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Note, when I say hydrogen-based atmosphere I mean replacing the 20% oxygen with hydrogen1.

Let's just say basic plant life2 has developed on our fictional planet (which has the approximately all the same parameters of Earth did). What colour would the plant's leaves be - what colour would chlorophyll be. Please ignore the fact that respiration would not work3.

According to here, chlorophyll is green because the red & blue wavelengths it delivers enough energy for photosynthesis - adding green would result in excess heat that could potentially damage the plant. This promotes me to think that the colour might stay green, as they would have the same parameters4.

1: The Carbon Dioxide could be replaced with Methylene (CH2), but this might mess up this further.

2: Around about the Jurassic period.

3: It probably wouldn't work due to the fact that energy is gained from oxidising the glucose sugars. However, this question is focusing on creating the glucose sugars.

4: There would still (initially, at least at the start) be carbon dioxide in the atmosphere. I know the fact that the oxygen is a byproduct would mean that the hydrogen atmosphere would be replaced with oxygen eventually. Thinking further, this would mean that the hydrogen would mostly play no part whatsoever.

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    $\begingroup$ There's a lot of things that you need to clarify before anyone can meaningfully answer this question. What do you mean by "generate hydrogen"? Why would hydrogen in that form be a by-product instead of oxygen (O2, specifically)? Are you asking us to make up a new process that produces that, or to assume that at some point, the laws of physics and chemistry are not what they are? $\endgroup$ – Harris Oct 23 '17 at 20:28
  • $\begingroup$ @Remi.b That's probably a better site. Moving it $\endgroup$ – Modelmat Oct 24 '17 at 7:30
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    $\begingroup$ Hello and welcome to Worldbuilding @modelmat. Your question is unfortunately much too broad, and it is unanswerable. Your question essentially says "If the entire foundation for aerobic life as it has evolved in the past 3+ billion years was different and essentially non-existent, how would we humans be different?". The simple answer is: you just broke (almost) all life on Earth. There would not be humans or any other kind of aerobic life. You cannot say that you change the foundation of things and then expect everything else that follows to be anything remotely like what is now. $\endgroup$ – MichaelK Oct 24 '17 at 8:20
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    $\begingroup$ Welcome to WorldBuilding! Your question covers a lot of ground, especially because there are multiple questions in the body like how plants would be different and how our bodies would be different, ... Please try to edit your post to narrow down the scope so we can help you. There is a character limit of 30.000 ;) You can always later ask related questions. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Oct 24 '17 at 8:28
  • $\begingroup$ Welcome! It's been remarked that you’re actively improving the question and might be interested in some pointers. Check out Lessons in writing questions. $\endgroup$ – JDługosz Nov 4 '17 at 7:28
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Land plants around us use (green) chlorophyll a and chlorophyll b by accident. There are lineages of photosynthetic organisms which use other pigments:

It just happened that in evolutionary history the green plants were more successful; it was not destiny, it just happened so.

The end of it is that you can make your leaves any color you want.

By the way, you may have noticed that urban planners are quite fond of decorative trees with purple leaves, mostly Prunus cerasifera (cherry plum-tree).

Prunus pisardii

(Purple-leaved Prunus cerasifera var. pisardii. Photo by Arturo Reina Sánchez, available on Wikimedia under the CC-BY-SA-3.0 license.)

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Im going to go with green

As far as I can tell the reason green light is rejected is fairly biologically arbitrary (why such a narrow band in the middle of two larger bands).

And in fact there are organisms that can use green light.

Chlorophyll is green because it makes it more effective at absorbing red spectrum light (which is more abundant than blue) this however isn't a biological imperative rather an efficiency adaptation. By sacrificing one narrow band it has access to a wider more common band.

On your world changing the composition of the atmosphere doesn't really change the spectrum of available light. Since red light is still abundant photosynthetic organisms can still be green.

This is ignoring the fact you completely changed the metabolic process of photosynthesis which in itself would more likely change the color.

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  • $\begingroup$ Note that photosynthesis does not rely on the green-colored molecule chlorophyll (swapping the magnesium atom for one of iron would work too and such a molecule would be very similar to heme, which performs a similar role as electron-donor), just that all photosynthetic plant life on this planet happens to use chlorophyll (and we don't really know why). Even red-leafed plants still use chlorophyll. See: the Reverse Krebs cycle for one bacterial method. $\endgroup$ – Draco18s Nov 10 '17 at 15:53
  • $\begingroup$ As I said it chose green as an efficiency enhancer and gained prominence probably for the same reason which is why I said it was arbitrary. But thanks for the added alternatives $\endgroup$ – anon Nov 10 '17 at 15:57
  • $\begingroup$ Sure, sure. I just did some poking around to see what info there was. $\endgroup$ – Draco18s Nov 10 '17 at 15:58
  • $\begingroup$ you may want to look at my answer to this worldbuilding.stackexchange.com/questions/63259/… basically every part of the spectrum is used by something, plants use a narrow section of the spectrum becasue the rest was already being used by halobacterium when plants evolved. $\endgroup$ – John Nov 10 '17 at 16:45
  • $\begingroup$ @John thanks for that added information, it doesn't really change my point though other than to provide examples of what I was referring to. $\endgroup$ – anon Nov 10 '17 at 17:15
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If I understand what you are trying to ask, the answer to your question would be related to the Methylene rich(?) atmosphere. Determine what frequency of light would be most able to reach the chloroplasts, and pick a color that would absorb the energy rather than reflecting it.

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