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I'm wondering about how to make my photosynthesising organisms all glow. Basically, I want to make bioluminescence a consequence of photosynthesis, as if the organelles themselves produce light in the chemical reaction of photosynthesis. I want to have all sorts of colours, too. From a deep, ominous red to a bright and vibrant blue-green. I want all my plants to glow, and I also want my photosynthesising animals to glow as well! How would I get this to happen?

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    $\begingroup$ The "chemical reaction of photosynthesis" happens in sunlight, obviously. To have the leaves glow visibly in sunlight is hard. Consider how hard it is to see the screen of your mobile phone in sunlight: and the screen of the mobile phone is designed to put out as much light as possible! $\endgroup$
    – AlexP
    Dec 15, 2023 at 22:49
  • $\begingroup$ @AlexP maybe a sort of dim duskish light level during the day due to atmosphere? $\endgroup$
    – alkahest
    Dec 15, 2023 at 23:56
  • $\begingroup$ news.mit.edu/2017/… engineered bioluminescence, might be a bit of help (I hope!) :) $\endgroup$
    – alkahest
    Dec 15, 2023 at 23:58
  • $\begingroup$ @AlexP Sorry, I should have clarified. This would happen mostly at night, the energy being released after the day. The glow would also be more obvious in the sea, and at close inspection. Consider the faint glow grass would give off in early-morning fog :) $\endgroup$ Dec 16, 2023 at 2:59
  • $\begingroup$ Bioluminence is a waste of energy if it's just for the sake of it, therefore it should have a purpose. What would be the purpose for the plant to have its leaves emit light during the night (day too as it's not a controlled process for plants, but daytime luminence IS useless)? Start with this. If you won't find the purpose, drop the idea. Yep Avatar's night was aesthetically great, yet it didn't serve other purpose than artistic, and IIRC plants didn't emit light there. $\endgroup$
    – Vesper
    Dec 16, 2023 at 4:13

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Use excited states of supporting chemicals

After all, if your planet's biochemistry does not exactly resemble what's happening on Earth, you can declare that there are both compounds that produce energy for the photosynthesizing organisms, and compounds that are required to support this process but that also have an excited state, turning excited by a blue or UV quantum, and release a red to green quantum upon returning to normal. Some fireflies emit light with a specialized compound that emits a quantum or "red to green" light depending on some yet unknown conditions regarding the molecule in its excited state; effectively this grounding can be used to explain why some of the plants emit more red and some more green light, while using one and the same compound that gets excited.

Additionally, you may propose that excitation of that compound can also happen by another mechanism, as do fireflies, in order to explain why the plants still emit light close to the dawn, with no excitation being performed by the sunlight. But remember, induced bioluminence is waste of energy, so it should have some purpose to run such a mechanism beyond being pleasant to human sight.

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For that you need the common ancestor of all extant plants to have evolved with the need to glow in the dark. Perhaps it coevolved with a pollinator that only comes out at night, and which is attracted by light.

There are bacteria that provide bioluminescence to deep sea creatures, so the genetics of this for plants are very possible. Or more likely the plants themselves don't glow, but their bacteria and fungal symbiotes do. By consuming these plants, animals would also consume the glowing organisms. This would provide them with some glow as well.

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  • $\begingroup$ You could add a mitochondria-like part to the plants? A unique cell that is so pivotal that any and all plants that wanted to survive got it, and it just so happens that it enables bioluminescence in plants that got it $\endgroup$
    – Demigan
    Dec 17, 2023 at 21:24
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Frame challenge: reflection

You specifically request chemicals, but let me suggest reflection instead.

The premise if your plant is at the start already a bit problematic. The energy of light is required to start photosynthesis, so if your plant glows in the sunlight it is also losing a lot if energy as light again. That suggests it is inefficient. It can't even be a catalyst, as sunlight needs to add it's energy to the chain of life. Most of all life on Earth eventually depends on it.

What you might be able to explain is that the photosynthesis is a bit like the sensors in the eye. The light hits them and excites them. We can then make a parallel with several wildlife eyes, like cats. Their eyes can seem to glow thanks to a reflective layer behind the eyes. This passes the light along the sensors again, which allows for better night vision. This doesn't apply to sunlight, but it is also much deeper in a glass body.

Your plants are hit with light. The photosynthetic cells react, but the light passes though the layer. The light is then reflected, hitting the photosynthetic cells again, after which part of the light escapes and creates a glowing surface.

Why reflection?

The reason for reflection is that this way the least amount of energy is lost. The transmutation of light into chemical energy and then back into light is a bigger cost than simple reflection.

The colours are then also explained. If plants were truly efficient, they would look black to our eyes. No one knows why they reject the green spectrum of light, making most photosynthetic cells appear green. Your photosynthetic cells allow certain wavelengths of light to pass unhindered, while trying to absorb as much of other wavelengths as possible. The wavelengths not absorbed are relected like any other, giving the plant a glow that might be seen in daylight. Like a coloured mirror reflecting light.

In any case you would never expect glowing animals in sunlight, but these would get you closest in the most plausible way.

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  • $\begingroup$ Plants do not "reject" the green part of sunlight. They just absorb it a little less than the blue part, and not-so-a-little less than the red part. If you have ever looked at a black-and-white photograph you may have noticed that foliage is dark. $\endgroup$
    – AlexP
    Dec 16, 2023 at 14:07
  • $\begingroup$ That would make a visible glow appear in the twilight hours, or among plants growing in a cave, where some light gets through the entry. Some plants may occupy this vacant niche of growing inside the cave and evolve chloroplasts with a reflective underside which may account for the glow. $\endgroup$ Dec 16, 2023 at 14:30

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