Change My View: Alien Plants Could Be Any Color

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Change My View: Alien Plants Could Be Any Color

Post by Backstroke_Italics »

There is a common belief among conworlders that the frequency of the peak output of a parent star dictates the color of photosynthetic organisms on planets around that star. This is because plants and algae on Earth use chlorophyll a and chlorophyll b, which together absorb the most light at frequencies between 400 and 500 nanometers, and 600 and 700 nanometers, leaving a gap that looks distinctly green. Plants and algae thrive using this pigment, reflecting a range of visible light that roughly matches the frequency of the peak output of the sun. To the hasty thinker, this may seem like an undeniable causal relationship. But when you are working with a sample size of one, and two things roughly align, we must be especially cautious of placing too much explanatory power on something that could, ultimately, be a coincidence.

As an analogy, insects have six legs. Other arthropods typically have more. Insects are tremendously successful on land, dwarfing the biomass of arachnids and crustaceans combined. Are we to conclude that having six legs gives a creature an advantage on land? It is entirely possible that this is a coincidence, and that the number of legs is superfluous. It is also possible that the number of legs is related indirectly to evolutionary success, say by freeing up appendages for other tasks. But we cannot say that having six legs is better adapted to land than eight or ten simply because one clade of six legged arthropods has proven successful on land. Similarly, green pigment may have had a large impact, a small impact, or no impact at all on the success of algae and plants. Using chlorophyll a as the primary photosynthetic pigment is an adaptation that may have evolved only once, meaning that we should be looking at every difference between ur-cyanobacteria and other photosynthetic organisms in our quest to find out what allowed them to be so successful.

A related fallacy is the assumption that evolution must be purposeful. If plants are green, there must be an advantage to being specifically green. Thus we have any number of ad-hoc explanations. Plants need to reflect light to prevent burning. But then why are plants in chilly, cloudy climates just as green as plants in the desert? Plants absorb high and low frequency light to give a more stable input that is buffered against sudden increases in intensity. But a spike in light intensity will increase high and low frequency light by the same factor as medium intensity light, so the fluctuation is the same either way. In short, it is working backward to look for a reason we merely assume must exist.

So what else can we do? What could explain why the green things were the things that were successful? Well, first of all there really aren't many other options. So-called “red algae” and “brown algae,” even the anaerobic “purple sulfur” bacteria, still have chloroplasts that contain chlorophyll a, and sometimes b and c. Nearly all known photosynthesizers can trace their photosynthetic ability to the symbiotic absorption of a bacteria that used chlorophyll. So we have to go very far afield to find a potential competitor. One possibility is Halobacterium, which uses the photosynthetic pigment rhodopsin. Rhodopsin absorbs light around 570 nanometers, exactly in the range that chlorophyll reflects. But these archaebacteria are highly adapted to hyper-saline environments. Their cell membranes have a unique structure to deal with the salt, and they form a crust of salt that regulates their exposure to UV, which in turn has a regulating effect on their reproduction. It's not surprising these things haven't colonized the land, and it's unlikely to happen now, when the terrestrial environment is already dominated by plants. But the fact that they use a pigment that absorbs light at the peak frequency doesn't seem to hold them back in their quest to go on living the saltiest life possible. It may be the case that we are looking at a sample size of one: one lineage of photosynthesizers was in a good position to make the necessary adaptations to life on land within the critical time period (and various adaptations to dominate marine environments as well). Green pigment may have aided in that process, or it may have been something else that gave these things an edge.

