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My world is essentially a Neptune-sized gas planet that got seeded with life and its surface layer filled with oxygen, therefore gaining a sizeable layer with Earthlike conditions.

Within this layer, life is diverse, but the largest lifeforms would be colonies of treelike plants that have connected branches and a translucent, photosynthetic canopy stretched between those branches. Think an organic version of the tensegrity sphere. The branches would be connected by non-rigid "ligaments". The life cycle would start with a single instance, which would clone itself, forming a mushroom cap shape with a flat, ring-shaped floor on the inside bottom with a circular hole in the middle. Gradually, all instances would grow in size, preserving the general shape but the center would die, decompose, and form a "soil" layer in the inner flat ring.

The young colonies would be buoyed up by updrafts as they fell, in clear skies the sun would warm the air inside the canopy enough to slow the fall, allowing it to heat up even more and maybe rise. The early stages are the most difficult. The majority of instances sink to the depths. As the branches, canopy, and connective structures are all on the outside, the volume of heated air also grows and it becomes easier to stay aloft as the colony grows larger. The colony is adaptive and can regulate the inside air temperature and pressure.

Could such a forest grow as large as a city? A continent? Could it actually support an entire ecosystem within itself of creatures that don't drift, fly, or float their entire lifecycle? Would it be strong enough to withstand turbulence and storms, or would it need to stay within a jet stream to survive? Would extremely large ones generally be stable?

I thought of this through pondering how a floating island would actually be possible. In the lore of my world, human colonists discovered this world and created aerostat colonies before a disaster forced a migration to these forests.

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    $\begingroup$ Welcome to Worldbuilding! Please take the tour and visit our help center to get familiar with the site. And have fun! $\endgroup$ – L.Dutch - Reinstate Monica Jul 20 '18 at 6:56
  • $\begingroup$ If you haven't seen it, check out en.wikipedia.org/wiki/The_Integral_Trees $\endgroup$ – Brizzy Jul 20 '18 at 7:09
  • $\begingroup$ For the last point I'd say: Evolution is usually driven by external pressure. So, land-dwelling creatures might evolve from flying ones if there is a clear advantage to it, or if it removes a disadvantage. Also it depends on the longevity of the colonies. It might be possible that flying creatures nest inside the colonies with an abundance of food so they grow larger in size, have no necessity to leave and with the the use of their flying skills reduced and the trouble getting into air increased then lose the wings in favor of arms or front legs. $\endgroup$ – Otto Abnormalverbraucher Jul 20 '18 at 12:47
  • $\begingroup$ @OttoAbnormalverbraucher I've considered the longevity aspect of it. If abnormal weather can occasionally tear apart large colonies, then I think there would be animals that inhabited the colonies for a portion but not their entire life cycle. $\endgroup$ – user199429 Jul 20 '18 at 20:59
  • $\begingroup$ I might be able to answer this when my phone is finished charging, but it probably won't be the prettiest as I'm terrible at typing on my phone. $\endgroup$ – SealBoi Jul 21 '18 at 0:15
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You need plants with hydrogen-filled bladders

I never thought I'd say that one day.

Anyway, my suggestion is that your plants evolve some kind of hollow sac, but I'm not 100% sure how they'd get that far without them (As in, past pond scum stage).

On the fictional planet Ilion, by a person I do not know the name of (He goes by Malicious Monkey on the Speculative Evolution forums), there is a clade of flora called Buoyphytes:

enter image description here

These use air-filled bladders to float on the surface of water bodies, which means they can both colonize the sea and photosynthesize efficiently. If these were filled with hydrogen, they could float in the air.

But how would this evolve on a gas giant? Well, here's my idea: the microbes that would become your plants already produced hydrogen as a metabolic waste product. When they eventually produced a colonial form that would go on to become a multicellular plant, their colonies were shaped like hollow spheres and they expulsed the hydrogen into the interior. When the balloon is full, they would open a hole in the colony to let some gas out. The evolutionary pressure for this adaptation could be that they increased their buoyancy to get higher than the other photosynthesizers and thus outcompete them.

For more on Ilion's Buoyphytes, have a look here: https://sunriseonilion.wordpress.com/ilion/species/red-plants/

"Setaceous Cetacean"'s moon called Solais ( http://s1.zetaboards.com/Conceptual_Evolution/topic/8121915/11/#new ) has a lot of balloon plants (though they aren't plants in the traditional sense).

To have a look at another "life on a gas giant" project, check out this one: http://s1.zetaboards.com/Conceptual_Evolution/topic/8143795/1/

Those are just some additional links to inspire you, I'd say they'd be quite helpful. I haven't looked as much as I'd like to at the last two, so hopefully they're relevant.

