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On a distant world, the atmosphere is different than ours - there is no oxygen but a high level of C02 like on Mars or the early Earth. This planet is otherwise similar to Earth but slightly warmer. The planet orbits a Red Giant as a sun. The soil is rocky, and high in Silicon.

If I want plants (photosynthetic plant-like multicellular organisms) on my planet, as similar as possible to Earth plants, how do I make them work? What would they look like and how does their chemistry function? There is no animal life, only alien vegetation.

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  • $\begingroup$ plants need oxygen too $\endgroup$ – John Aug 2 '20 at 21:24
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    $\begingroup$ Hello Edmond301. I apologize for the VTC, but there's no science to back up any answer. The question is 100% hypothetical. Consequently, you have no basis for selecting a best answer of than "I like that one best," which is opinion-based. When you have a moment, please take our tour and read our help center to better understand how the site works. Thanks. $\endgroup$ – JBH Aug 2 '20 at 21:24
  • $\begingroup$ What is the total atmospheric pressure? Mars has mostly CO2 but it is very much lower pressure than on Earth. Round about 0.00628 atm. Something that grows in a very low atmosphere will be very different to something that grows in a higher atmosphere, even supposing lots of CO2. $\endgroup$ – puppetsock Aug 2 '20 at 21:24
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    $\begingroup$ "What would they look like?" They would look great. They would look most beautiful. They would look like tangled webs. They would look for themselves. They would look like alien plants. They would look like ineffable manifestations of the power of love. They would look well-adapted to their environment. They would look like green magma. They would look satisfied with their lot. They would look somewhat like overgrown mosses. They would look like a luxuriant rainforest. They would look like a strangely illuminated meadow. They would look like a vegetable mass of tentacles. $\endgroup$ – AlexP Aug 2 '20 at 21:26
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    $\begingroup$ Atmosphere pressure? Water? Considerations about be only vegetable-like but not really a vegetable, looking like more to fungi kingdom than vegetable? Cyanobacteria seems the most plausible thing on Earth. $\endgroup$ – Rodolfo Penteado Aug 2 '20 at 21:55
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Anaerobic glycolysis.

Under circumstances of low oxygen, aerobic eukaryotes can switch to glycolytic metabolism. Humans can do this too, temporarily.

https://en.wikipedia.org/wiki/Glycolysis

Glycolysis is an oxygen-independent metabolic pathway. The wide occurrence of glycolysis indicates that it is an ancient metabolic pathway.[5] Indeed, the reactions that constitute glycolysis and its parallel pathway, the pentose phosphate pathway, occur metal-catalyzed under the oxygen-free conditions of the Archean oceans, also in the absence of enzymes.[6]

Anaerobic glycolysis is less efficient than aerobic, yielding a third of the energy per glucose molecule. Also there are byproducts that must be dispensed with or regenerated to glucose; lactic acid and ethanol are examples.

Maybe the original plants used glycolysis back when they could make sugar but were not assured of plentiful oxygen. They can still do it today.

Low Oxygen Response Mechanisms in Green Organisms

Low oxygen stress often occurs during the life of green organisms, mostly due to the environmental conditions affecting oxygen availability. Both plants and algae respond to low oxygen by resetting their metabolism. The shift from mitochondrial respiration to fermentation is the hallmark of anaerobic metabolism in most organisms. This involves a modified carbohydrate metabolism coupled with glycolysis and fermentation.

So your plants would be glycolytic. That is totally legit biochemistry and not fiction. For a fiction I like the idea that they store the end product as either ethanol or lactic acid, regenerating sugar from it when there is lots of water and sun to use. Those plants might be harvested for their storage products in addition to their sugars.


It occurs to me that plants in a low oxygen environment might store the oxygen they produce in their tissues, just as they store the carbohydrate product. Gaseous oxygen is reactive and difficult to store though could be stored as bubbles in an aquatic plant.

Animals store oxygen using heme molecules. Hemoglobin is one. Myoglobin stores large amounts of oxygen that whales use during their deep dives. Your plants could have similar pigments that capture the oxygen product of photosynthesis and keep it handy for aerobic metabolism when needed.

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Cyanobacteria consume carbon dioxide and produce oxygen as a waste product. They require water, sunlight and a few nutrients, but they do not require any oxygen at any point of their photosynthetic cycle so they could be a good starting point.

One possible candidate for your “plants” if you want to base them on Earth life forms would be Nostoc pruniforme which grows in gelatinous spheres with a smooth surface like a plum. These are typically a few mm or cm across but can get as large as 15cm or more in some cases.

Nostoc pruniforme

That said the possibilities within chemistry are vast beyond reckoning so there is no reason why given sufficient evolutionary pressure on an alien world with an alien biogenisis that something like cyanobacteria should not evolve into more complex plant like shapes and forms as well as developing resistence to the oxygen it is itself producing as a waste product.

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Do what aquatic plants do

Here on Earth, plants need oxygen for respiration, just like animals and fungi. Very many plants have very successfully adapted for life under water, although plants cannot extract oxygen from water.

How did they do it? They make their own oxygen through photosynthesis.

Some aquatic plants are very popular with people who keep aquaria, both as a source of oxygen for the fish and as landscaping material.

Ceratophyllum demersum   Hemianthus callitrichoides

Left, Ceratophyllum demersum, a popular aquatic plant. Photograph by Totodilefan, available on Wikimedia; public domain. Right, Hemianthus callitrichoides, an aquatic plant widely used in aquascaping. Photograph by Ranjith-chemmad, available on Wikimedia under the Creative Commons Attribution-Share Alike 4.0 International license.

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The Earth's primitive atmosphere was Carbon dioxide and methane. When the first photosynthetic organisms absorbed carbon dioxide they converted it to oxygen. With few animals to consume it, the atmosphere literally converted.

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  • $\begingroup$ The problem lies in the "multicellular" part. $\endgroup$ – Mary Aug 17 '20 at 3:42

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