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On Earth, all plants are green because they contain the pigment chlorophyll, which photosynthesizes by absorbing all light except green light.

However, it is quite possible - if not likely - that alien planets would have plant life (As in, organisms functionally similar to Earth's plants) containing different pigments, especially since green is an non-optimal colour.

There are several common pigments which could be used as plant colours, but their shade differs depending on the spectral type of the star. The cooler the star, the darker the hue, meaning that F-, G- and some K-type stars have the potential support the widest variety of colours (The coolest stars make all the pigments black or near-black, and the hottest ones make them whiteish).

The star which my planet orbits is a G-type star, so it's got a good selection of possible colours.

These are the plant colours, with their pigments, which I have considered having (Definitely green):

  • Green - Chlorophyll (Confirmed)
  • Purple - Bacteriorhospin
  • Yellow/Orange - Carotenes
  • Pink - Phycoethryn
  • Blue - Phycocyanin

So, my question is: Can a planet support flora of different pigments, living alongside eachother, without one pigment outcompeting the others?

I don't see how pigments could "occupy different ecological niches", because their job is just to absorb light. And since some colours absorb more light than others, I'm not sure if green, orange, pink and blue plants could live in the same ecosystem.

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    $\begingroup$ Worthwhile pointing out that not all plants on earth are green. There are multiple plant varieties with red leaves, some with yellow-white, and even some with variegated leaves with multiple colours on at once! $\endgroup$ – Joe Bloggs Jun 24 '18 at 12:09
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    $\begingroup$ You should consider that the greenness of most (definitely not all) plants on Earth is an accident, the result of chance. It just so happened that the lineage of algae which eventually gave rise to upper plants used green chlorophyll. The problem is not to simply absorb light, but to make the energy of light available in the complicated chemical dance which results in converting carbon dioxide and water into carbohydrates; simply absorbing light and as thermal energy is useless. $\endgroup$ – AlexP Jun 24 '18 at 12:52
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    $\begingroup$ Another thing worth noting is that plants on Earth typically have multiple pigments that are tuned to different wavelengths -- this is, among other things, used as a kind of sense organ, e.g. to detect dusk (where red wavelengths are generally stronger than usual) vs midday (where bluer wavelengths are more common). See, e.g. phytochrome. $\endgroup$ – Jules Jun 24 '18 at 19:17
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    $\begingroup$ While plants appear green to us, that is largely an accident of the relative sensitivity of our eyes to the green wavelength. More red light is actually reflected and/or fluoresced than green. $\endgroup$ – pojo-guy Jun 24 '18 at 22:31
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    $\begingroup$ @pojo-guy Source for that? $\endgroup$ – Jonas Schäfer Jun 25 '18 at 8:04

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I would say yes, if one of the pigments was not the result of the need to attract light. Suppose the planet had a secondary source of nutrients ( possibly older, e.g. geothermic vents) that was much more prevalent than on Earth. That source could influence the pigmentation of the plants that feed on it.

In order not to be relegated into niches like pools of sulfuric water or deep waters, the plants that feed on that secondary source would have to survive in the oxygen-rich environment generated by the other plants (something that the Earth anaerobic organisms can't do well, by definition).

We could imagine that some genetic family could have made the breakthrough of, e.g., turning some part of their reproductive system into a "respiratory" organ: it would fuel a chemical reaction to recreate the chemical formula that was originally best for their metabolism.

That would make them functionally closer to animals, but they would still be plants. In fact, they could to some degree take the place of animals, by consuming the oxygen of the other plants, and reconverting it into CO2. There would be a "balance of power" between the two sorts of plants, and each feeding the other in the ecosystem.

Such plants getting nutrients from the soil and breathing oxygen would be highly "irregular", but the Cambrian explosion showed that many solutions could exist and in nature anything that works can buy survival... at least for a while. I am sure any biologist or a specialist of organical chemistry might find a number of serious objections with such a scenario, but at least it would sound plausible to a layman.

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  • $\begingroup$ Another possible secondary source of nutrients is predation, in which case the plants may evolve to be whichever color attracts the most prey. In the real world, carnivorous plants often have red accents. $\endgroup$ – DoctorDestructo Jun 25 '18 at 18:07
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    $\begingroup$ Yes, that might make sense. Predation might buy them additional energy. The color could attract prey; or conversely, these plants could start displaying the dye of their preys (like flamingos eating shrimp). Or the two effects could be mutualy reinforcing, if the plant was mimetizing as a member of the species it is predating upon. $\endgroup$ – fralau Jun 25 '18 at 18:20
  • $\begingroup$ @fralau your last idea remind me of a bee orchid. $\endgroup$ – SealBoi Jun 25 '18 at 20:08
  • $\begingroup$ “if one of the pigments was not the result of the need to attract light” — In other words, flowering plants on Earth. $\endgroup$ – Konrad Rudolph Jun 26 '18 at 12:49
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Green is the most common color for plants, but they are not exclusively green already on our planet.

