Hurried Thoughts: You Can't Feed a Society with Mushrooms

If you spend much time perusing forums where people trade worldbuilding ideas, you've probably come across someone asking about a fully underground society—or one without access to sunlight for some other reason—that therefore can't grow conventional crops, and wondering what they might subsist on. As you might have also seen, the most popular response is to suppose that such a society could instead cultivate mushrooms or some other fungus for sustenance, as they don't appear to rely on sunlight for their growth. It's become something of a common trope in an era where we've become a bit inclined towards skepticism of depictions of such societies without discussion of their food sources, and also become a bit of a pet peeve of mine.

Let's not bury the lede here: this doesn't work, for fairly straightforward thermodynamic reasons. Fungi don't get energy from nowhere, and any closed system with constant activity will be losing energy to heat and so can't sustain itself forever (or even particularly long where there's a major human and biological component). But I think these arguments from fundamental physical laws can sometimes feel a bit abstract and unintuitive, so let's think a little more about why people think mushrooms could serve as a staple food source and what it misunderstands about their different ecological role from plants and what food really is for.

We derive two things from our food: nutrients and energy. We don't tend to think of the distinction much (aside from a vague recognition of the difference between calorie-rich and nutritional food) because the energy comes to us contained within the chemical bonds of nutrients: sugars, fats, and sometimes proteins. I think it's thus easy to think of food as fundamentally a material which must be consumed to replace what we expel as waste, with nothing really lost in the process. But that's only partially true: when we break down sugars or other nutrients to release energy, the material remains, and in principle—if we had the energy and proper biochemical equipment—it could be retained and synthesized into whatever else we need to construct our bodies, no further food required.

Indeed, that's largely what plants do. They derive any materials they need mostly from the air and water, with a bit of nutrients from the soil, use energy from sunlight to convert it into organic molecules they need to construct their bodies, and then mostly retain it, with little waste on balance aside from oxygen and perhaps water depending on how you look at it (they let it evaporate out of their leaves to help pull more water out of the soil). Some microbes are even more thrifty, with alternative forms of photosynthesis that don't produce oxygen or any other type of waste (though they produce far less energy).

We, as heterotrophic animals, cannot directly harvest energy from inorganic source and have to derive it all from our food. That energy is used and then lost as heat, unrecoverable by us or other life, and we need that energy both to run our bodies and to actually do that synthesis. Any given portion of food generally contains less energy than we would need to reconstruct the material in that food into our own body mass, so in order to meet our energy needs we are constantly consuming more material and nutrients than we require and we just dump the extra as waste (really we could probably get away with deriving a lot less nutrients from our food than we do, but if we're eating so much food for energy anyway we might as well pick up any nutrients we encounter on the way to save us the energy cost of synthesizing it ourselves).

Mushrooms and most other fungi are, like animals, heterotrophs, feeding off of dead organisms and deriving any energy and nutrients they need from them (some fungi have close symbiotic relationships with plants or algae and so are a bit trophically ambiguous but at any rate are dependent on photosynthesis and sunlight and so irrelevant here, and some oddball microbial fungi might derive energy from radiation). Because they might be more edible to humans than the material they feed on, they can provide us a way to recover nutrients that would otherwise be unavailable to us. But they do not produce energy, and so cannot contain any more than was in their food.

Indeed, they will contain substantially less; extraction of energy from food and then storage of that energy isn't a terribly efficient process, and the fungi will be using much of what they gain for their own purposes. The typical rule of thumb is that with each step up the food chain, about 90% of an ecosystem's energy is lost (the herbivores in an ecosystem together only contain about 10% of the energy in the ecosystem's plants, carnivores feeding on those herbivores only 1%, secondary carnivores only 0.1%, etc.; fungi and similar detrivores are often considered to sort of lie to the side of the food chain or trophic pyramid because of their unique role of recycling nutrients, but thermodynamically can be thought of as a predator eating food like any other). The actual statistic varies a lot in nature and a well-managed system could probably do far better, but even so each step of feeding organic material to fungi or eating the fungi will come with a steep loss in energy.

