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The planet is a cold ammonia world. Its ocean is mostly made ammonia, with some traces of ices (mostly water ice) at the bottom of the ocean. The ambient temperature is 225ºK, and the pressure is 4 atmospheres. The star is of G5V type and the planet has a semi-major axis of 1.2 AU.

The plants in question spend their lives completely in the air (so they never go to the ground) and have lost root mechanisms to get ammonia and other key nutrients from the ground. The atmosphere does have lots of common elements and nutrients in its atmosphere, but the atmosphere doesn't hold key trace elements for this life like metals or salts. So, without going to the ground, how do these floating plants get their trace nutrients such as metals and salts?

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  • $\begingroup$ FYI: Our planet has plants like this (minus the ammonia ecology) extension.psu.edu/…. $\endgroup$
    – Atog
    Commented May 24 at 23:07
  • $\begingroup$ This is really a brainstorming question; there's no physical problem with trace amounts of relatively heavy atoms floating in a fluid. Other folks have pointed out air plants, but I would also point out some jellyfish also live in surprisingly-resource-poor fluids. Maybe there's some sort of obstacle you want to add which frustrates otherwise-straightforward airborne biology. $\endgroup$
    – Corbin
    Commented May 26 at 4:57

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Something needs to get those trace elements into the air. Some examples:

  • Strong winds/hurricanes/tornadoes
  • Volcanoes/tectonic activity
  • Regular comets sprinkling dust
  • Meteor impacts
  • Industrial pollution from a civilization below
  • Fires in more terrestrial plants

I don't think I want to live there!

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    $\begingroup$ Beat me to it. Adding thermal geysers, such that seems to exist on Pluto or cryovolcanoes on Ganymede. while they may be mostly nitrogen etc. they could be a delivery system for dissolved nutrients. $\endgroup$
    – Gillgamesh
    Commented May 23 at 12:37
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They eat them.

Carnivorous plants are hardly unknown in the real world. They derive their energy primarily from sunlight but, as in your question, the trace elements they can't get from their soil come from insects or other prey instead. Your floating plants could either wait passively for insects to come to them, as earthly carnivorous plants do, or perhaps stumble across them in their drifting from place to place. You could imagine a sort of bottom-feeding plant with root-like tendrils that ensnare morsels as the plant drifts over a surface, drawing them into a mouth for digestion.

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    $\begingroup$ Some plant species could even forgo insects and hunt relatively nutrient poor but significantly easier to catch other plant species. It's a plant eat plant world out there ... somewhere. $\endgroup$ Commented May 24 at 16:10
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Dust on the winds

Your world has some easily accessible patches of the trace elements on its surface, likely in the middle of a large desert. These rocks formations that contain the trace elements are slowly being worn down by streams of sand, blown by the winds. After that, the trace elements are lifted up by the dust storms, and slowly deposition down somewhere else on the world.

Kinda like how we can find desert sands you find on a car sometimes, while living half a world away.

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    $\begingroup$ It's thought that dust from the Sahara fertilizes the Amazonian jungles. (NASA article) $\endgroup$
    – R.M.
    Commented May 23 at 13:57
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birds poop on them

Or some other, more alien, kind of flying creature. They eat nutrient-containing bugs, seeds etc. (or alien equivalent) and then fly up and build their nest on a nice convenient floating plant, leaving all the associated detritus to slowly break down and be absorbed. It's a nice symbiotic relationship - the plants provide a nesting site far away from non-flying predators, while the birds provide nutrients for the plants.

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    $\begingroup$ Came here to mention symbiosis ! +1 ! $\endgroup$
    – nick
    Commented May 24 at 8:13
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Airborne pineapples

I present to you the Tillandsia genus, in the Bromeliaceae family, the same as pineapples.

They are also commonly known as air plants because they obtain nutrients and water from the air, not needing soil for nourishment.

In South America, it is very common to see plants growing on electric wires, rocks and even glass, all year round. I suggest you search images related to "bromélias fio elétrico" for visuals.

They get any trace nutrients from air impurities generated from rains, winds and other atmospheric phenomena. They also can develop some mutualism with birds and insects.

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  • $\begingroup$ That is so cool. $\endgroup$ Commented May 23 at 20:30
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They don't need them


Unless you're proposing that these plants have been transported to your world via a portal from some other planet, chances are good they have lived all their lives in this environment. As did their progenitors and their most distant ancestors. In other words, your plants have in some way evolved to live in, survive, and even thrive in the atmosphere of this cold ammonia world.


I propose that the atmospheric plants in your world represent either a distinct subkingdom of plants that arose from land bound ancestors who found a way to thrive without metals in contradistinction to other plants that do need those elements; or else that no plants in your ecosystem need those elements.

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    $\begingroup$ Yeah, that is a possibility, but it's kind of hard to just use the CHON elements plus some sulfur from hydrogen sulfide in the atmosphere, and the small amount of dilute salt and minerals in the precipitation (and just for reference, my other plants do use these elements). However, I agree that some floating plants would make that change, so if they do still use the elements in question, it will be in extremely trace amounts and only when absolutely necessary. $\endgroup$
    – Neil Iyer
    Commented May 23 at 3:46
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Not "in the air" per se but air has "impurities"

In our world the air has solid particles in suspension. Moreover rain is not pure water, but brings with it some impurity.

The idea is that the wind can sweep dust off the ground and the dust can be used by the plants by filtering the air or possibly by absorbing them from precipitations.

I don't know if this violates the clause in the question that stated that such substances are not "in the air", but I'd find it acceptable.

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    $\begingroup$ If the seas are mostly Ammonia, there IS going to be some in the air. Even if it hasn't reached the boiling point, evaporation is going to take place. And that means, that it will be brought down in the acid rains. Even further along that line, because of the lack of water evaporation, ammonia is the thing that will rain from the skies. $\endgroup$
    – vinzzz001
    Commented May 23 at 12:50
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    $\begingroup$ Here on Earth there are plants which DO get all trace nutrients from the air. Epiphytes aka airplants, which grow on trees but which are not parasitic, with roots that merely attach them to the tree's (cellulosic) bark.It's also notable that the entire Amazon basin owes its fertility to very fine dust raised by sandstorms in the Sahara and blown across the Atlantic on prevailing winds. $\endgroup$
    – nigel222
    Commented May 23 at 13:26
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I'll try to add something that hasn't already been suggested.

Symbiotic relationship with other life forms

Insects could collect metallic dust while foraging on the ground, and transport them on the plants. Maybe the insects use the plants as a safe place to avoid predators? maybe they are bioluminiscent and attracts them?

Rain

The (ammonia?) rain in this world could react with some of the metals to create gasses from which the plant extract metal from.

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Real world plants that grow in scant/unique environments obtain nutrients via various means

  • Carnivorism: Be it whether using adhesive, moving, water filled traps to ensare insects/small mammals and consume their proteins/mineral rich bodies
  • Detritus consumption: many plants especially airborne varieties from branches/rocks have ability to catch rainwater in pool which in turn acts reservoir for small organisms whose nitrate/mineral rich excrements accumulate at bottom as rich sludge.
  • Roots: Most comprehensive root systems in waterborne plants are already adept with smaller diameter roots to accumualte necessary micronutrients. Same rivers/streams/ponds/lakes also serve as habitat for organisms who wastes feeds and wash downriver
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