Change My View: Alien Plants Could Be Any Color
Posted: 12 Oct 2020 01:55
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.
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.