Fungi do not need light to grow. They also can and do grow in feces.

Is it possible for a human colony to provide their caloric needs by growing mushrooms using their own poop? If so, what's the minimum size of a colony which produces enough poop to grow a sufficient amount of mushrooms to feed themselves?

For consistency, you can assume:

  • A 2000 daily calorie intake per person
  • The environment is heated through external means so there's energy input into the system, it's not closed (if that matters)
  • You may ignore the mental and physical health impacts of living without sunlight and around so much human poop
  • If no existing edible mushrooms can grow in human poop, then there's been genetically modified mushrooms which can. No other qualities of the mushrooms have been changed. Existing mushrooms are preferred in answers if possible though
  • Vitamin needs are satisfied via supplements. Bonus points though if answers can address these needs through the mushrooms
  • 1
    $\begingroup$ If you're allowing genetically modified mushrooms, then you can wave your hands and say that your folks have creates a supermushroom that solves their nutrition needs. We can't tell you whether the folks in your world have created such a supemushroom or not. That's something you'll need to decide for yourself. $\endgroup$
    – sphennings
    Jul 20, 2022 at 13:07
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    $\begingroup$ While not directly feeding the people, the Talaromyces Flavus is a fungus that feeds on iron. It could be really useful for your civilization to use this fungus in their mines to weaken the rock(using a rotating schedule) to ease the mining process. You might even be able to extract the iron from the fungus too. $\endgroup$
    – vinzzz001
    Jul 20, 2022 at 13:41
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    $\begingroup$ No, because your biome lacks Primary production. However, if there is a constant influx of biological material (ex. someone above keeps dumping their poop into the cave), it can work. $\endgroup$
    – Alexander
    Jul 20, 2022 at 18:35
  • 2
    $\begingroup$ If there is an electricity source, couldn't you use artificial lights for plants? Or does it have to be mushrooms? $\endgroup$
    – qwr
    Jul 20, 2022 at 22:26
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    $\begingroup$ Technically your question isn't an exact duplicate of Best Foods/Plants to Grow in Generational Spaceship?, but that question is what you should have asked. $\endgroup$
    – JBH
    Jul 21, 2022 at 3:02

3 Answers 3



The problem here is that there are no calories going into the system. Plants extract calories from the sun, but fungi extract calories from whatever they live on. The mushrooms must have fewer calories worth of food than the feces that went in, because the fungi use the calories to live and grow. There would rapidly be less calories for humans, less feces for mushrooms, and the system would quickly grind to a halt.

Mushrooms are also low in calories and typically vitamins. They can or do contain vitamin D, Zinc, and potassium. They are an incomplete and generally somewhat low source of protein. Here is where genetic engineering would come in handy. You can add the enzymes and pathways to allow your fungi to produce more and more varieties of amino acids to provide complete protein. You might even be able to make the fungus out of a digestible starch like a plant, but that would vastly increase the caloric needs of the fungus and only exacerbate your calorie problem. Raising the fungi with more minerals and the engineered ability to absorb and sequester them Could make the mineral content balanced and complete. But the fungi would still need a source for those minerals.

Heat from outside is an extremely poor energy source. Besides the fact that fungi can't use heat for energy, heat is already an extremely disorganized state of energy. Unless you have an epic source of heat (like the sun) it will be the lowest energy state entropically.

  • As a side note, plants provide oxygen, so I hope your colony has an external source of oxygen. They also fix carbon, which will be decreasing as people and fungi respire CO2.

But wait, there is SOME hope.

If you have geothermal heat or any other source providing electrical power, and are growing something photosynthetic and inedible (like an algae) in tanks using light, this photosynthesizer can be fed to the mushrooms, the mushrooms will have an energy input (rotting the algae) and this becomes possible.

Based on the question, I assume just growing plants for food is counter to the goals of the question.

If your colony has an external source of biomass (like alien plants, if it is on another world), a fungus could be engineered to break down the alien sugars and amino acids (or equivalents) and use these to produce food humans could eat. I've used something similar in some of my stories.

In the somewhat unlikely category, you could have a non-renewable supply of energy. If you have a fungus that can utilize some chemical in the rocks for energy, then you have a (non-renewable) source of energy. Then you would need to provide a stream of the mineral to the fungus, or constantly expand the fungus to new deposits. If the fungus can use petroleum, and you have access to oil or tar pits, the fungus could consume those for energy. But for this kind of activity, bacteria usually do the job of utilizing exotic chemical sources of energy better. You might need something more akin to a lichen (but with bacteria as the symbiote) for your fungus.

