The organisms in the oxygen based ecosystem we have today is perfectly adapted to each other. The ocean is filled with water, and on land it falls from the sky. Plants, algae and cyanobacteria split water into hydrogen and oxygen. Carbon dioxide is absorbed to produce sugars. Animals, fungus and the other non-photosynthetic organisms, as well as plants at night, use oxygen and release carbon dioxide back into the atmosphere.

This cycle is based on water as both an electron donor and a substance all living things needs to survive. And it is based on carbon and oxygen, where the consumers release carbon into the atmosphere, making it available for the primary producers, and the release of the waste product oxygen into the atmosphere, a molecule all complex forms of life depends on.

But there are other photosynthetic forms of life, bacteria that use a different electron donor than water. Are there any of these, if given the chance, that could have the potential to form a complex ecosystem regarding available electron donors, with consumers that produce an atmospheric waste product the producers requires to live, which in turn produce their own waste product the consumers depends on? Of course, the producers would also form the foundation of the food web, but that goes without saying.


You are aware that there exist photosynthesizers that do not use oxygen. You could read up on those.

An example is purple sulfur bacteria. https://en.wikipedia.org/wiki/Purple_sulfur_bacteria

The purple sulfur bacteria (PSB) are part of a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria... Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as their reducing agent, and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide, or thiosulfate (as well, some species can use H2, Fe2+, or NO2−) as the electron donor in their photosynthetic pathways.[4]

The waste product consumed by the PSB is H2S or hydrogen sulfide. Hydrogen sulfide is produced by sulfur reducing bacteria. Just as we reduce oxygen with our metabolism and produce water, these bacteria reduce oxidized sulfur compounds and produce H2S.


Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S).[1][2] Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

In these ecosystems, sulfur fills the role of oxygen. In an anaerobic environment like a sewage treatment lagoon, sulfate reducers break down solids and generate H2S. Purple sulfur bacteria then use the H2S and sunlight to do photosynthesis. H2S can be a gas too, if your question mandates a gas atmosphere.

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    $\begingroup$ Sadly chemistry is not my strongest side. So the photosynthetic organisms absorbs the H2S that is released into the atmosphere by sulfate-reducing organisms, which the producers use to create nutrients for themselves and release their own waste product which is used in the metabolic process of the consumers in a sulfur cycle? $\endgroup$ – Trond Jansen Sep 22 '19 at 2:39
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    $\begingroup$ @TrondJansen Yup. The waste product in question is solid elemental sulfur. $\endgroup$ – Logan R. Kearsley Sep 22 '19 at 3:23

Yes. In fact, there are quite a few options.

Willk has already mentioned sulfur. In this case, primary producers produce solid sulfur as an anabolic waste product, which consumers must eat along with the rest of their food, rather than breathing in. (Unless, of course, they are from the planet Sar, which is hot enough that sulfur exists as an atmospheric gas, and molten copper chloride stands in for water.) Actual sulfur producing bacteria tend to accumulate crystals of sulfur in their cells, rather than releasing it all directly into the environment, so you could expect sulfur-producing plants to do the same, as they have even bigger excretion logistics problems than unicellular photosynthesizers do!

Some real-world bacteria can also perform carbon fixation using free hydrogen directly, in environments where free hydrogen exists. And there are organisms that generate hydrogen from the anaerobic respiration / fermentation. So, theoretically, there could be a cycle there; however, in practice, if you have a lot of hydrogen in the air, as well s carbon dioxide, they will spontaneously react over time (or not so spontaneously, as organisms can get energy by catalyzing the reaction themselves, which is exactly what methanogens do on Earth) until one or the other is depleted.

In a sulfuric acid world, plants could acquire hydrogen from sulfuric acid, producing solid sulfur trioxide or gaseous sulfur dioxide as a waste product, which consumers would then eat or breathe in place of diatomic oxygen. Such worlds are also likely to have a lot of hydrochloric and hydrofluoric acid around, which given sufficiently energetic light to work with, or photosystems which can accumulate energy from multiple photons (or work around it by just generating ATP / the local equivalent until there's enough of that around to power the reaction) could also be split to acquire hydrogen. I would not, however, expect the release of straight Cl2 of F2 gas, however, as those are highly reactive (maybe on a really cold world around an F-class star...)--rather, I'd expect to see them bound up in metal complexes (just like iron-oxidizing bacteria do with oxygen), or halocarbons--gaseous carbon tetrachloride and carbon tetrafluoride. Unfortunately, those are very stable chemicals, so they won't be very useful for completing an ecological cycle with consumers. Rather, you'd expect them to be feedstock for further exotic anabolic processes--extra sources of carbon and less-reactive forms of halogens.

