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Could an alien underground macroscopic ecosystem including human sized plant-like organisms evolve around lava as an energy source? These organisms could use heat and/or light from the lava to synthesize organic molecules. Would these organisms favor a particular shape? Would they be more likely to hang from ceilings than to grow from the ground? Could the presence of anything in the environment encourage this, such as reflective elements? Assume life could have evolved on the planet's surface originally or simultaneously.

It seems like this would encounter a number of problems such as:

  • As calculated in this post, lava gives off very strong light, and humans at least have to be no closer than around 8 or 10 meters and at a sharp angle from the surface of the lava.
  • Lava needing to have a constant relatively unchanged presence that could reasonably be adapted to across evolutionary time spans. What could prevent the lava from cooling that would also keep it in the same place and allow organisms to thrive?
  • How quickly energy could attenuate from the lava source, this could force life into tighter spaces than other cave environments?
  • The organisms might need a heat potential across which to do work. This could maybe take the form of cool hollow shafts in the rock to expel heat with, or the plants prefer to grow around corners or towards the energy source so that further back parts of the organism are cooler?
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  • $\begingroup$ Can we use fungi instead of plants? Giant fungi-like structures? $\endgroup$
    – Mikey
    Apr 1, 2017 at 2:23
  • $\begingroup$ They don't have to be too similar to plants, but they need the following qualities: -sessile, non-moving -Some kind of energy fixing by photosynthesis, thermosynthesis, or one degree removed from energy fixing. -macroscopic sizes $\endgroup$
    – Stephen Schroeder
    Apr 2, 2017 at 14:40

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Real cold lava ecosystem. Does it have to be hot lava? Because that burns stuff up. There are cold lava ecosystems.

https://microbewiki.kenyon.edu/index.php/Deep_subsurface_microbes#Crystalline_Metamorphic_and_Igneous_Rocks

The basis of this ecosystem is organisms which use hydrogen liberated from water using iron or sulfur. From the link "these ecosystems can exist indefinitely without any input from the surface."

Heat as energy source. Re using heat I could imagine that an organism which spanned a heat differential could cycle some molecule back and forth between the hot side and the cold side. The molecule assumes some energetic configuration on the hot side then at the cold side can be catalyzed to give up its energy to generate ATP. The organism might be a slime living on a geothermal heated hot rock with its top side in cool flowing water. Such environments are not uncommon but it might not be obvious that a slime was using an exotic metabolic path like this.

Iron oxidizing bacteria do something like this - harness the interface between two environmental conditions. They grow where water with reduced iron in it comes out of the ground. The iron will oxidize on its own pretty soon out in the air. The bacteria capitalize on this by oxidizing the iron inside their own cells and harnessing the energy released. The strands of this bacteria look rusty because they are making rust.

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  • $\begingroup$ It looks like this could indeed make for a self contained ecosystem, especially since they excrete organic compounds. Maybe these lithoautotrophic microbes in underwater pools or shafts could support macroscopic fungi? And the igneous rock food source could be replenished by lava? $\endgroup$
    – Stephen Schroeder
    Apr 2, 2017 at 14:50
  • $\begingroup$ These things could totally be the basis of an ecosystem and given that they are getting energy from lava, new lava would replenish their energy source. I think a big problem for an ecosystem like this is low nitrogen and phosphorus; especially nitrogen. Available organic nitrogen is limiting for lots of ecosystems; in the deep earth even gaseous nitrogen might be in short supply. Methods for bringing nitrogen to this ecosystem offer interesting narrative side roads. $\endgroup$
    – Willk
    Apr 2, 2017 at 19:19
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No you could not have a macroscopic underground ecosystem powered by lava.

At their most extreme hyperthermophiles can survive temperatures of 105C. Lava starts giving off visible light at temperatures ten times that.

The closest real world examples to what you are talking about are hydrothermal vents. These only support multi-cellular life up to temperatures of 80C.

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  • $\begingroup$ If organisms were further away from the lava would that be better, or would this space/distance create some inherent problem? $\endgroup$
    – Stephen Schroeder
    Mar 31, 2017 at 19:53
  • $\begingroup$ An expansion...Lava isn't simply molten rock, it's also gas. Horribly toxic gas. Any underground system that includes open lava is going to contain horribly toxic gasses in the air. $\endgroup$
    – Twelfth
    Mar 31, 2017 at 19:54
  • $\begingroup$ Would it be necessarily toxic to things that aren't human or even from earth? $\endgroup$
    – Stephen Schroeder
    Mar 31, 2017 at 19:56
  • $\begingroup$ Nothing about the gasses is necessarily toxic, though they can certainly be toxic to humans. $\endgroup$
    – sphennings
    Mar 31, 2017 at 20:02
  • $\begingroup$ Another post I think I'm going to include in the OP that goes into the math of a 6-foot person approaching lava, although it sounds like very few macroscopic organisms if any should be closer than 8 feet. $\endgroup$
    – Stephen Schroeder
    Mar 31, 2017 at 20:31

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