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Apparently the Biology stack doesn't like theoreticals. :P

I'm looking into the sci-fi trope of floating plants via some sort of equivalent of a kelp's pneumatocysts, and the biggest issue to overcome seems to be that of lift gas storage. Methane is a big enough molecule that many organic substances can hold it securely, but produces little lift. Hydrogen and helium provide much more lift, but will permeate through just about anything, including most metals, polymers and even many dielectrics. The closest thing I can find would be a naturally-produced rubber via plant sap and ammonia, but I fear that its permeability would by far outstrip the hydrogen-production ability of hydrogen-producing algae. Is there a lightweight, naturally-secreted substance that can hold helium or hydrogen more effectively than organic rubber?

Note: I am past the needs of this question asking about the feasibility of lift gas: science is currently trying to figure out how to ramp it up to industrial scale. Likewise, I'm not asking for the evolutionary pathway to this end product like here and here. Just organic substances that fit my use-case.

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    $\begingroup$ Similar questions that would be good to reference (but don't answer your question) are here and here. A really interesting question is here. $\endgroup$
    – JBH
    Commented Jun 3, 2020 at 15:45
  • $\begingroup$ I'm sure that evolution could produce such a thing if there was sufficient evolutionary pressure in that direction and there was no other way of achieving that end. Nature is remarkably adaptable, capable of feats of engineering that far surpass what humans can achieve in many areas. Perhaps a fine spider silk material sealed by some form of secretion that formed a gas tight layer. Urethane perhaps, although not formed naturally there is no reason to suppose it couldn't be generated if required. Nature would probably come up with something even better, chemical possibility is almost endless. $\endgroup$
    – Slarty
    Commented Jun 3, 2020 at 20:14
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    $\begingroup$ Hydrogen is easily available and useful, but it's also extremely flammable. Imagine a forest fire in this world. $\endgroup$
    – NomadMaker
    Commented Jun 4, 2020 at 4:24
  • $\begingroup$ Next time you fillet a fish try injecting helium into its swim bladder and see what happens $\endgroup$
    – Frank
    Commented Jun 4, 2020 at 7:16
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    $\begingroup$ @Frank Yeah I'm never going to be in a position to inject helium into a fish's swim bladder. Can you describe what happens for the less vertebrate-inclined? :D $\endgroup$
    – Qix - MONICA WAS MISTREATED
    Commented Jun 4, 2020 at 13:18

3 Answers 3

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In general, semi-permeability is a necessary aspect of living things. Part of the definition of life is that it needs to be able to grow and metabolize things, and you can't do that without a way of getting things in and out of yourself. Even when biology does create an impermeable membrane, it tends to then take those membranes and fill them full of proteins to make holes through which the things they need can pass.

Because lifting gasses like H2 and He are small, symmetrical, and non-polar they can pass through pretty much any protein bypassing the lock and key mechanism that proteins use to filter larger organic compounds.

That said, sometimes life secretes things that are not alive such a mucus, hair, venom, and silk, but these things also tend to be made largely out of proteins (turns out you use what you know, and life KNOWS proteins).

Your best bet for an organic zeppelin will probably involve resin. Unlike most other biological secretions, natural resin is designed to create a continuously impermeable polymer which is designed to protect plants from wounds by keeping things out; so, your float bladder could be coated with something similar to resin to prevent the escape of gases.

There is also the question of how much hydrogen can this organism actually get from water. 1 gram of hydrogen has a volume of 11.12 liters at STP. Since water is about 11% hydrogen by mass, this means that, by splitting a single millilitre of water you get about 1.2 liters of hydrogen which creates about 2.4 grams of lift. This means that your floating flora or fauna will be able to easily adjust its altitudes once neutrally buoyant, but filling up from empty will probably take landing in a body of water. It will also be able to do some re-inflation using it's own stored body water, but will still need to regularly land for more water.

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  • $\begingroup$ Right, that's why I gave the rubber example. It can hold hydrogen, but I'm unsure if the amount of hydrogen the appropriate amount of algae can produce would be enough to replace the lost hydrogen. $\endgroup$
    – Carduus
    Commented Jun 3, 2020 at 16:11
  • $\begingroup$ Because lifting gasses like ... He2 I feel there must be a typo right there. $\endgroup$
    – Adrian Colomitchi
    Commented Jun 3, 2020 at 16:28
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    $\begingroup$ Oops, yes that was a typo. As my daughter likes to say, "noble gases don't like hugs." $\endgroup$
    – Nosajimiki
    Commented Jun 3, 2020 at 17:19
  • $\begingroup$ Estimating the upper bounds of evolution is always a shot in the dark. Since we do not have any examples of allege who's selective fitness relies on this, we have no good examples of how fast such an organism can inflate. Best you can do is calculate how much water you need to split and if the organism has a way of metabolizing enough energy to do so. The first of these questions I've added to my answer, the latter is much to subjective to answer unless you define all sorts of stuff about what this organism's size, diet, etc. $\endgroup$
    – Nosajimiki
    Commented Jun 3, 2020 at 17:56
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Actual gas cells in actual airships were made from the outer membrane of the intestines of cattle.

Lifting gas is held in balloons or gas cells at a pressure only very slightly above the presure of the surrounding air. There is very very little pressure differential pushing it to escape. Any kind of barrier will work, especially if made airtight, for example by waxing.

In real historical airships, the gas cells were made of goldbeater's skin, which is produced from the outer membrane of the intestines of cattle:

To manufacture goldbeater's skin, the gut of oxen (or other cattle) is soaked in a dilute solution of potassium hydroxide, washed, stretched, beaten flat and thin, and treated chemically to prevent putrefaction. (Wikipedia)

Fun factoids:

  • Around 1912 the Germans realized the advantages of goldbeater's skin for the construction of gas cells, and started using it enthusiastically for their Zeppelins. (It was a British monopoly before that.) A worldwide crisis of goldbeater's skin soon followed, as the amounts required for use in airships vastly outstripped the available supply: a typical small-ish WW1-era Zeppelin required about 200,000 sheets of goldbeater's skin, coming from the intestines of about 80,000 oxen.

  • A large Zeppelin, such as USS Shenandoah or the LZ 130 Graf Zeppelin, used about 750,000 sheets of goldbeater's skin, requiring the sacrifice of 150,000 heads of cattle.

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Let it leak out. But recapture it.

Your lift bladder contains hydrogen. Squirmy little hydrogens do leak out of the membrane. But on the outside of the bladder membrane are vascular channels which contain hydrogen-binding proteins. Escaping hydrogen is recaptured and cycles back to the lift vessel interior.

A reservoir of circulation protein bound hydrogen is also handy for quickly adjusting the size of your lift bladder without wasting hydrogen by adjusting the rate of hydrogen capture / deposition. The size of your lift bladder is dynamically regulated.

Some hydrogen gets away. That is OK. You can regenerate hydrogen from water to match your losses.

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    $\begingroup$ Might be naive here, but wouldn't a gust of wind be all that's necessary to displace enough of the hydrogen and bring the plant downward and potentially in harm's way? $\endgroup$
    – Qix - MONICA WAS MISTREATED
    Commented Jun 4, 2020 at 13:20
  • $\begingroup$ @Qix - the hydrogen is internal. It is inside the creature in a lift bladder and that is surrounded by more creature (and vascular channels). $\endgroup$
    – Willk
    Commented Jun 4, 2020 at 22:21
  • $\begingroup$ Oh I see, it uses a balloon effect. For some reason in my head you were describing a propulsion mechanism. My bad! $\endgroup$
    – Qix - MONICA WAS MISTREATED
    Commented Jun 6, 2020 at 13:47

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