People have been trying to imagine elaborate alien biologically possible ecosystems for a while. A lot of people seem to both want but ignore one of those fundamental aspects of our own ecosystem, algae and photosynthesis in general. They also tend to want these alien environments to be hospitable for humans (always more fun if we are in the story).

What I want: is a hard-science chemically plausible alternative to photosynthesis.


  • Can use any reasonable natural source of energy
  • must store said energy in a reactive compound (equivalent of sugar)
  • must produce oxygen as a waste element (any form as long as its unbonded with another element)
  • may use [CO2 and water] but cannot use light at all. May use light but not CO2 and water
  • May assume the environment is chemically and thermally different than Earth, though please stick to agents that would occur naturally in geology preferably in abundance.
  • The organism doesn't have to be carbon based
  • For every reactant used; Must describe the conditions needed for that reactant to be present (only 1 is needed preferable whichever is more reasonable with the others) EX: [ if Fe203 is a reactant, can be found as a solid on a surface with an oxygen atmosphere]
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    $\begingroup$ looks familiar.... worldbuilding.stackexchange.com/questions/96207/… $\endgroup$
    – Willk
    Commented Oct 27, 2017 at 21:18
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    $\begingroup$ @Will its a question intended to help people like that, by attacking a small critical component of what they are asking for. $\endgroup$
    – anon
    Commented Oct 30, 2017 at 14:21
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    $\begingroup$ Hard-science "build me components for an alien biosphere"? What I'd like to say to that can't be posted anywhere with standards about such things. I can think of a dozen redox reactions and ionisation pathways that work but only the basic chemistry not the biochemistry that supports it or that it supports; that's a Doctoral thesis not a worldbuilding question. $\endgroup$
    – Ash
    Commented Oct 30, 2017 at 17:57
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    $\begingroup$ @anon Basically any redox reaction would work in that case. The case for any of them would them simply depend on availability of the reactants the the chemical binding energy needed. $\endgroup$ Commented Oct 30, 2017 at 18:27
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    $\begingroup$ I'd like to point out that there is no need to change the chemistry in any way. You just need to create the electric potential in some way other than light hitting a pigment and everything else can stay the same. Triboelectric or piezoelectric effect might work. A powerful magnetic field with either the field or the planet rotating might be more interesting. Volcanism on otherwise frozen planet might allow thermoelectric effect. I have absolutely no intention of expanding any of this to answer, so if anyone wants to do so, they should feel free. $\endgroup$ Commented Oct 30, 2017 at 20:50

6 Answers 6


You asked for hard science, so here it is.

The key process in photosynthesis is the Joliot-Kok cycle. This is what "splits" water and produces $O_2$, along with the $H^+$ and $e^-$ that are used to create high-energy molecules. Here is the original proposal in a paper by Kok, and here is a link to the full text if you have access. The mechanism is complex redox chemistry, but is nicely summarized in the below diagram from this SE question, which actually got the diagram from here. Joliot-Kok cycle diagram. Another SE question shows us where the light plays a role- when moving between the different $S$ states. As soon as we have $e^-$ and $H^+$, we have energy in chemical form that's often captured in cofactors such as $NADH$, $NADPH$, or $FADH_2$. These are all high-energy forms of their oxidized states- $NAD^+$, $NADP^+$, and $FAD$, respectively, and can be thought of as a kind of battery that gets charged by $e^-$ and $H^+$. In a sense, the "real" goal of photosynthesis is producing $e^-$ and $H^+$ from light energy which can then be used to "charge" the cofactors. So let's look at a couple ways to do this.

As a note, the cofactors aren't magic materials- just large organic molecules. $NAD^+ = C_{21}H_{27}N_7O_{14}P_2$, $NADP^+ = C_{21}H_{27}N_7O_{17}P_3$, and $FAD = C_{27}H_{33}N_9O_{15}P_2$

Simple (but realistic) solutions:

1) Thermosynthesis

There's no reason that the energy required to split water MUST come from light- that's just the way that current biology does it. With a different suite of enzymes and a different cycle, biology could extract energy from a wide variety of sources. In this case, thermosynthesis would rely on heat instead of light:

$H_2O+heat => O_2 + 4H^+ + 4e^-$

This mechanism would be different from the Kok cycle because you'd have a thermally activated alternative to P680 in the middle rather than photoreactive. So that's one solution- thermally activated P680.