Here's the dirty little secret of photosynthesis. A molecular pigment absorbs energy, causing its electrons to become excited, i.e. move to a higher energy state. This allows them to be captured by an electron acceptor. An electron donor resets the process. For plants this means chlorophyll passes an electron to a pyridine nucleotide, which eventually passes it to a phosphate molecule, which starts a chain-reaction that ends with the synthesis of ATP. Water is the electron donor, which creates the need to dispose of diatomic oxygen. But take a look at those first two steps. What sort of molecule can absorb light leading to electron excitement? What sort of molecule can act as an electron acceptor? It turns out, the answer to both questions is “just about anything.” Any molecule that is opaque at a given frequency can experience electron excitement when exposed to sufficient amounts of light at that frequency. Any reducing molecule can act as an electron acceptor. In terms of the physics and chemistry, photosynthesis does not require exotic materials. The main requirement is simply that a pigment, an electron accepter, and an electron donor be arranged together and exposed to light. You could use retinal pigment, a peroxide electron acceptor, and a sulfide electron donor (in fact these are all roughly attested). The challenge is in how you synthesize these things, arrange them so they can interact, keep them from spontaneously reacting with one another, protect them from decay, ensure the proper pathways to ATP, etc., not in finding a molecule that will make the chain possible. Chlorophyll is basically a porphyrin that's had a magnesium atom shoved into it, i.e. an already available molecule that could be easily converted into a pigment. Plenty of other molecules would have done the job, but porphyrin is abundant and simple to synthesize and keep stable.

Given such a wide variety of options, why do we assume that whatever molecule ends up in the one major attested lineage of photosynthesizers must be special in some way? Because it reflects light at a suspicious range of frequencies? What wavelength of light could plants reflect that would not trigger our famous pattern hunting instinct? Red? Blue? We must acknowledge the fact that we do not know for certain that photosynthetic organisms face a strong pressure to reflect the peak frequency of their star's output. We have a sample size barely above one, with no clear causal relationship and every indication of coincidence.
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aliensdrinktea
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Re: Change My View: Alien Plants Could Be Any Color

Post by aliensdrinktea »

I think it's likely that the majority of plant life on a planet will utilize whichever pigment(s) takes the most advantage of the star's peak output. It's evolutionarily advantageous. However:

1) Non-green plants exist in nature here on Earth. Plants with red or purple leaves are not terribly uncommon.
2) Black plants photosynthesize more efficiently than green ones, yet they are exceedingly rare here on Earth.

I don't see why alien plants should necessarily be a certain color when Earth plants don't even obey that "rule", though I do think it's worth keeping in mind what would be most advantageous.
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eldin raigmore
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Re: Change My View: Alien Plants Could Be Any Color

Post by eldin raigmore »

There is a theory that chloroplasts and Cyanobacteria (blue-green algae) evolved at a time when the Earth’s ecosphere was dominated by rhodobacteria — photosynthesizing prokaryotes whose light-capturing pigment was rhodopsin or something else red or purple or purplish-red.
The Cyanobacteria were able to survive and thrive in a layer of the ocean underneath the layer dominated by the rhodobacteria — I think I saw that claim. At least I saw an illustration that looked as if it could be supporting that claim!

....

However; I do not think anything photosynthetic is likely to be white or silver.
Now it’s your turn; change my mind about that!
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Re: Change My View: Alien Plants Could Be Any Color

Post by Khemehekis »

As I summarized in this post, I've read a lot of conflicting things about plant colors.

I want to give my conplanet Keitel blue or blue-green plants, but now I don't know what to believe!

Any advice on what kind of planet (star type, distance from sun, etc.) would be ideal for blue plants?
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Re: Change My View: Alien Plants Could Be Any Color

Post by WeepingElf »

aliensdrinktea wrote: 12 Oct 2020 02:451) Non-green plants exist in nature here on Earth. Plants with red or purple leaves are not terribly uncommon.
Yes, but these also have chlorophyll in their leaves, and it is that which their photosynthesis is based on. It is just that they have additional red or purple pigments which have a larger impact on the leaves' overall colour.
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Re: Change My View: Alien Plants Could Be Any Color

Post by Pabappa »

WeepingElf wrote: 12 Oct 2020 14:47
aliensdrinktea wrote: 12 Oct 2020 02:451) Non-green plants exist in nature here on Earth. Plants with red or purple leaves are not terribly uncommon.
Yes, but these also have chlorophyll in their leaves, and it is that which their photosynthesis is based on. It is just that they have additional red or purple pigments which have a larger impact on the leaves' overall colour.
I believe you, of course, but it begs the question, ...how do such plants manage to breathe at all, if chlorophyll is blocking out all light other than green wavelengths, and the other pigments are blocking out green wavelengths?

