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I want to create a world for my colonists that's bleak and unwelcoming as well as alien and strange.

So to that end I wanted to make mushroom-like fungi the major flora on the planet. I've read Where do mushroom forests thrive? for some inspiration. I don't need forest-sized growths, but to make it more alien I thought having a calcium carbonate structure would give a ghostly white appearance, similar to deep-sea coral that have made this trade-off compared to soft coral that uses chitin.

But what would give rise to this? What is gained for swapping chitin for calcium carbonate?

The only thing I can think of is that it would be necessary if there were extreme winds, but even then I'm not sure.

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    $\begingroup$ Chitin is a structural polymer (a polysaccharide, to be specific) whereas calcium carbonate is just a mineral. It will have very different structural properties, and the two can't be simply interchanged. What you're interested in here is more of a sort of saprotrophic land coral than a fungus; a wholly new thing that deserves its own name! A heavy rock-based structure won't lend itself to overhanging muchroom shapes either... more blobs and columns and spires. Just a thought. $\endgroup$ – Starfish Prime Feb 8 at 16:29
  • $\begingroup$ @StarfishPrime that's a really fascinating point! I'm going to have to work on that $\endgroup$ – Pureferret Feb 8 at 17:44
  • $\begingroup$ Do you want the pure calcium carbonate (or mineral mix), or organic chitin reinforced by calcium carbonate (a lot of calcium carbonate in molluscs' shell, a little in crustaceans' shell)? $\endgroup$ – Alexander Feb 8 at 18:51
  • $\begingroup$ @Alexander I'd honestly consider any of those, so long as there a significant amount $\endgroup$ – Pureferret Feb 8 at 18:55
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    $\begingroup$ Note that fungi are heterotrophic organism and the fruiting body (mushroom) has only reproductive function. This is the reason why mushrooms are generally small. If you want mushroom-based flora, you probably want autotrophic fungi that can fix air CO2 from some form of radiation energy, such as sunlight or other forms. In that case, you could make them similar to trees, with some sort of central permanent structure that includes roots down to soil protected inside the skeletal body and semi-pernament structures on top of it that function similarly to leafs. $\endgroup$ – Colombo Feb 9 at 3:17
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Moderately elevated calcium and CO2 levels.

Chitin is a relatively expensive skeleton to use. It is a decorated version of cellulose, which contains energy like starch; think of how wood burns. All that energy has to be diverted from the metabolism of the organism that makes the chitin.

By contrast, calcium and carbon dioxide in water (such as "hard water" from the tap) can spontaneously produce calcium carbonate, if the product of the concentration of calcium and carbonate are high enough.

CO2 + H2O ⇋ H2CO3 ⇋ H+ + HCO3- ⇋ 2H+ + CO32-

So under the right circumstances (high calcium, high CO2, alkaline conditions) the skeleton is free! (True, under those conditions calcium and/or carbon dioxide are being removed from the environment.) Note, however, that if your soil doesn't have abundant calcium, these fungi cannot make it, unless they evolve to do nuclear transmutation.

The Achilles' heel of this approach is that any acidic environment will reach these "cell walls" and turn carbonate to bicarbonate to CO2 and release it, destroying the skeleton. Calcium carbonate skeletons are best hidden inside a body or else used in gentle seas that might naively have imagined no one was going to acidify them in a planetary ecological catastrophe.

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    $\begingroup$ So..... stalactites on the underside of mushrooms making a white surface? Brilliant! $\endgroup$ – Trioxidane Feb 8 at 17:12
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    $\begingroup$ "in gentle seas that might naively have imagined no one was going to acidify them in a planetary ecological catastrophe." Ooof. $\endgroup$ – Pureferret Feb 9 at 10:34
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Size

There is a reason that after a certain size creatures use bones. Weight and strength. The weight for effective strength in chitin exceeds a threshold, making the creature too heavy and possibly collapse on itself. Bones on the other hand give structure and points for your body to be grafted to. This reduces weight by huge amounts.

Your mushrooms can be of a huge size that makes a simple chitin ineffective. To still be viable, they slowly grow a skeleton like structure. This can happen automatically, where hormones of cells experiencing certain pressures triggers the growth, possibly on the underside of the mushrooms. It might not be fully clad on the underside by bone, but bonewhite tendrils throughout with varying thickness can be chilling already, possibly contrasting some colour. Alternatively this bone structure can seep partially into the surrounding cells, making them bone white (and posdibly hardened) as well.

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    $\begingroup$ A webbing of bone around the body of a mushroom the size of a horse would be quite a sight. $\endgroup$ – Whitehot Feb 8 at 15:58
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    $\begingroup$ Now imagine the mushroom dying off and decaying, leaving only the bone in place $\endgroup$ – Anderas Feb 9 at 6:41
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If the environmental conditions make it more convenient to use calcium carbonate than chitin, it makes sense that natural selection will favor it.

For example if calcium carbonate is readily available in the environment on this world, while chitin needs to synthetized, it can make sense that calcium carbonate is preferred as "cheaper" material.

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    $\begingroup$ Limestone is mainly composed of calcium carbonate, and form quite unsettling structures given the proper weather conditions and erosion, which would be great for a bleak and unsettling alien world theexplorer.ro/wp-content/uploads/2018/02/… $\endgroup$ – Whitehot Feb 8 at 15:56
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Edibility, or more precisely the lack thereof

Humans have an enzyme, chitinase, which digests chitin. Calcium carbonate is a lot harder (no pun intended, well not much) to use as a food source.

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    $\begingroup$ I would imagine that on an alien planet, native live forms may have evolved the ability to digest or at least break down calcium carbonate. $\endgroup$ – Pureferret Feb 9 at 10:23
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Chitin isn't as strong as bone, so the mushrooms would need bones for support for larger sizes under stronger gravity. And yes, it will help if you've got a very windy planet. Another reason why would be if we had a planet that's got calcium-rich soils, an alkaline environment, and higher carbon dioxide levels than Earth. These fungi could produce calcium carbonate via their internal water supply, and use it to build skeletal structures using a mechanism similar to the mending of broken bones in humans. And because a mushroom that bleeds would be creepy AF. The increased carbon dioxide would also make the planet, assuming an Earthlike albedo, orbital radius, and solar intensity, a bit warmer than Earth, and the white could help to reflect excess radiation.

Of course, the fungi would need a way to absorb calcium, and the metabolism of fungi produces carbon dioxide, so they would hold onto it for use, and then release it. Probably the mycelium absorbs calcium.

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