  • 6
    $\begingroup$ “inedible (like an algae)” if you can grow algae, why not grow edible algae? $\endgroup$
    – Topcode
    Jul 20, 2022 at 14:12
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    $\begingroup$ @Topcode Because their premise is the people living on fungi. If they can eat the plants directly, that's a whole different question. Maybe all they have is fungi and inedible plants. $\endgroup$
    – DWKraus
    Jul 20, 2022 at 14:17
  • $\begingroup$ "But the fungi would still need a source for those minerals." Minerals can be recycled ideally forever. But your point about low-entropy energy still stands. $\endgroup$ Jul 20, 2022 at 22:09
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    $\begingroup$ 👍 for pointing out that without some alternative source of energy going into the system this is just the old cannibalism question with one added step 👌 $\endgroup$
    – Pelinore
    Jul 20, 2022 at 22:36
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    $\begingroup$ Mushrooms that do have Vitamin D need to be exposed to sunlight to develop it. $\endgroup$
    – Allan
    Jul 21, 2022 at 13:43

There are no perpetual motion machines. The usable energy recovered through each cycle is lower than 100% and the lost energy must be replaced somehow from an outside source or else your usable energy will eventually drop to zero. It need not be sunlight but by it cannot, by definition, be poop from humans who eat nothing but the mushrooms.

The environment is heated through external means so there's energy input into the system, it's not closed (if that matters).

The system is not closed but the food cycle is. You either need an organism capable of biologically using that heat or machinery to convert it to a form biologically accessible (i.e. to power grow lights). Otherwise, the food cycle is indeed closed.

It also requires a heat differential to extract usable energy and this tends to be difficult since most heat you find is low-grade, low differential, waste heat.

  • $\begingroup$ The question mentions that there is energy input via heating, hence they don't consider the poop-mushroom-poop cycle to be closed. In how far does this impact your answer? $\endgroup$
    – Schmuddi
    Jul 21, 2022 at 7:55
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    $\begingroup$ @Schmuddi: Due to thermodynamics, probably not. You'd need a heat differential, And extracting energy from a heat differential through biological means is hard - not entirely impossible though. We know about some bacteria that live in hot water sources, warmed by magma. But those provide very little biomass, compared to what humans would need. $\endgroup$
    – MSalters
    Jul 21, 2022 at 10:06
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    $\begingroup$ @Schmuddi In addition to the heat differential mentioned by MSalters, that energy needs to actually enter the food cycle. You either need an organism capable of biologically using that heat or machinery to convert it to a form biologically accessible (i.e. to power grow lights). The question mentions neither therefore OP is mistaken and the food cycle is indeed closed. $\endgroup$
    – DKNguyen
    Jul 21, 2022 at 13:21
  • $\begingroup$ Thanks for the edit, that makes perfect sense to me. $\endgroup$
    – Schmuddi
    Jul 21, 2022 at 16:50

Not with fungi, but with chemolithotrophic organisms

As mentioned above, your ecosystem has no organism capable of creating organic matter from minerals. Plants can do it using light, so they are photolithotrophs. If you have an external source of energy, like geothermy, just grow plants under electric lamps.

However there are organisms that do not require light to reduce carbon into organic molecules, called "chemolithotrophs": on Earth these organisms are only bacterial and require specific environments like deep-sea hydrothermal vents because these are a source of reduced sulfur/iron/hydrogen that they need.

In your setting, you would need to have such chemolithotrophs in an easily accessible environment near your troglodyte humans. And you would need them to be edible, as well as growing in sufficiently thick mat. Alternatively, you could consume giant worms that live in symbiosis with chemolithotrophic bacteria.

As for the fungi idea, I think (did not check) that the fungi living in caves obtain their organic matter coming from the outside, like bat poop.

  • 1
    $\begingroup$ It should be noted that these organisms, while very well adapted to their environment, are extremely slow metabolising compared to humans, for obvious reasons. They only really live in the extreme environments where faster organisms can't survive. Even if you could breed them and harness the foodstock (which isn't edible for humans, but we can handwave that with a multi-stage process), I wouldn't even hazard a guess as to how much of them you'd need to sustain even a single human :D It's an interesting idea, but it's hard to see how it could compete with green plants and electric lighting. $\endgroup$
    – Luaan
    Jul 23, 2022 at 15:04
  • $\begingroup$ Ah, didn't think about that. However, the reasons for slow growth are not so "obvious" to me... With the right amount of supply and high enough temperature, why not? Are redox potentials too unfavorable? Some other chemical limitation? $\endgroup$ Jul 24, 2022 at 10:40
  • $\begingroup$ It's really tricky; higher temperatures are quite a challenge for Earthly life. But just to get a little scale perspective, consider that anaerobic organisms feeding on the same food as us get around 10-20x less energy from the food - and that's something that can completely outcompete these lithovores and most other extremophiles. That's how awesome (and dangerous) oxygen is as the last electron acceptor. But the other part of the equation is how plentiful oxygen is. Even when you get the right stuff in the rock, the concentration tends to be really low. $\endgroup$
    – Luaan
    Jul 24, 2022 at 19:35

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