Can a human colony be self-sustaining without sunlight using mushrooms?

Question

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

Answer 1

NO:

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.

Answer 2

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.

Answer 3

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.