Sorry about the poor formatting, I'm typing on my phone. I hope I answered your question well, and thanks for asking.

Edit: Just found out that Malicious Monkey is a she. And her name is Emily Holland, to give her credit by her real name.

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  • $\begingroup$ I'd say that the answer to all of your questions about size, stability etc would be yes, at least if you go with this approach. $\endgroup$ – SealBoi Jul 21 '18 at 23:26
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    $\begingroup$ Methane or ammonia would also work as lifting gasses, and there are plenty of real-world examples of macroscopic organisms that have have gas bladders, and ones that produce methane. $\endgroup$ – Logan R. Kearsley Jul 21 '18 at 23:31
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    $\begingroup$ Hydrogen as a metabolic waste product from plants makes me think of what might happen if the atmosphere prior to life arising was rich in methane. As pointed out in this question, photosynthesis of methane + water -> carbohydrates + hydrogen is about 4 times as efficient as Earth's CO2-consuming, oxygen-producing pathway. $\endgroup$ – Someone Else 37 Jul 22 '18 at 2:01
  • $\begingroup$ @user199429 No problem, glad I could help. $\endgroup$ – SealBoi Jul 22 '18 at 4:54
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Planes fly by forcing air against their carefully aerodynamically-sculpted bodies in ways that generate lift. Bernoulli is overrated.. There are a few ways that an organism of significant size could generate large amounts of lift.

They can do it the way planes do it: the colonies are ambulatory, and they propel themselves along at speed so they fly forever. Like some sharks, if they ever stop moving, they sink and die. The precise mechanism of how they do this is up to you, but heat differentials and gas production from metabolism would combine with clever valves and air bladders nicely.

They can take advantage of the naturally highly chaotic and dense atmosphere's raging currents. Basically, the plant colony would be a giant sailing vessel. The entire structure would have control surfaces that are constantly in adjustment to maintain altitude. Colonies would occasionally break apart in a raging storm. This would be one mode of reproduction.

Buoyancy can also be manipulated. The atmosphere will not be uniform throughout, in the same way the ocean has multiple layers. In the case of a gas giant, denser gases will naturally separate out and be found at higher depths. This is for the same reason that if you take a bunch of ball bearings and foam peanuts, put them in a box, and shake it, all the ball bearings will end up at the bottom. So the plants can rise or sink by adjusting their density. Gas bladders may be involved. The plant may have an ability to adjust the density of its bulk as it grows, to adjust buoyancy as well.

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Many plants have seeds designed to disperse through the air. It would seem reasonable that in the right environment this evolutionary process might scale.

I would think a smaller lower gravity planet or moon might harbor this kind of life due to the reduced gravity. Perhaps with the winds being produced by tectonic stresses resulting from orbital interference. Like IO, whose gysers are believed to be caused by the gravitational interference with other moons for example. Frequent near misses with other moons might cause thermodymic changes that would cause steam gysers and frequent high winds. Perhaps a highly elyptical orbit causes one planet or moon to scoop up atmosphere from the others periodically.

Spider silk is now known to take advantage of static electricity to allow small spiders to fly. It might be possible for a plant to develop a similar process, where millions of hairs hundreds of meters long develop a charge and help pull them into the air as storms charge and discharge.

Probably most of them would have symbiotic relationships with other species in order to get their nutrients. I think they would look like a web of tumbleweeds or kudzu with chunks of spiderweb like moldy bits interspersed. The mold would be responsible for converting nitrogen and minerals like the bacterian on legume roots.

There is a type of fern that is essentially airborn, getting most of their moisture directly from the air. They sell them sometimes at junk shops in beach towns, I don't remember what their called.

I would think they would need to evolve a way to deal with water adhesion. If they benefits from floating, then they would want to shed weight and condensation, and in a misty environment would be a problem. Maybe a cohabiting bug or animal would like things dry, and so keep any condesation off the web of plants. Perhaps these friendly species might migrate in great clouds to and from the gysers as they approached breeding season.

Different species would probably compete in different ways, so the forrests would form waves shapes, where certain types pulling others down, or vice versa depending on whether their primary energy source was from below or above. Perhaps periodically forming conduits for lightning discharge that create great structural columns of dead wood, that the other plants and species feed off of.

Just a few thoughts.

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  • $\begingroup$ The question is specifically about living in a gas planet itself, which is why it is asking about large floating biomasses. $\endgroup$ – Dayton Williams Jul 22 '18 at 2:58

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