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Ceraon is a moon with advanced megafauna and flora. The moon is pretty small, so gravity is only about one fifth of that on earth.

Much of the life-preserving atmosphere is retained through a great number of sedentary organisms that form translucent biodome-esque "atmospheric balloons", allowing other life on Ceraon to flourish inside.

This planet's ecosystem, and the balloon-organisms, has just started being studied by earthling scientists, so detailed knowledge of "why"s and "how"s are still limited.

My question is, what is the largest size these atmosphere-preserving balloon-organisms could achieve, realistically? For instance, would one's the size of a football stadium be too unrealistic?

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    $\begingroup$ If gravity isn't in the equation, size is limited by food. What do those guys eat? $\endgroup$ – Raditz_35 Jun 27 '17 at 12:21
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    $\begingroup$ Must each balloon be a single organism - or can it be a collective colony of smaller organisms (same concept as a coral)? $\endgroup$ – G0BLiN Jun 27 '17 at 14:47
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    $\begingroup$ Also - what exactly is the difference between the "life-preserving atmosphere" inside these balloons and the surrounding atmosphere (different pressure? different mixture of gasses? radiation? other factors)? $\endgroup$ – G0BLiN Jun 27 '17 at 14:50
  • $\begingroup$ @Raditz_35 their energy would be from photosynthesis and/or energy from other organisms, it's not put in stone. $\endgroup$ – Fred the John Jun 27 '17 at 20:33
  • $\begingroup$ @G0BliN they could be like corals or a single organism, whichever was more convenient for great size! The bubbles would preserve atmospheric pressure, capture gases produced by it and/or other organisms, probably block harmful radiation too. Possibly it's all to protect other organisms it has some symbiotic relationship with. $\endgroup$ – Fred the John Jun 27 '17 at 20:34
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Largest wooden Dome in earth gravity: The Superior Dome, which IS a football stadium.

There are also several Air-Supported structures, which might be highly relevant for Atmosphere-retaining Creatures. Under Earth pressure and gravity, several of these are sports stadiums too: List of Air-supported structures

In combination: I do think that a sedentary(!) organism the size you want is quite feasable, especially in low gravity.

If you don't want them to be plants, maybe they're filter-feeders - the membranes between their bones(?) keep in most of the air, but there is a slow outward flow that carries detritus and microorganisms against them, to be then eaten by symbiontic bacteria which turn that into a sludge that is absorbed and processed by the membranes (or drips onto the "support beams" or just thicker veins along the membranes and is absorbed there). Maybe symbiontic algae live there and use their own photosynthesis to produce food for the megacreatures?

Fascinating idea in either way :)

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Sessile organisms can get pretty big. The largest organisms in the world, by mass and area, are a 100+ acre aspen grove in Utah and a 2.4-mile-wide honey fungus in Oregon.

Of course, those are sitting in or on the ground. It gets more complicated when you have to suspend most of the organism as a roof up in the air. Since these things are holding pressure in, though, the mass of roof can in fact be held up by the air pressure inside. In fact, I'd expect these things to grow so that the areal density of the bubble material produces weight that exactly balances air pressure, thus minimizing tensile load on the structure. That way, in ideal conditions, the structure could grow arbitrarily large- even encompassing the entire world.

You will, however, need to deal with dynamic instabilities in the roof structure, and since this is a living organism, you'll also need to worry about transporting nutrients throughout the structure. Both of these constraints mean that the size of your balloon-dome creatures will be restricted by their internal structure; do they have tendrils or columns or something in the interior, which could be used to anchor the roof, damp vibrations, and transport water and food from the ground? Or are they anchored and supplied solely from the outer edges?

If they have internal support, there's no reason they couldn't get arbitrarily large. Otherwise, the precise limits are going to depend on details of the material strength of the organism's tissues, and the efficiency with which is can transport water and nutrients. Anchoring area at the edge grows linearly with radius while the area of roof tissue to be fed grows quadratically, so eventually that will catch up an impose a maximum size (fortunately, you're not limited like trees, since most of the transport will be horizontal, rather than vertical, and the lower gravity helps with the vertical component). You'd pretty much have to make up the precise numbers, but personally I'd find a football-stadium sized enclosure to be entirely believable.

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    $\begingroup$ Minor nit-pick - if the balloon itself is not living tissue (such as hair, scales, shell, or a secretion such as cobwebs), the nutrient/water transport limitation may be lifted. $\endgroup$ – G0BLiN Jun 27 '17 at 14:57
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    $\begingroup$ I like this concept very much and like this answer: /arbitrarily large/. I envision these things as huge solar balloons. Their main activity would be to trap gas molecules and move them to the inside. Their nutrition could all come from the activity of the captive garden ecosystem they have inside. They could reproduce on the day of maximal atmospheric pressure, budding miniballoons off the top each with its own garden starter within. They warm up until buoyant and float away. $\endgroup$ – Willk Jun 27 '17 at 15:01

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