A chart of overall flow of carbon (black), energy (red), and other nutrients (green) through a simple human-centered open ecosystem. I'm excluding cycling of oxygen, nitrogen, and water for simplicity.

We can perhaps start to see how the whole misconception comes about then: if you think of food mostly in terms of material, it might make sense that, if a society collects together all its waste and any remains when they die and then feeds it all to fungi, then perhaps all material could be completely retained and recovered as edible material, allowing the inhabitants to simply eat the same food over and over again. Though really, even in this flawed perspective, it wouldn't work out: most of the carbon we consume from our food isn't ultimately expelled in solid or liquid waste, but exhaled as CO₂. Fungi can't convert CO₂ back to organic material any more than we can (and in fact produce some as well as they digest food); in a sealed subterranean system with just humans and fungi, that CO₂ would just build up to hazardous levels. A more open system with air circulation to a surface world with photosynthetic life may not have that issue, but would still be constantly losing carbon without an outside source (the oxygen in CO₂ is lost to biological circulation as well but that can be at least partially replaced with oxygen from atmospheric O₂ and water, which you'd probably expect to have greater supplies of than organic carbon).

But the bigger issue is that food isn't just material. The energy we extract from food is gone from the waste; some still remains in compounds our digestive systems can't break down, which is why it's worth it for fungi to feed on it at all. Exactly how much is a bit tricky to pin down and likely varies greatly with diet, but typically the total recoverable energy is less than 1/10 our caloric intake (~0.15 kg/day waste, 1/4 of which is dry fecal matter, with about 20 MJ/kg energy, comes to ~0.75 MJ/day, compared to a typical diet of ~8-12 MJ/day). And again, only a portion of this energy will ultimately be recovered in edible mushrooms.

A closed ecosystem with fungal recycling.

We also contain some unused energy in our bodies when we die that could be recovered, but again not much compared to our total needs. A similar proposal you'll sometimes hear is that a society might subsist only on cannibalism or that an ecosystem might lack plants or other autotrophs (organisms that produce energy and nutrients from inorganic sources) and instead have a circular food chain of predators feeding on each other, and this might actually be preferable here, because by cutting out the fungal middleman, we're more efficiently recovering energy from remains (the edible parts anyway; fungi would still help to recover any energy or nutrients in inedible material, so perhaps a hybrid system would be ideal for the most enduring—if still ultimately doomed and presumably a bit miserable—version of a closed society in a bottle). But the caloric content of a typical human body—somewhat more than 100,000 Calories—could only sustain one other individual for at most a few months (slimmer rations would last them longer but leave the starved consumer's body with less leftover calories to pass on to the next person). An alternative species with a slower metabolism like reptiles might last longer if they remained largely dormant between meals, but ultimately we can be sure—based on fundamental thermodynamics again—that no adult animal contains enough energy to allow a newborn offspring to grow to equivalent size.

A cannibalistic closed ecosystem.

So not only are mushroom cultivation or cannibalism insufficient to sustain a closed society, they don't really recover enough energy to allow one to even get going; a cannibalistic society would have to halve its population every few months at most, and fungi recover so little extra calories from waste or inedible leftovers that they barely extend that timeline.

I sometimes hear it proposed that perhaps some plant matter or other organic material could be gathered from the surface to help feed the mushroom cultivation operation, but because the fungi aren't actually adding any energy to the equation, the energy content of that gathered material must be at least equivalent to that of the crops that would be required to food the society's population directly, and really—because of the energy lost in that trophic transition from plant feedmatter to edible mushroom—it would have to be substantially larger. Perhaps a society could grow regular crops and then use any inedible portions or leftovers to feed fungi in an attempt to maximize the production of limited farmland, but the actual gains would be marginal; we've pretty well optimized many of our crops to devote as much of their energy production to their edible portions as possible, and it would probably be more efficient to try and recover the nutrients present in any inedible leftovers in a form that could fertilize the soil, as crop growth is usually more restricted by soil nutrient availability than by energy production (given Earth's generous allotment of sunlight, anyway).