Naively I would have to assume that these plants must be still mostly green and that the leaves with other colors serve to attract insects or other animals, similar to what flowers do with their blossoms. But I havent looked it up.

Likewise, regarding the study of the plants with black leaves .... the scientitsts seem to have worked on the assumption that the black plants were photosynthesizing on all wavelengths, and had a novel pigment that enabled them to do that rather than being ordinary green plants with a black coating. Surely a study wouldn't have been done if the black pigment were just there for show, ... right?
I'll take the theses, and you can have the thoses.
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Re: Change My View: Alien Plants Could Be Any Color

Post by LinguistCat »

Pabappa wrote: 12 Oct 2020 16:07
WeepingElf wrote: 12 Oct 2020 14:47
aliensdrinktea wrote: 12 Oct 2020 02:451) Non-green plants exist in nature here on Earth. Plants with red or purple leaves are not terribly uncommon.
Yes, but these also have chlorophyll in their leaves, and it is that which their photosynthesis is based on. It is just that they have additional red or purple pigments which have a larger impact on the leaves' overall colour.
I believe you, of course, but it begs the question, ...how do such plants manage to breathe at all, if chlorophyll is blocking out all light other than green wavelengths, and the other pigments are blocking out green wavelengths? ...
It's probably better to think of what colors a pigment absorbs instead of blocks in this case. Chlorophyll absorb red and blue ends of the visual spectrum, and scatter green. Other pigments absorb green and sometimes other wavelengths. Usually from what I've heard, these secondary pigments either add to the ability of the plant to photosynthesize by using the green, OR block out wavelengths that are too strong to the point that the plant would burn or scorch if it accepted all of the light's energy.

The other issue regarding purple photosynthesizers on early Earth though, is that they are hypothesized to have existed before green photosynthesizers and chlorophyll evolved, and that chlorophyll probably evolved to grab the wavelengths those purple photosynthesizers didn't make use of from lower in the water column.

That would imply that life on a planet will evolve life that will use the peak wavelengths of the light on their homeworld and block or scatter the rest of the light so they aren't overloaded with energy. Then, if a secondary coloration comes about, it will likely absorb those other wavelengths that the first group does not use. Which becomes the main form of photosynthetic life might depend on how efficient these processes are. You might even get a world where both forms are common and work in different niches.
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Re: Change My View: Alien Plants Could Be Any Color

Post by Backstroke_Italics »

Great contributions, all. Thank you.

All plants and algae derive from an ancestor that used chlorophyll a. Since then various lineages have added up to four variants of chlorophyll, lost their ability to photosynthesize, evolved new non-photosynthetic pigments that make them appear red or brown or black, or even developed novel photosynthetic pigments.

Yes, it's true that some photosynthetic pigments other than chlorophyll are used in some plants and algae, but all them are basically chlorophyll-adjacent pathways. They absorb energy from a certain wavelength, transfer an electron to a chlorophyll molecule, which then makes its way to the electron acceptor. To my knowledge, no plant or algae relies solely on these secondary photosynthetic pigments. Most non-green pigments like carotinoids are non-photosynthetic. This is not because they cannot be used for this purpose (remember any molecule that absorbs enough energy to experience electron excitement could suffice), but because they simply aren't.

Black leaves are not utilizing every wavelength for photosynthesis, even if they absorb light at every wavelength. Remember, photosynthesis is a machine with specific parts. An electron acceptor and an electron donor must be present, as well as access to the greater ATP pathway, for photosynthesis to occur. It doesn't happen just because a leaf has blocked some rays.

The theory of the “Purple Earth,” that chlorophyllous cyanobacteria evolved in the context of preexisting rhodopsinous bacteria, is completely speculative. I class it as one of numerous ad hoc explanations that may be true, or may not, with no clear evidence either way. I still find the “plants don't want too much energy” argument unconvincing for a number of reasons. It's not hard to come up with possible explanations of green pigment. But in the absence of any ability to test our hypotheses, we cannot attribute explanatory power to any of them.

