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I've been thinking about worlds like Keplar-62e, thought to be covered in deep oceans, and wondering about floating reefs. Could a bio-structure substantially similar to terrestrial coral hold enough gas pockets, preferably in the form of bio-generated Hydrogen, to float neutral-buoyant close to the surface or does the math just not hold up?

Edit: Pumice has been pointed out to me, how embarrassing, but I'm looking for something with less porosity and greater bulk strength to form a floating reef that will largely withstand extremes of weather etc...

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    $\begingroup$ Why hydrogen? Any gas is so much lighter than water that the difference betwen hydrogen, oxygen and carbon dioxide is irrelevant for purposes of buoyancy. And since there are buoyant rocks, I cannot see a reason why the skeleton of a coral colony couldn't be buoyant too. $\endgroup$ – AlexP Aug 21 '17 at 18:46
  • $\begingroup$ @AlexP Because hydrogen would be the least pocket for the most buoyancy, which would reduce structural issues arising, also it's one of the easier gases to justify in a biological system. And here's me with a background in Earth Science and I didn't even think of Pumice! I've been wracking my brain about this for at least a week, thank you. $\endgroup$ – Ash Aug 21 '17 at 18:53
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    $\begingroup$ The difference in the buoyancy of hydrogen and nitrogen or carbon dioxide in water is minuscule, much smaller than the difference in buoyancy caused by the variations in water salinity or temperature. And N2 or CO2 are even easier to justify in biological context; that's what fills the buoyancy chambers of the nautilus for example. And where I live pumice or pumice-like artificial stones are used as an abrasive in the bathroom -- it's an everyday material. $\endgroup$ – AlexP Aug 21 '17 at 18:58
  • $\begingroup$ @AlexP Hmm, live and learn, Nitrogen huh, how is that even a thing? I'll look at it later, time for bed. $\endgroup$ – Ash Aug 21 '17 at 19:01
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    $\begingroup$ Reality-check does say it contrasts with science-based and hard-science, but consider them as three points along the scale from internal consistency to fully sourced published scientific papers. You never want more than one of the three on a question. $\endgroup$ – Separatrix Aug 22 '17 at 13:32
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No

Coral is sensitive to depth and temperature, very sensitive to temperature. Ocean currents aren't so forgiving.

Basic requirements 18-27m, clear warm waters.

That would require them not to just float but to be neutrally buoyant at a specific depth, not easy to do when you're basically a rock. Staying in warm water is also difficult when you're a at the mercy of the currents. Your coral is very quickly going to find itself at the centre of one of the gyres (a.k.a. the garbage patches) or in cold waters at the end of a warm water current.

Coral already has difficulty surviving storms and a big storm can do a lot of damage to a reef. Without a solid rock base, and at the mercy of the rolling waves, your reef isn't going to survive long even just in deep ocean before the real storms hit. Flexibility is the key to surviving a storm, an attempt to build a large coral reef like structure will result in large waves breaking it up then the subsequent waves smashing it against itself until there's nothing but sand.

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  • $\begingroup$ So floating yes, surviving no. $\endgroup$ – Ash Aug 22 '17 at 13:32
  • $\begingroup$ Yeah, that was essentially my thought too. Making it float is simple, but it just makes it far more vulnerable to the weather and currents compared to sitting on the sea bed. $\endgroup$ – Matt Bowyer Aug 22 '17 at 13:54
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    $\begingroup$ But the OP is asking about coral-like organisms, not corals as they exist on Earth. So there are, for instance, diatoms which have cell walls of SiO2, and which can form colonies. There are also examples of floating islands on Earth, e.g. en.wikipedia.org/wiki/Floating_island $\endgroup$ – jamesqf Aug 22 '17 at 17:58
  • $\begingroup$ @jamesqf, those floating islands are inland, there's no reason to think they'd survive ocean waves. I've ignored some important features of coral to get this far, one being that corals are territorial and have little battles over space. $\endgroup$ – Separatrix Aug 23 '17 at 7:31
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    $\begingroup$ @Separatrix: There are free-floating mats of seaweed, e.g. oceanservice.noaa.gov/facts/sargassosea.html Doesn't seem like it would take much of an evolutionary nudge to send them down a path that evolved into free-floating islands, perhaps incorporating harder parts from coral-like organisms. Because we aren't limited to Earth corals, but alien organisms that are somewhat coral-like. $\endgroup$ – jamesqf Aug 24 '17 at 17:21
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I have found floating coral skeleton fragments while beachcombing, the largest perhaps 2-3 kilos. The coral polyps themselves were long gone, so the coral was "dead" but all four fragments I've found over the years had since been colonized by algae and gooseneck barnacles. I've never cut one open, but it's been theorized that these fragments had once been washed ashore elsewhere, dried out and somehow hollowed out inside, and then somehow again washed back into the sea. Very interesting phenomena.

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  • $\begingroup$ So floating reef-like structure colonising old dead coral is not impossible, cool thanks for that. $\endgroup$ – Ash Dec 25 '19 at 17:16
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Give it gas sacs

The Portuguese Man o' War is a colonial organism that superficially resembles a jellyfish. One of its component units forms a gas sac that allows it to float at the surface of the water. When it is in danger, the man o' war can deflate its sac to sink below the surface. It has no other means of propulsion.

Corals are also colonial organisms. Each polyp has a hard skeleton but inside is a soft animal. It is possible in theory for one polyp out of a certain number to develop into a large floating balloon, allowing the organism to move. It could use these to float and sink depending on conditions like weather and temperature. It may evolve like this in order to avoid large, slow-moving coral eaters that are bound to the sea floor, like giant starfish.

How would such an organism form a "floating reef"? I'm picturing a kind of branching and interlocking lattice, somewhat flat and with many "holes" that can be filled with the inflated polyps when the reef wants to float - like a giant sheet of bubble-wrap where the spaces in between the bubbles are hard coral. A person may be able to walk around on it provided they only step on the "hard" parts and avoid popping the balloons, and also avoid distressing the colony to the point where it would sink to escape.

However, the thickness of these "reefs" would be limited. Real corals grow huge because the living polyps grow on top of layers and layers of dead ones. But these floating corals cannot do this, otherwise there will be a certain point where the living balloon polyps can no longer lift the layers of skeletons they grow on. At some point the colony will have to break apart or shed their lower layers.

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