We could instead conceive of a scenario where there are no available crops, and instead all plants on the surface are inedible but can be fed to fungi to produce edible mushrooms, but again that works us back around to needing to gather far more plant matter to compensate for the energy lost in fungal growth than we would need if we could eat the plants directly, so we've shifted pretty far from the concept of a restricted subterranean society. Really, this scenario isn't too different from pasturing: feeding plant matter to animals that can process it into edible meat. This tends to require orders of magnitude more land area than directly growing an equivalent amount of food in crops, but one of the historical advantages was that it could be practiced in areas with harsh climate or poor soil otherwise unsuitable for intensive agriculture, and often at low labor cost compared to cultivating the same area because the animals do all the work of gathering the plant material (this is less true of modern factory farming, but the motivations and priorities have shifted over time). To cultivate mushrooms in the same way instead requires the cultivators to gather the material themselves. To give you a sense of the scale of the operation, meeting one person's caloric needs would require harvesting around 10 kilograms of mushrooms per day (frying or otherwise drying the mushrooms can reduce how much mass you'd need to scarf down every day, but I'm not even going to worry about how well this diet would meet your other nutritional needs).

An open ecosystem requiring fungal processing (using waste for plant fertilizer instead may be preferable).

If we want to avoid such an enormous surface operation, we need an alternative energy source. There are some options there, but they aren't amazing. Geothermal activity at hot springs and deep sea hydrothermal vents famously provides a supply of energetic compounds that chemotrophic microbes can subsist on for energy, sometimes serving as food sources for larger animals, and we might expect to find similar sources of energy underground in volcanically active areas. Deep sea vents in particular lack any sunlight (some food does sink down from surface water above, but little in comparison to what the vent microbes provide) so give a good benchmark for what these systems can sustain: based on measurements and estimates, the total production of fixed carbon (i.e., the mass of carbon that is converted from inorganic compounds like CO₂ to organic material imbued with energy we could extract) of a large collection of vents might be around 10-100 tons per year;  dry food is typically around half carbon by mass and we each need about 1 ton of dry food per year, so depending on how optimistic we are about how much of that carbon we could harvest as edible food, we're looking at enough to sustain something between a large family and a small village.

We could also harvest geothermal energy directly: The largest geothermal energy plant complex on the planet can produce about 1,500 megawatts of electricity, though most don't manage more than a few hundred MW. Taking that high estimate and assuming we devote it all to growing lights, we could produce the equivalent of average sunlight levels for an area of around 500 hectares, perhaps 1,000 ha if we're careful about producing just the most useful wavelengths of light. Modern intensive farming can produce as much as 50 tons/year of potatoes (no need to bother with mushrooms here), though much of that is water weight and we'll need a somewhat more diverse range of crops to actually meet our nutritional needs; so call it enough food on balance to feed 10 people/ha optimistically, for a sustainable population of 10,000. But if we're more conservative about available energy, how efficiently it could be used, and how productive our agriculture is, we could easily fall back down to a population of a few hundred.

Given no access to sunlight or other sources of energy at the surface, sustaining more than a tiny population ultimately requires a fuel source. Fossil fuels generally produce around 30-50 megajoules of energy per kilogram when burned, about half of which can be captured as electricity, so to match that largest geothermal plant requires a steady supply of about 80 kg of fuel per second, or over 2.5 million tons/year, which is a quite the extraction operation for a small community (and in an enclosed space would easily suffocate everyone through CO₂ production). Uranium and thorium fuel in nuclear reactors produces around 80 million MJ/kg of energy, about 1/3 of which is captured, so would only be consumed at a rate of a bit over 1 kg/year for the same rate of power production, which is far more manageable but still implies a substantial fuel stockpile necessary if we want a society to carry on for centuries, and perhaps grow to more than a few thousand or have some spare power to occasionally turn a bedroom light on rather than devote everything to crop growth. Still, large uranium ore deposits can have over 10,000 tons of uranium (over 100,000 tons for the very largest). Sticking with the somewhat optimistic estimate of crop growth, a population of 1 million devoting only half of their power production to food could last on such a deposit for tens of thousands of years.