I am looking forward to seeing what else people have to say on this topic!
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Re: Change My View: Alien Plants Could Be Any Color

Post by Salmoneus »

There's actually been some research done on this more recently, with computer simulations. It turns out that the benefit of green chlorophyll is its inefficiency - or, to put that differently, its consistency. The energy available for a band of wavelengths is highest in the green part of the spectrum; howevy, this energy supply is very peaky (as peaks tend to be). Small variations in the input - cloud conditions, shadows, the changing angle of a leaf, time of day, time of year, latitude - result in big variations in output as you, as it were, slide back and forth from one side of the ideal peak to the other, unpredictably, across multiple timescales (seconds, hours, weeks, presumably on an evolutionary scale also millennia). The organism's energy needs, however, tend to be fairly consistent - while largescale predictable cycles can affect behaviour (eg winter; for some plants also day/night), smallscale unpredictable cycles are too... well, smallscale and unpredictable... for the organism to respond to effectively (fruit don't just suddenly grow more slowly whenever a cloud goes by!).

If you want to get a more or less steady output from your energy system, this creates a dilemma: what output level do you 'set' the system at? If you set it high, you can use as much energy as possible at peak input times, and that's great. But then all the time you're not at peak, you don't have enough input to drive your system, and everything switches off, and that's bad. On the other hand, if you set it low, you can keep the system turned on as much as possible. But then at peak input times, you're taking in far more energy than you can actually use - and letting more energy into your engine than you can use tends to damage your machinary (in a factory machine, the problem is "heat"; in a human, it's "cancer"; I'm not sure what the plant version is).

The sensible way around this, unless you build a super-responsive plant (and those are called animals and have in most cases had better things to do than sit around photosynthesising), is to avoid a peaky energy supply. If you want a steady output, it's good to hook it up to a steady input. Photosynthetically, that means avoiding the peak of the energy curve (where a small suboptimality in your setup results in big percentage drops in power), and instead taking from the flatter parts of the curve (where your equipment doesn't have to be as precise). This results in less power overall, but a more reliable and consistent power source. Photosynthetically, in earth atmosphere and conditions, that means green plants.


If you assume that you are building a system that is optimised to provide a constant power level from terrestrial lighting conditions, your perfect system has an absorption spectrum, according to the models, remarkably close to that of actual terrestrial plants. I.e. your photosynthetic panels should be green.


Helpfully, researchers have also simulated what your panels should look like in other conditions. Specifically, the ideal spectrum is different if you're underwater, and if there are other receivers stealing light above you. Altering the conditions to match the conditions experienced by purple bacteria and green sulphur bacteria, the same model again gives predictions that are very close to the actual spectra for purple bacteria and green sulphur bacteria.




But of course, the "everything's just a coincidence" hypothesis can never be disproven, for any scientific result.
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Re: Change My View: Alien Plants Could Be Any Color

Post by Salmoneus »

Incidentally, additional limbs are useful because they increase stability, which is more important at small sizes (because stability through corrective response is much harder, the smaller you are), and manoeuvrability. However, additional limbs impose a severe metabolic cost: an extra pair of legs requires a substantial increase how much oxygen and food you need, and how good your system has to be at distributing these things - two human legs together comprise around 40% of the total human body weight. (the arms comprise only another 10% or so, because they're not weight bearing, and hence much weaker than our legs). Having fewer limbs can also make you quicker, though it does pose additional problems with cooling (because fewer legs means your legs work more of the time). [this is why rabbits, for example, hop when they need to escape quickly, but switch to running on four legs for more sustained movement - two legs is quicker, but is harder to sustain without further evolution (for which see: kangaroos, humans)].

There is therefore a trade-off beteween stability/manoeuvrability and cost. The ideal balance point varies particularly with two factors: the size of the organism (smaller things need more limbs), and the amount of oxygen available (more oxygen means limbs are cheaper, so you're more likely to want more of them).


Unless you have a huge amount of oxygen (and food!), you're unlikely to see spiders the size of dogs, once dogs have evolved. A dog-sized spider would require vastly more food and oxygen than a dog-sized dog (or a spider species that evolved to lose some of its legs), and its slight increase in agility would struggle to compensate.

[however, the more legs you have, the less it costs to add extra legs, because each leg is propotionally lighter and less load-bearing. Whether you have 40 o 60 legs is much less important than whether you have 4 or 6...]
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