than that happening.
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.
Would be potentially very useful for timber or biomass production. I doubt people would trust eating it.
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.
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.
I have seen some black plants around where I live.
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...).
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.
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...
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...
[1] https://www.google.com/search?client=safari&sa=X&sca_esv=923...
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.)
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
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!
Who knows, maybe that's why the retinal photosynthesis evolved first though.
And that’s why many humans don’t like eating their greens, because even plants don’t like greens.
Near me there is a plum tree with purple leaves
Not mentioned in the article...
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)
That's really neat!