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Somewhat related to How many plants would be needed to produce oxygen enough for 20 humans?

Suppose a number of humans are closed in a large hermetically sealed room on Earth with a glass (or other see-through) roof material that allows sunlight through:

Would it be possible to create a symbiotic relationship between humans and plants / trees whereby the humans would exhale enough CO2 for the plants to survive whilst the plants would produce enough oxygen for the humans to survive?

Would (for example) one person and one beech / oak tree work (or some combination of trees that don't shed leafs seasonally and continue absorbing enough CO2 whilst emitting enough oxygen all year around)? If not, how many humans and how many trees / plants would work?

I am thinking if a disaster strikes Earth and the atmosphere becomes unbreathable whether such a system could theoretically work?

Also, this set-up would be highly relevant for Mars whilst also being very highly relevant for any type of self-sustainable orbiter or spaceship (and therefore relevant for space exploration).

Thank you for any responses or tips,

Jan Stuller
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    It can be done, although our attempts so far have stumbled. See Biosphere – CuteKItty_pleaseStopBArking Jul 03 '21 at 12:32
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    There should be not only enough oxygen for the humans and CO2 for the plants. there should not be too much CO2 and oxygen for the humans. More than 50 % oxygen at 1 bar is too much when breathed during days and weeks. – Uwe Jul 04 '21 at 09:27
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    There is no way that deciduous trees like oak and beech could support human life in a sealed environment, in any number, unless the humans find a way to stop breathing for a few months while the trees are dormant with no leaves in winter. (And if you try to make deciduous trees grow continuously all year round, that will also fail.) – alephzero Jul 04 '21 at 13:01
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    @PcMan Presumably it can be done, we just haven't figured out how to do it yet. ;) So claiming it can be done is somewhat premature, a bit like claiming that fusion powered rockets can be done... – PM 2Ring Jul 04 '21 at 13:11
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    A closed mini-ecosystem has to do a lot more than recycle water and convert CO2 to O2. Eg, it has to deal with other gases produced by living organisms, especially methane. It's a complex juggling act to keep all the vital elements circulating nicely, and not turning into toxic sludge. Especially if you don't want to expend a huge amount of energy to keep the system running. – PM 2Ring Jul 04 '21 at 13:45
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    @PM2Ring IMO, sustainable production of air, water, and food in a closed system is the most significant challenge in long-term space exploration, and it remains as perpetually out of reach as fusion-powered rockets. – David Hammen Jul 04 '21 at 13:55
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    @DavidHammen Agreed. And you don't actually need fusion power to live or travel in space or on another planet. But if we can't learn how to do closed ecosystems we won't have long-term exploration, or independent orbital habitats. (Bases on planets or moons may be able to "cheat" by bringing in fresh materials & tossing out the sludge). I know you know this, I'm just mentioning it for the benefit of other readers. – PM 2Ring Jul 04 '21 at 14:08
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    @PM2Ring Stealing words from Inigo Montoya (as portrayed by Mandy Patinkin), "Hello. My name is outer space. You killed the Earth. Prepare to die." – David Hammen Jul 04 '21 at 14:23
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    I have voted to close. This question isn't about space exploration !? Much more a biology question I think. You don't need space science to answer the question. – Cornelis Jul 04 '21 at 17:03
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    @PM2Ring Of course it can be done. We have one working example, with a proven 4.5-billion year stability. It's called "Earth". It's just that our attempts to scale it down a bit have met with mixed success so far – CuteKItty_pleaseStopBArking Jul 04 '21 at 22:13
  • Trees grow, sequestering carbon. In nature other trees burn or die and decay, releasing carbon. Without some equivalent of the latter processes, your tree would reduce the CO2 to low levels. I don't know what happens to plants in that case. – Keith McClary Jul 05 '21 at 02:05
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    @Cornelis: have edited the title and the body a bit to address your concern: I do think the question is highly relevant for space exploration, because (for example), plants are meant to play a significant role in oxygen production and CO2 extraction for any future Mars settlements (see this design here, for example). – Jan Stuller Jul 05 '21 at 11:14
  • That's right, I just thought your question would be better suited for Biology SE. One doesn't need any knowledge of space exploration or planetary science to answer it. – Cornelis Jul 05 '21 at 13:29
  • @alephzero: thank you so much for your response: and out of curiosity, how many humans could an oak or a beech tree support during the "leaf season", i.e. when it has leafs? – Jan Stuller Jul 07 '21 at 13:54