Finishing the equation (this is identical to the light-independent reactions of photosynthesis):

$2H^+ + 4e^- + 2NAD^+ => 2H^- + 2NAD^+ => 2NADH$

2) Photosynthesis without water

Alternatively, one could use a different electron acceptor. This came up in the WB question that inspired this question (if we recurse much more we'll have to move to meta), and the solution was nitric and nitrous oxides, one of the most powerful electron acceptors in nature. It's plausible to imagine these nitrogen oxides taking the place of water in the normal photosynthetic pathway, producing $O_2$ and $N_2$ as a result. The researchers didn't have a mechanism for this, but it supposedly produces $O_2$ that is then used to oxidize methane. So that's another solution, looking something like

$2NO + light + H^+ => N_2 + O_2 + 2e^- + H^+$

Finishing the equation (again, identical to the light-independent reactions of photosynthesis):

$H^+ + 2e^- NAD^+ => H^- + NAD^+ => NADH$

Crazier ideas

This is Worldbuilding- let's stretch the limits of plausibility. Where else can we get energy from? Mechanical movement. My vision for this is some kind of kelp-like organism being tossed about by waves or tides, similar to the theoretical wave/tidal energy extractors. As the stalk of the kelp is stretched, it pulls on a long molecule. There are a couple ways we can get energy out of this.

3) Conformational changes

This is like what happens in your eye- a long molecule is unkinked (double bond switches from cis to trans) except we're using mechanical energy to straighten it. As it does that, it forces a conformational change in the molecule that pulls a hydride ($H^-$, or those all-important $H^+ + 2e^-$) off of water- starting a redox chain similar to the Kok cycle. The $^+OH$ would then be attacked by another water, forming hydrogen peroxide- this could decompose into $O_2$ and $H_2$ in the reverse of the normal process. Feasible? Not really. Good fiction? Maybe. Here's your formula:

$2H_2O + mechanical force + NAD^+=> H^- + ^+OH + H_2O + NADH => H_2O_2 + H_2 +NADH$

4) Radical chemistry

Similarly, we could use that mechanical force to tear apart a bond, creating two radicals. I'm imagining an $O-H$ bond, forming some alcohol radical and $H_{(rad)}$. The hydrogen radical would react with something like $FAD$. $FAD$ is another one of those cofactors that modern Earth biochem already uses, and it has a low energy state as $FAD$ and a high energy state when it's reduced to $FADH_2$. $FAD$ accepts two radical hydrogens in this mechanism, so it's perfect for our use. The alcohol radical would attack water to form a peroxide and proceed as above. Can I imagine it actually working? No. Will it help suspend disbelief for a fiction novel? Probably. Here's the equation:

$2RCOH + mechanical\ force + FAD +H_2O=> 2H_{(rad)} + 2RCO_{(rad)} + FAD + H_2O => FADH_2 + 2RCOH + H_2O_2 => FADH_2 + 2RCOH + H_2 + O_2$

where RCOH is a generic alcohol- perhaps ethanol ($CH_3CH_2OH$) or propylene glycol ($C_2H_6OHCOH$)

5) Beta radiation

This type of radiation produces a positron, the antimatter particle to an electron. When a positron and an electron collide, they annihilate. If that happened to a water molecule or something similar, it'd make a hydrogen radical that could get snapped up by FAD, forming a hydroxide ion. Not really sure how to get oxygen out of this, but perhaps it could be catalyzed into sodium peroxide and then into sodium hydroxide and oxygen gas. Equation:

$2e^+ + 2H_2O + FAD=> 2H_2O_{(rad)} + \gamma\ rays + FAD => 2H_{(rad)} + 2^-OH + FAD => FADH_2 + 2^-OH$

6) Gamma radiation

You specified "cannot use light" but I felt like the gamma-radiation eating fungi and bacteria deserved a shoutout. This light wouldn't be coming from a sun, it would be coming from a radioactive source probably deep within the Earth. Not sure if that counts, but I'll include the references here and here just in case. Their basic formula is the same as photosynthesis, albeit with much higher energy photons:

$2H_2O + \gamma\ rays => O_2 + 4H^+ + 4e^-$

To finish the equation, we use a cofactor yet again:

$4e^- + 2H^+ + 2NAD^+ => 2H^- + 2NAD^+ => 2NADH$

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    $\begingroup$ Now I know why it's called hard science. $\endgroup$
    – Vylix
    Commented Oct 31, 2017 at 1:36
  • $\begingroup$ The usage of light was merely to prevent people from some moderate rehashing of normal photosynthesis. Also for the sake of laymens could you elongate your enzymatic shortcuts (NADH FADH RCO) They don't need to be inline but perhaps at the bottom for reference (just so the chemistry doesn't look as simple as it appears). The crazier ideas are cool because you could envision other organisms to create the conditions needed to support this thus resulting in an alien ecosystem. $\endgroup$
    – anon
    Commented Oct 31, 2017 at 14:09
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    $\begingroup$ Holy mother of... kinetosynthesising kelp?? Awesome. $\endgroup$
    – Joe Bloggs
    Commented Oct 31, 2017 at 17:31
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    $\begingroup$ @anon I'm not sure how you want people to solve this problem without complex organic molecules... photosynthesis requires even MORE complex organic molecules and I've left out all of the enzymes. I'll finish out the first two answers so you can see what I mean. If you're asking for a way to use energy to build sugar molecules in any kind of way, then I'd recommend an organic/physical chemistry course and vote to close as too broad. $\endgroup$
    – Dubukay
    Commented Oct 31, 2017 at 18:58
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    $\begingroup$ @anon photosynthesis took a while to evolve on earth, it is still around and the base of the food chain because it is sustainable not because it is the simplest. You can have a realistic biological reaction or you can have one made of only simple molecules not both. $\endgroup$
    – John
    Commented Nov 1, 2017 at 3:34

There are probably some good theoretical processes, but there are 2 natural processes that come to mind, Chemosynthesis and Retinal.

Chemosynthesis takes an acid, heat, and CO2 to make sugar, water, and it strips the anion off the acid.

The retinal cycle uses sunlight and beta-carotene to produce retinal, oxygen, and some energy. It's also the basis for the purple earth hypothesis and is thought to be a precursor to modern chlorophyll.

If emitting Oxygen isn't a hard requirement, you can modify the chemosynthesis reaction to emit any kind of element you want, so long as it can form an acid in the presence of hydrogen. The more electronegative it is, the higher intensity of input energy you need. It's possible you could even do it with a complex acid like sulphuric (H2SO4) and have a secondary reaction that might produce some oxygen off the byproduct.

  • $\begingroup$ except it must produce oxygen, and are you saying if I so wanted through chemosyntesis I could release helium. $\endgroup$
    – anon
    Commented Oct 30, 2017 at 14:19
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    $\begingroup$ "so long as it can form an acid in the presence of hydrogen" I don't think helium fits that particular description. You'd really need something from the nonmetals groups 4 through 7, though a more complicated molecule like acetic acid isn't out of the question. $\endgroup$
    – John Kossa
    Commented Oct 30, 2017 at 18:45
  • $\begingroup$ Just found the link to the original paper on the Purple Earth hypothesis (Free article!) definitely worth checking out: ncbi.nlm.nih.gov/pmc/articles/PMC1334761 $\endgroup$
    – Dubukay
    Commented Nov 1, 2017 at 3:59

Some bacteria employ chemosynthesis to reduce carbon dioxide and generate organic matter if oxygen and hydrogen sulphide are present. Other bacteria generate their own oxygen in the absence of light using nitrite. So if elements of both are present with the oxygen producer in excess, this should mimic photosynthesis and produce both oxygen and organic matter in the absence of light.

Nitrites in nature
Nitrites form part of the Nitrogen cycle https://en.wikipedia.org/wiki/Nitrogen_cycle And are present to some extent in most soils.

Sources of Nitric Oxide
Nitrites from the Nitrogen cycle can be reduced to nitric oxide by Xanthine Oxidoreductase (XO) Under aerobic conditions:

2NADH + NO2- > XO > NO + 2NAD+ + H2O

Nitric oxide is also formed at high temperatures lightning.

N2 + O2 → 2NO

Production of Oxygen in the absence of light
The bacteria Candidatus Methylomirabilis oxyfera converts nitic oxide into free nitrogen and free oxygen.

2NO > N2 + O2
http://www.kegg.jp/kegg-bin/show_organism?org=mox https://www.mpg.de/621120/pressRelease201003241

Production of carbohydrates
Chemosynthesis is the use of energy released by inorganic chemical reactions to produce food. Chemosynthesis is at the heart of deep-sea communities, sustaining life in absolute darkness, where sunlight does not penetrate. Such as here.

Typicaly utilizing reactions such as this.

CO2 + 4H2S + O2 -> CH20 + 4S + 3H2O

Hydrogen sulphide (H2S) occurs naturally as a product of sulphur containing rocks and magma in contact with sea water under high pressure and occurs in some deep sea vent effluents.