Cyanobacteria, for example, uses other pigments together with chlorophyll, and same do red algae (from which the obvious name).

Chlorophyll is just the most widespread, but there are examples of other colors, like the Cordyline australis

Coming to your question

Can a planet support flora of different pigments, living alongside each other, without one pigment out-competing the others?

The answer is definitely yes.

enter image description here

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    $\begingroup$ Arguably with chlorophyll being "the most widespread", it has indeed "out-competed" the others, no? $\endgroup$ – Lightness Races with Monica Jun 25 '18 at 20:12
  • $\begingroup$ Or perhaps a more banal explanation being that green is the dominant color because chlorophyll was the first highly successful microorganism. Had Bacteriorhospin been the first, plant life on earth may have been mostly purple instead of green. Evolution isn't exactly an "equal opportunity" system. $\endgroup$ – Neil Jun 26 '18 at 12:14
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    $\begingroup$ @LightnessRacesinOrbit In an evolutionary context, “outcompete” doesn’t merely mean “surpass”, it means “displace competitors by pushing them to extinction”. Green leaf plants may have outcompeted others in many places of the world, but not in all. $\endgroup$ – Konrad Rudolph Jun 26 '18 at 12:53
  • $\begingroup$ @KonradRudolph Aha! That explains it :) $\endgroup$ – Lightness Races with Monica Jun 26 '18 at 12:56
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Lifetime/Growth rate trade-off

The reason plants do not absorb in the green is to avoid damage. As a PhD student working on organic solar cells, I can tell you that finding organic materials that can sustain light exposure over extended periods of time is very challenging.

Perhaps on your planet different lifetime/harvest strategies could co-exist: some plants would be long-lived, with sub-optimal absorption (pretty much like ours). Other would harvest at higher wavelengths, grow very rapidly, reproduce quickly and die. Depending on their predators, either of these two strategies might be optimal.

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  • $\begingroup$ What colours, do you know, would correspond to the various degrees of longevity? $\endgroup$ – SealBoi Jun 24 '18 at 13:57
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    $\begingroup$ This is not easy to answer a priori. As far as I know, our knowledge about lifetime of organic materials under light exposure is very empirical. This would also depend on the spectrum of the light that reaches your planet, which in turn depends on (in order of importance) : the temperature of it's star, the planet-sun distance and the planet's atmosphere composition. $\endgroup$ – Alexis Jun 24 '18 at 14:06
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    $\begingroup$ If I were you I would simply assume that evolution on your planet has found pigments of its own (not necessarily those you listed above). This is not too bold of an assumption as organic materials are very diverse. Then the general trend is that the smaller the wavelength, the larger the energy and hence the more damage it causes. But it also depends on the very nature of the pigment, so you have some freedom about it. $\endgroup$ – Alexis Jun 24 '18 at 14:11
  • $\begingroup$ Is the blue absorption of chlorophyll part of the moeity of the hydrocarbon tail, or is it from the Mg coordinated porphyrin? (Does it contribute to energy production, is it a safety filter, or incidental by-product?) Would a green absorbing dye automatically also absorb in the damaging UV range? (The planet could have different UV level, different trace metals, and more pigments related to advertizing or camouflage.) $\endgroup$ – amI Jun 25 '18 at 23:20
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There are two situations I can foresee in which two or more pigments might co-exist:

  1. niche lighting; if one pigment takes up red light preferentially while the rest of the spectrum passes through then other photosynthesisers, lower in the biosphere, may use pigments with high efficiencies on other wavelengths to get all they can out of the light that makes it through the red-hungry canopy layer.

  2. matching efficiency; if there are two or more photosynthetic pigments with the same performance efficiency at a given light intensity then it's possible that both/all of them would compete on an even footing with each other until something finds a better pigment.

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As noted in other answers, multiple colors will be equally viable and thus exist side-by-side if there are multiple ways to achieve equivalent energy. One way that might occur is a planet in a binary star system. The two stars could have each a different spectrum. As the planet turns, the planet would see different suns at different hours. Some plants may efficiently harvest light from A while others harvest B.

