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Basically, in the event of a blotting out of the sun á la an impact winter, what sort of food source (if any) could be used to sustain humanity?

In my universe, underground/makeshift greenhouses would not be infeasible, but more desirable would be the ability to farm some sort of edible food on the surface. Would fungi / what would be able to survive in these lowlight conditions?

Then, let's say we'd be able to give them a limited amount of exposure to artificial light. How much light would they need per day?

Also, In the Metro 2033 series they grow fungi in the subway tunnels.

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  • $\begingroup$ There's the implied question "can humans live on that fungi"? Will they be able to get all their essential nutrients? $\endgroup$ – Schwern May 9 '16 at 5:35
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Some light is required to get edible mushrooms to fruit. Depending on the mushroom a week or two in a month long grow cycle. Selective use of UV lamps will add vitamin D. Read all about this in exhaustive detail in the UN Training Manual on Mushroom Cultivation and Growing Mushrooms from Penn State.

The next problem is most edible mushrooms grow on organic matter, where is this coming from? The obvious choice is "night soil", but I don't know how well edible mushrooms will grow in that. Furthermore, this is a closed loop, humans eat the mushrooms, the mushrooms eat human waste, and it will eventually run out of energy.

If you get them growing, you can't live on mushrooms alone. You'll slowly die of malnutrition. Looking at the nutrition information for oyster, Crimini, and white mushrooms you'll see great holes in their nutrient balance. Particularly the essential vitamins. This leads to blindness (A), nerve problems (E), bleeding (K), anemia (Folic Acid), and neurologic dysfunction (B12).

Such a setup may be able to use cavefish for supplementary protein and nutrition, but cave ecosystems are extraordinarily fragile and likely would not yield much. You'd be better off using some power for lights to grow supplementary vegetables high in the missing nutrients.

Finally, mushrooms are a poor source of calories. They're mostly water. Depending on the variety, 100g of raw mushrooms will get you 20 to 40 calories. Compare to 100g of black beans will get you 130 calories.

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Fungi don't possess chloroplasts. They are saprophytic, living by taking in organic matter, such as dead wood or dead soil organisms. That's why you find them left-over food.
All of this information is quite neatly summed up in the common saying "They treat us like mushrooms - keep us in the dark and feed us BS". But if you want references, any elementar biology textbook will do.
But for more information on mushroom composition, here's a good link: http://pubs.acs.org/doi/abs/10.1021/jf020608m.
Mushrooms and other fungi are especially valuable for their micronutrient and nutraceutical benefits. To get enough calories, though, you need photosynthesizers. And for those, you need visible light, and plenty of it.

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Basically all fungi can survive low light condition, because they dont need light. You can see them grow under trees shadowed by them. People grow edible fungis at farms or in homes underground literary now. (because they usually also need high humidity)

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