H2S and CO2 are present in the environment. The only missing element is Oxygen which is is provided from the nitrogen cycle via reduction of nitrite to nitric oxide by Xanthine Oxidoreductase, followed by conversion to free oxygen and free nitrogen by Methylomirabilis oxyfera type bacteria.

I propose that the Methylomirabilis enzymes and synthesis processes occur in isolation from their usual uses (methane oxidation) and instead are provided to a symbiont chemosynthetic species who use H2S, CO2 and the O2 provided to produce carbohydrates. In return the Methylomirabilis symbiont is provided with a proportion of the carbohydrates produced.

Other bacteria that produce oxygen in the absence of light and utilise a range of inorganic materials can be found here.

Other chemosynthetic bacteria work with different inorganic media such as hydrogen or ammonia to produce organic materials so these could also be used (see above link to chemosynthesis).

Further details of Methylomirabilis oxyfera can be found here.


I'm can't find it in the short time I have to answer this question, but I ran across an article a couple years ago about a lady that found a small lake/pond in an out-of-the-way spot here on Earth that has micro-organisms that replaced their carbon phosphorus atoms with either sulfur or cyanide arsenic. The carbon phosphorus source in the area was so low and the alternate was so prevalent that they somehow made the switch.

So, they somehow ceased to be carbon based life forms, basically. Again, it's been years since I read the 1 article, but I do remember that she was POed about being misquoted by almost all news media as having said that she "found alien life on earth", when she said something more like "finding life on earth that is alien to our way of thinking about it".

The image I remember being with the article is a lady with a steaming pond behind her that looks like it's salt rimmed. If I'm remembering any of this right, I seem to think the area around the pond is completely barren of flora.

While searching for that article, I found what appears to be a similar article, about researchers in a deep mine that are trying to figure out how some micro-life is living without light and with high concentrations of sulfur.


I'm sorry I can't be more use, but I hope it's a starting point. At least this is some science, if nothing else.

EDIT: I found the original article, plus the hard science to back it up. I also found an article that tries to debunk it. I don't have the chops to understand it, so I'll let you fight/figure it out. I got some of the details wrong from memory, so the comment by @Ash was instrumental in finding these articles.

The lady's name is Felisa Wolfe-Simon.




Pasting the possible relevant hard-science from the ScienceMag.org article:

Arsenic (As) is a chemical analog of P, which lies directly below P on the periodic table. Arsenic possesses a similar atomic radius, as well as near identical electronegativity to P (5). The most common form of P in biology is phosphate (PO43–), which behaves similarly to arsenate (AsO43–) over the range of biologically relevant pH and redox gradients (6). The physicochemical similarity between AsO43– and PO43– contributes to the biological toxicity of AsO43– because metabolic pathways intended for PO43– cannot distinguish between the two molecules (7) and AsO43– may be incorporated into some early steps in the pathways [(6) and references therein]. However, it is thought that downstream metabolic processes are generally not compatible with As-incorporating molecules because of differences in the reactivities of P and As compounds (8). These downstream biochemical pathways may require the more chemically stable P-based metabolites; the lifetimes of more easily hydrolyzed As-bearing analogs are thought to be too short. However, given the similarities of As and P—and by analogy with trace element substitutions—we hypothesized that AsO43– could specifically substitute for PO43– in an organism possessing mechanisms to cope with the inherent instability of AsO43– compounds (6). Here, we experimentally tested this hypothesis by using AsO43–, combined with no added PO43–, to select for and isolate a microbe capable of accomplishing this substitution.

  • $\begingroup$ For what it's worth, I remember this news story too. For a few days prior to the announcement, it was touted as an important new piece of evidence in the search for extraterrestrial life (since life even on Earth didn't need to conform to the processes we have come to expect). $\endgroup$ Commented Oct 27, 2017 at 22:14
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    $\begingroup$ guys this is a hard science tagged question, with criteria explicitly asking for chemical formulas $\endgroup$
    – anon
    Commented Oct 30, 2017 at 14:25
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    $\begingroup$ Won't be Cyanide, it's a basic organic molecule, try Arsenic. $\endgroup$
    – Ash
    Commented Oct 30, 2017 at 16:22

So since you asked for a non-light and/or non CO2 reaction I'll go both, I'm going to assume that we can have biological structures that exploit the Seebeck Effect and use thermal gradients to liberate electrons for chemical reduction. Lifeforms exploiting a thermal gradient in this way will probably be relatively thin and wide with a thermal absorbing side and a heat dissipating side and will take advantage of either infrared insolation from sunlight, rather than any of the visible spectrum light used by conventional photosynthesis, or heat from undersea volcanism or other geothermal activity.