For example: At one sunrise you might see the green plants unfurl, and then at the other sun's rise, the other color plants unfurl. Then when the first sun sets those plants furl up for their "night", while the other plants have the second sun all to themselves. Some plants might have both types of pigment or may even change color through the solar cycles to optimize for the light at that moment.

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  • $\begingroup$ Welcome to worldbuilding. Please note the OP is not asking how to get different colors. $\endgroup$ – L.Dutch - Reinstate Monica Jun 24 '18 at 15:01
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    $\begingroup$ Hmmm. I was basically going to post this same answer. @Katie, I’m going to edit this answer to be more directly an answer to the question. I feel you should get credit for this answer, it just needs better phrasing. $\endgroup$ – SRM - Reinstate Monica Jun 24 '18 at 21:52
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To have them all be roughly equally prevalent, you need each to have some specific advantage that doesn't edge out all the others, otherwise random variance will cause one to eventually be the dominant form. Plants on Earth are mostly green because green light accounts for the largest amount of solar energy coming through the Earth's atmosphere (well, the largest amount that hits the surface), and therefore the most likely source of over exposure (and thus degradation) of the pigments.

So, the easiest option here is that something causes a variation in the spectrum of light hitting the planet's surface. Possibilities I can think of for this include:

  • Something about the atmosphere varies based on the time of day or the season. As a result, certain pigments are better either at different times of day, or during different seasons. Seasonal variance is probably more likely here as it already happens to a slight degree on Earth and allows for a more sensible life cycle for the plants.
  • Something about the star itself varies on a periodic basis. Orbiting a binary with two different star types (G and something else) would do this, and I would argue that this is actually the most plausible option. Having either an F type or K type star as the other partner would give the best overall stability of the spectrum reaching the planet while still providing enough variance to periodically favor one variety over the others, although I don't know anything about how astronomically possible a G/K or G/F binary system would be.
  • Possibly the most interesting, but least plausible: The planet is on a long elliptical orbit around the star (like a comet). Distance from the star impacts the energy of the various frequencies, and thus you get 'seasonal' differences in what pigments are best. Such a planet would have a very hard time supporting life as we know it however.

Alternatively, make it such that green is still the 'best' color in terms of lifetime, but the others have some other advantage not related to energy conversion. Maybe they make the plant more resistant to some types of disease. Maybe they indirectly encourage certain species to more actively pollinate that plant. Maybe they are toxic to certain herbivores, and therefore give the plant an edge in areas where they are prevalent.

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There shouldn't be any issue. However, there are a few paths here based on some unknowns:

Can differently colored plants interbreed?

If they can, then it's likely that they will mix genes to a point of turning into an averaged color over millions of years. You'd expect the color to fade towards the predominant color mix.

Are differently colored plants found in the same region, or are they separated?

If they're separated, then the color muddying won't happen as fast, but it should eventually still happen (birds carrying seeds, humans traveling, even the wind would carry an errant seed now and then.

Are there notable survivability differences between the plants?

If, for example, pink plants need less C02 than green plants, they're capable of living in areas with less C02 prevalence.

In areas where green plants can survive, pink plants would have excess resources that they can dedicate to growing larger in size. This can be a cause for slightly varied plant life: pink trees, green shrubs.

Different survivability factors can be a cause for the separation. Inventing some traits to prove the point:

  • Pink plants need less oxygen => High altitude locations are predominantly pink.
  • Orange plants dramatically drop in efficiency in the cold => The tropics are predominantly orange.
  • Blue plants die in warmer environments => Cold regions are predominantly blue.

And so on. The separation doesn't need to be perfect, i.e. other colors can still exist in the same region, but you'd expect a particular color of plant with a particular genetic "skill" to be more prevalent in the region that matches said skill.

But if they can interbreed, and they're in the same region together, they will over time mix their genes enough to lose their individual distinct qualities (including color).

I notice you mentioned:

I don't see how pigments could "occupy different ecological niches", because their job is just to absorb light.

But that is somewhat contradicted when you said:

On Earth, all plants are green because ... containing different pigments, especially since green is an non-optimal colour.

If you extend your observation that green is non-optimal yet ubiquitous, you will find that just because a trait is predominant doesn't always mean that it was the key to the creature's genes increasing their evolutionary prevalence.
How did green become the predominant plant color, if it's not the most optimal color? Because the ancestor plants that were the best survivors just happened to be green. They were optimal species for reasons other than their color.

Similarly, the plant color traits I mentioned are not necessarily caused by the color. Maybe the cold-resistant plants just happen to be blue. Maybe the low-oxygen plants just happen to be pink. There is no inherent requirement for one trait (e.g. survivability factor) to be caused by the other (e.g. color. They can be coincidental.

