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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.

IllustriousMagenta
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    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? – Harris Oct 23 '17 at 20:28
  • @Remi.b That's probably a better site. Moving it – IllustriousMagenta Oct 24 '17 at 07:30
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    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. – MichaelK Oct 24 '17 at 08:20
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    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] to learn more about the site. Have fun! – Secespitus Oct 24 '17 at 08:28
  • 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. – JDługosz Nov 04 '17 at 07:28
  • Your title is not a full sentence. Rather than what exactly? A descriptive title helps people judge what to expect when they read your question. I took the liberty to change the title a bit and format your question to make it easily visible on the first glance. I vote to reopen. Thanks for sticking around, I hope your question gets reopened and you get the answers you are looking for. You can always later post additional new questions and link to the old ones, incorporating what you learned into the newer ones. Have fun on the site! – Secespitus Nov 04 '17 at 08:50
  • @Secespitus, isn't this still too broad? (a) all biology still exists, (b) all photosynthesis produces hydrogen, (c) what would be different? That's still unanswerable. If we reduce this to one plant, then this might be answerable. But as written, this is basically asking, "what would life be like with a hydrogen atmosphere?" (Answer: burned to a crisp after the first lightning bolt, and that's assuming the physicists are wrong.) – JBH Nov 04 '17 at 17:12
  • Seems more like a question for chemistry, but even there I doubt you'd get an answer. I think you'd need to do a lot of simulation & experimental work to design a plausible photosynthetic pathway that works on say CH4/H2 reactions rather than CO2/O2, and guessing what color that might be? Difficult :-) – jamesqf Nov 04 '17 at 18:00
  • I'm completely restarting this question, with some google research. – IllustriousMagenta Nov 04 '17 at 22:21
  • @Modelmat, allow me to make an observation about your 4th point... The leaves are not the ones that changed the composition of atmosphere, but the phytoplankton instead. Unless your planet also has photosynthetic forms of life that are not trees with leaves, maybe the hydrogen would not be replaced. This question has some answers with insights and references on oxygen, if you're interested: https://worldbuilding.stackexchange.com/questions/95893/fantasy-map-that-has-minimal-land-at-the-equator-what-does-this-mean-for-rainfo/95898#95898 – Thai Nov 10 '17 at 02:05
  • basically a duplicate of this https://worldbuilding.stackexchange.com/questions/63259/what-colour-is-the-foliage-if-the-sky-is-violet/63264#63264 the answer is the color of the sky does not effect the color of plants. – John Nov 10 '17 at 16:41
  • "The hydrogen atmosphere would be replaced with oxygen eventually". Hydrogen and oxygen love each other so much that as soon as they are in contact any tiny pretext is enough to make the combine and produce water. Essentially you cannot have significant amounts of hydrogen and oxygen in the atmosphere at the same time. – AlexP Nov 11 '17 at 04:07

3 Answers3

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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.)

AlexP
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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.

anon
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  • 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. – Draco18s no longer trusts SE Nov 10 '17 at 15:53
  • 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 – anon Nov 10 '17 at 15:57
  • Sure, sure. I just did some poking around to see what info there was. – Draco18s no longer trusts SE Nov 10 '17 at 15:58
  • you may want to look at my answer to this https://worldbuilding.stackexchange.com/questions/63259/what-colour-is-the-foliage-if-the-sky-is-violet/63264#63264 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. – John Nov 10 '17 at 16:45
  • @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. – anon Nov 10 '17 at 17:15
  • @anon except green is not more efficient it is less efficient, it also access a narrower portion of the spectrum. Plants are just stuck with it becasue the bacterium that they formed a symbiotic relationship with happened to use green. – John Nov 12 '17 at 19:39
  • @John Makes me wonder if we genetically engineered a plant that used a different color spectrum (e.g. absorbs green, reflects red) if it would out-compete the natural species. I suspect not, but I'm not a biologist. – Draco18s no longer trusts SE Nov 13 '17 at 18:55
  • depends on what portion of the spectrum you consider "green" halobacterium use the entire middle of the visible spectrum, reflecting only two narrow bands on the ends, if plants had formed a sybiotic relationship with them instead we would have pepto bismol colored plants. – John Nov 13 '17 at 21:31
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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.

Brian Lami
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