Fusion fuels with similarly efficient reactors could produce perhaps 3-8 times as much power per mass. Water on Earth is about 0.0035% deuterium by mass and deuterium fusion could release 570 million MJ per kg of fuel; presuming about 1/3 of that was captured as electricity and we have good filtering the same society as above could be sustained with an external water source of about 4,000 tons per year.

As a final consideration, we could abandon conventional agriculture altogether; Some space exploration planners have proposed that rather than passing energy through the inefficiencies of natural photosynthesis, it could instead be used to synthesize compounds to feed chemotrophic microbes; the overall efficiency of conversion of input electricity to edible calories could conceivably be over 30% (incidentally, that same source estimates an overall efficiency for crop growth of 0.4%, about 5 times my estimate, but basically assumes that all light directly hits a leaf and the energy captured is all devoted to edible food production). Given a daily energy consumption of about 10 MJ (2390 Calories) per person, that implies a supportable population of hundreds of thousands to millions for a large geothermal power source and annual fuel consumption of about 180 kg/person for fossil fuels, 0.14 g/person for uranium, and 0.02 g/person for deuterium (extracted out of 0.55 kg/person of water).

In any of these scenarios, feeding waste and bodies to fungi or other detrivores could still be sensible; not to produce food, but to recover vital nutrients, as those still matter. Much as mushrooms can't produce energy from nowhere, plants or chemotrophic microbes don't get material from nowhere no matter how generous their energy supply; the bulk of their mass is ultimately derived from air and water (including exhaled CO₂, recovering that carbon as fungi cannot do), but some vital nutrients like phosphorus for iron must be drawn from the soil or other substrate, and will be depleted eventually if the nutrients within food matter are continuously removed and not replaced. Feeding waste and bodies to detrivores helps break it down into simple nutrients that can be used to fertilize the soil. And nutrients truly are not lost at any point in the process: the same phosphorus or iron atom can cycle between soil, crop, food, human, waste, and fertilizer over and over, essentially forever. Thus, if all nutrients can be perfectly recovered—a big if, admittedly—a closed system really can last as long as you like with only an energy input (so long as the total biomass—i.e., the population—remains the same, which may be tricky).

As one final thought for today, recognizing food's dual role as energy and nutrient source opens up some interesting avenues of exploration for speculative biology: what if an organism couldn't get both from the same place? On Earth, carnivorous plants do actually consume animals purely for nutrients while still sourcing at least most of their energy from photosynthesis, but this is only competitive in very nutrient-poor environments (because extracting nutrients from even moderately enriched soils is far easier than predation). Some microbes can also subsist on varying mixes of organic and inorganic energy and nutrient sources, but so far as I can tell, no predators source only energy from their prey; life on Earth stores essentially all its energy in molecules rich in organic carbon, our most vital nutrient, so there's not really reason not to take those nutrients while we're getting energy (often you can't get both energy and carbon from the exact same molecule—because the same act of energy extraction also releases the carbon as CO₂—but because our energy needs are so much larger, if you're already eating enough food for energy you don't need much more to meet your carbon needs as well). Our need for various other minor nutrients complicates matters, and we can take at least a few like sodium from inorganic sources, but the bulk of our diet is food that simultaneously meets our needs for energy and carbon and other major biological elements. Much the same will probably be true of any ecosystem where all organisms share a common biochemistry.

But what if they didn't? Say a planet hosted two biospheres, each based on very different and largely incompatible biochemistry. What if a predator from one biosphere developed a method to break down some of the nutrients from the other and extract their energy? It might have an advantage hunting across biospheres, on prey never adapted to defend against anything like it, and so meet much of its energy needs that way, but would still need to supplement its diet with some prey from its own biosphere to meet its nutritional needs. Any number of complex ecosystems could probably be conceived of for such cases of partial biochemical incompatibility, but I'll leave that as an exercise for you for now.