Like the sulfuric acid world, but more plausible, while I am not aware of any Earthling organisms that do this, photoautotrophs could also acquire hydrogen (as carbon, and sometimes oxygen) and reducing potential from simple organic molecules, like methane, methanol, ethanol, acetate, etc., with more heavily oxidized organic molecules as the waste product. For example, in a world with a CO2/methane atmosphere, plants could rip hydrogen off of methane to produce ethane, ethylene, and/or acetylene gas as byproducts, which would be breathed in by consumers to regenerate gaseous methane for producers to consume and repeat the cycle.

Of course, acetylene is a pretty good energy storage molecule all by itself, and ethane is a good place to start building longer alkane and alkene chains, so as in the case of the sulfuric acid world these really aren't "waste" products like oxygen so much as they are additional useful products of photosynthesis, of which there is sometimes an excess which is useful to other organisms.

In a world with a slightly more heavily reducing environment, you can expect a decent amount of ammonia to be available. Stripping hydrogen from ammonia is easier than stripping it from water (although if you only go part way, you get some very energetic molecules, like hydrazine--the ammonious equivalent to hydrogen peroxide), so it would not be unexpected for that, rather than the less-abundant hydrogen sulfide or the more tightly-bound water to serve as hydrogen donor and source of reducing potential. The waste product in this case is nitrogen, which is famously not easily breathable,as dinitrogen is a very stable molecule that does not like reacting with anything. Except, it does react slightly exothermically with hydrogen to give you back your original ammonia, completing the cycle; there are no (known) organisms on Earth which can acquire energy through nitrogen reduction, because Earth is a highly oxidizing environment, and nitrogen-fixing bacteria have to expend more energy than ammonia production gains them in order to acquire the necessary reduction potential in the first place, but that situation does not hold in this hypothetical environment. So, your consumers would presumably perform hydrogenic fermentation and nitrogen fixation for a positive energy yield in both processes, closing the nitrogen-ammonia cycle instead of the oxygen-water cycle.

And in an even more strongly reducing environment, where excess hydrogen has destroyed all CO2 in the atmosphere, leaving behind free hydrogen, methane, water, and ammonia, your producers will be producing waste hydrogen rather than waste oxygen or nitrogen, and looking to acquire chemical oxdizing potential rather than reduction potential for anabolic processes. The consumers will not excrete any single gaseous molecular species to close the cycle, but the whole gamut of fully-reduced water, methane, and ammonia to resupply the producers with raw materials.

And of course, as a final note: in none of these cases should you necessarily expect glucose specifically, with its specific elemental ratios, to remain the go-to energy storage and structural molecule produced by alien photosynthesis. It wouldn't even be stable on a sulfuric acid world, and other types of molecules--like alkenes or organonitrogen compounds--will be competing for some of its functions in exotic chemical environments. Heck, even on Earth, there are organisms that get most of their energy from metabolism of fats and/or proteins rather than sugars, and the components of those cycles may end up more important than the basic oxygen-water cycle or its local equivalents.

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    $\begingroup$ A solid reply. Animals, or the equivalent of animals and fungus, needs to eat anyway, so if their "oxygen", for the lack of a better word, is taken in with the food, it doesn't have to be in gas form. It is maybe even better if it accumulates in organsism, which at least on land is restricted to the ground, rather than being spread in the atmosphere.The plants can obviously not do the same, so their source would come from the air or through their roots. $\endgroup$ – Trond Jansen Sep 22 '19 at 20:32

Green sulfur bacteria are anaerobic and photoautotrophic. They use sulphide ions as electron donors. Some species live around deep sea hydrothermal vents from which they feed on hydrogen sulphide. They are so efficient at harvesting light that they can even grow in the absence of sunlight in the very weak radioactive glow from geothermally heated rock.

They also form one component of the food chain for more complex organisms that live in the vicinity of the deep ocean vents.

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