2 Answers2

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This is being done with biosphere 2 and several similar projects and found to be very complicated. Specifically in a small sealed system there is little buffering or inertia available if one element is over or under producing or just having a seasonal change so die offs are hard to avoid.

For this reason the plants on the ISS and proposed future projects generally have some form of life support of their own to keep them optimally healthy, with that mechanical/chemical life support system buffering the changes in throughput.

Trees are sub optimal choice as life support in terms of CO2 processed per unit mass (unsourced 'seven or eight trees worth per person') with systems such as MELiSSA using algae and 'grasses' (rice/wheat) along with a heated reaction chamber and a bacterial nitrogen processing bed to reduce the footprint and improve control. Algae and Bacteria respond much better to being slowed down or speed up as humans and other elements change production than trees do.

GremlinWranger
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    Biosphere 2 didn't quite work as planned. Another way to say it: It was a failure. – David Hammen Jul 03 '21 at 16:17
  • Thank GremlinWranger, great answer! – Jan Stuller Jul 03 '21 at 19:56
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    @DavidHammen. The first mission gathered enough data to make the second one possibly succeed, except we can't have nice things – Mad Physicist Jul 04 '21 at 05:35
  • @DavidHammen I'm not sure if you were just having a bad day. I'm sure you're aware that scientific development and exploration quite often don't work how they were planned, but it doesn't mean they were a failure - they can still provide a ton of useful data and help push us forwards. I'm afraid your comment just comes across as a bit grumpy and ignorant (which, having read your previous answers and questions I can see you aren't). – Lio Elbammalf Jul 04 '21 at 09:48
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    @LioElbammalf I'm with David Hammen, possibly because I am old enough to have watched the train wreck that was the financial back end of Bio 2 as it happened and deleted parts of my answer on it 'being successful at separating people from their money' as irrelevant to the question. Certainly the science got lost a bit in pursuit of 100% efficiency and PR (we already know a lot about human malnutrition). – GremlinWranger Jul 04 '21 at 10:16
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    @LioElbammalf The first of the two Biosphere 2 expeditions ran into several severe problems. Oxygen levels eventually fell from 20% to 14%. That was equivalent to living atop a 13000 foot mountain. The people inside lost a lot of weight, partly because of the reduced oxygen levels, and partly because the limited food they grew placed them on a near starvation-level diet. Food and oxygen were eventually smuggled in. Yet another issue was small isolated group dynamics. People who serve in submarines or in the ISS are carefully screened for psychological issues. Biosphere 2 did not do that. – David Hammen Jul 04 '21 at 13:04
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    The second expedition was even more shambolic, thanks in part to actions by the then acting CEO Steve Bannon. (Yes, that Steve Bannon.) Failure is a one word summary of Biosphere 2. A two word summary is "hot mess." – David Hammen Jul 04 '21 at 13:07
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    Yes this makes sense; algal blooms can happen quickly in response to changes in conditions. New forest growth is much slower. – uhoh Jul 05 '21 at 00:27
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Terrestrial land plants are not sufficient producers of spare oxygen. Marine Bacteria, algae produce majority of Earth's freely available oxygen. Terrestrial plants produce barely fraction. So membrane bioreactors with algae cultivation produce sufficiently available oxygen.

early experimentation with BIOS-1 they demonstrated you can produce all the oxygen an adult male. Would require you woukd need for one person from approximately 20 kilograms of water and algae. However it would require sufficient sunlight thus would have to be spread over 8 square meters of surface area, about 90 square feet.

LazyReader
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