So the Seebeck Effect gives us a potentially non-solar biochemical energy pathway but what are we going to do with it? To start with this creature needs certain elements to a degree that traditional organisms don't, particularly Aluminium and Silicon for the thermocouple structure that constitutes its power plant, note it needs roughly equal amounts of these two elements. In nature both Aluminium and Silicon occur primarily as rock forming mineral oxides, Al2O3 for Aluminium and SiO2 for Silicon, reducing these compounds to form it's structure our thermo-plant liberates approximately the same weight of oxygen as what it takes up in Aluminium and Silicon, some of this oxygen will be kept to build energy storage compounds like ATP but most of it will be released into the atmosphere.

  • $\begingroup$ I don't think you can bond aluminum and silicon into a reactive compound that easily, you have a start but your missing the sugar alternative. $\endgroup$
    – anon
    Commented Oct 31, 2017 at 13:39
  • $\begingroup$ @anon ATP is a sugar alternative I didn't cover the chemistry for that pathway since it's an established terrestrial storage pathway. $\endgroup$
    – Ash
    Commented Nov 1, 2017 at 13:05

Chlorine Worlds

Life forms on all known chlorine worlds exhibit the same fundamental biochemistry. This, together with some consistent aspects of cell morphology, is considered to be a strong sign of common descent. As on more typical worlds, life on chlorine worlds produces chemical energy from sunlight by using it to reduce available hydrogen-bearing compounds. Just as on terragen-style worlds, the most common hydrogen donor is water, simply because it is so abundant. This form of photosynthesis releases oxygen. However, chlorine worlds also have a large stock of hydrochloric acid, and photosynthetic organisms also make use of that resource, and release chlorine. The splitting of water and the splitting of hydrochloric acid both release hydrogen ions and high-energy electrons, which are then used to produce carbohydrates and other organic compounds. The usual carbon source is carbon dioxide. There are therefore two dominant kinds of photosynthesis on chlorine worlds:

2HCl + CO2 ---> CH2O +Cl2 in which hydrochloric acid and carbon dioxide are consumed and organic compounds and chlorine produced, and H2O +CO2 ---> CH2O + O2 the far more common process familiar from Terragen and similar biochemistries.

Most photosynthetic organisms actually prefer to use hydrochloric acid if it is available, but the availability of water makes it the more common donor. Release of chlorine ultimately often leads to oxygen in the atmosphere in any case, since the chlorine reacts with water to release oxygen and produce hydrogen chloride again. The combination of the various oxygen and chlorine releasing photosynthetic pigments is typically purplish-black to human eyes. In the light of a typical K type star, that colour is quite nearly black. Respiration on chlorine worlds is the reverse of photosynthesis, and most organisms are capable of using either chlorine or oxygen as fuel. So, an animal on a chlorine world breathes out not only carbon dioxide and water but also hydrochloric acid. Chlorocarbons are abundant in the biosphere, and participate in many biological pathways (unlike the case on more typical garden worlds, such as Earth, in which natural chlorocarbons are present but relatively rarely produced by biological activity). Some particularly resistant chloride polymers are used by land-dwelling life forms to protect themselves from excessive concentrations of hydrogen chloride or from pure water, either of which is harmful to their tissues.

Vitriolic Type Worlds

Many vitriolic worlds have photosynthesizing lifeforms. The exact biochemical pathways can vary, but the basic process is similar on most of these worlds and somewhat familiar. Sunlight drives cellular processes which combine CO2, sulfuric acid and basic silicones into energy-rich silicone polymer "sugars" and release free oxygen. Cellular respiration is, of course, the opposite; silicone "sugars" are "burned" with oxygen to produce CO2, sulfuric acid and waste silicones. The silicone substrates are commonly solid, but sometimes liquid, and generally are not produced in sufficient quantities to raise removal difficulties even for complex multicellular lifeforms.

Sulfur compounds are nearly always of great abundance and importance in Vitriolic biochemistry, owing to their chemical usefulness and great abundance in the environment. Metals are also much more frequently utilized than in Terragen biochemistry owing to the great affinity of sulfuric acid for dissolving them. It should also be noted that the extreme heat of Vitriolic worlds is not an obstacle for local life, it is nearly always a requirement. Many reactions are dependent on the high energy such heat brings, even with the assistance of enzyme-equivalents. As such, temperatures much below 100°C begin to slow down most reactions a great deal.


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