And this is how different colors could occupy different niches:

  • Initially, there are several colors of plants occupying the same niche.
  • Let's assume these plants do not interbreed.
  • Over millions of years, one species of plant has adapted to the niche better than any of its competitors.
  • Over another million years, this plant slowly takes over the niche and basically turns into a monopoly. So far, it has maintained that monopoly.
  • This plant just happens to have a certain color.

In other niches, the same thing has happened, but because one niche has nothing to do with the other, the second niche's monopoly ca be maintained by a plant of a different color.

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Answer: Sure, it's perfectly possible.

The key point is that the photosynthetic pigment is merely one part (albeit an important part) of the whole photosynthetic pathway, and it's the whole photosynthetic pathway that is important and which evolves. And right here on Earth in the vascular plants, we have two significantly different photosynthetic systems: C3 and C4.

(See Wikpedia's C3 article and C4 article for details.)

The point is that C3 and C4 plants have co-existed on Earth for millions of years. (Most plant species are C3; the biggest group of C4 plants is the grasses. C3 species tend to do better with moderate temperatures, high CO2 and abundant water. C4 species tend to do better in hot dry environments with less CO2.)

So while it may be difficult for a new pigment to evolve, assuming that the both pigments are roughly comparably efficient, them coexisting over a long period would be nothing special. Been there, done that.

(Of course, we're "only" talking millions of years here. Even a relatively small absolute advantage might push one or the other to complete dominance over a hundred million of half-billion years. But each pigment having advantages over the other in some environments (like in the case of C3 and C4) might prove stable over a much longer time.)

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Different minerals, different colours.

Although many pigments have more to do with chemical structure, some pigments do derive their colour from elements within them like iron or copper. In any case, different regions might be rich in different minerals and plants might be adapted to whatever is present, whether it gives them their colour directly, or by changing their approach to photosynthesis.

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Not ALL plants on earth are green, although these are the most widespread. A long, long time ago, we did learn in school about algae, there are of four colors: green, red, brown and blue. The blue ones seem related to cyanobacteria, one of the oldest lifeforms on Earth! So, it is a fact that here on our own planet, besides the most well-known chlorophyll a and chlorophyll b, there are also other colours of pigment that effectively photosynthesize.

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    $\begingroup$ Welcome to Worldbuilding, Cristian loan! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 25 '18 at 13:42
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Yes, a planet can harbor plants of different colors. We know because the Earth does.

The color of light changes underwater. Therefore at certain depths red pigments are more efficient than green ones at gathering light, and red algae take profit from that. In any other planet different circumstances may make change the color of light in different places leading to different pigments of photosynthetic organism.

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Your plants have more than one pigment in the same plant.

There is one easy (relatively) way to do this, plants on your planet don't have one pigment they have two or more in the same plant, if they each catch about half of different but roughly equal parts of the spectrum this should work. For example one blue and one orange, or cyan and orange so the default color is not black. It would be unlikely to evolve in the first place but not impossible and once it did occur having more coverage and controllable variability in absorption will have benefits. This will also allow trace pigments to affect the color more easily since there is less of each pigment.this pigment plant will be relatively stable allowing for a lot of color diversity.

keep in mind plants likely won the "pigment competition" for reasons having nothing to do with color, our best guess is salinity tolerance or by forming a symbiotic relationship with early eukaryotic cells first, so your bi-color plants would be the a similar case it just happened to have a bi-color system just like early plants just happen to be green when other factors made them win.

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  • $\begingroup$ One reason plants on earth are green is that plants do absorb the red and the blue ends preferentially. $\endgroup$ – Sherwood Botsford Jun 26 '18 at 17:10
  • $\begingroup$ but from the a single pigment and only the far ends of the spectrum. $\endgroup$ – John Jun 26 '18 at 18:36
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Remember that colors are not an absolute but a subjective interpretation of electromagnetic waves.

The human eyes and the parts of the human brain which interpret their signals are the result of millions of years of evolution. Our color perception is the way it is because being able to separately filter frequencies of 440 THz (red), 560 THz (green) and 640 THz (blue) has proven to be useful for surviving in our environment.

That means other planets will generally seem less colorful for the human eye, because we did not evolve to perceive and differentiate the color nuances of that planet.

This is, by the way, also the reason why many telescopes operate in ultra-violet or infra-red. Just because these are colors of light our eyes can not see at all does not mean there is nothing interesting in space which shines in those colors.

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