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On Earth, the vast majority of the biosphere is ultimately dependent on a large number of autotrophic organisms that produce usable energy in the form of glucose by using photosynthesis. However, on worlds with thick, dense atmospheres or covered in massive sheets of ice, sunlight may not be available to life forms living on either the surface or in ice-covered oceans.

Life, of course, does not need a base of photosynthetic organisms to exist. Chemotrophic life evolved before phototrophs did, and is perfectly capable of living in the most barren and inhospitable environments we've found, like the insides of rocks deep beneath the surface of the Earth. Non-chemotrophic autotrophs do, however, enable a biosphere to expand beyond what can survive on what may be a limited amount of available oxidizable nutrients.

Are there any alternate means by which autotrophic life could harvest energy? In an otherwise habitable world without sunlight, is there an alternate way in which non-chemotrophic autotrophs could evolve?

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  • $\begingroup$ I'm a bit confused with some of the terminology. Aren't many chemotrophs also autotrophs? $\endgroup$ – HDE 226868 Jul 7 '16 at 21:49
  • $\begingroup$ My first thought is heat. Maybe this organism consumes chemicals available in its environment, but uses heat from other sources to power the process needed to break down its food. $\endgroup$ – AndreiROM Jul 7 '16 at 21:57
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    $\begingroup$ @HDE226868 I had the same question, and apparently its a bit fuzzy. The best I could come up with is that an autotroph generates energy bearing organic molecules like fats and sugars as part of their metabolic cycle. Lithotrophs generate energy by using rocks as their fuel (reducing agent). The sites I saw said that generally lithotrophs are autotrophs, but didn't go into anything about the exceptions. My guess is that there are some lithotrophs which use the rocks for fuel without converting them into what we would call "organic" energy bearing molecules. $\endgroup$ – Cort Ammon Jul 7 '16 at 21:58
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    $\begingroup$ Some of the sites I looked at had a term lithoautotroph, which was clearly a mix of both. Those sites also pointed to lithotrophs as "borrowing electrons" from the rocks around them, suggesting how non-organic their metabolism may be. $\endgroup$ – Cort Ammon Jul 7 '16 at 21:59
  • $\begingroup$ I'd always though of chemotrophs as being different from autotrophs, but I guess that's wrong! I'll update the question. $\endgroup$ – ckersch Jul 7 '16 at 22:00
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Based on the groups I see that we have defined there are two categories of autotrophs:

  1. Chemoautotroph
  2. Photoautotroph

This is simply because the most common forms of energy readily available to organisms are solar energy and chemical energy.

Theoretically, if an organism just needs energy to survive, you could engineer an organism that uses any form of energy a planet has. For example Geothermal Energy. If there are no other types of energy around, an organism could probably transfer dissipating heat into the energy required for life. If we are able to harness heat energy for powering fans, perhaps an organism could similarly use that energy to survive.

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To answer this question I think it is most useful to look at how life as we know it utilizes energy and go from there. This is of course isn’t going to cover all conceivable forms of energy capture but we’ll try to get pretty creative anyways.

To begin with, organisms are composed of chemical energy. By chemical energy we mean the energy stored in the form of atomic arrangements, chemical bonds that contain energy essentially. From the DNA, the RNA, the proteins, the membranes, all of it was formed by chemical energy and runs on chemical energy. It’s pretty hard to imagine life as we know it that doesn’t involve chemical energy of some sort since that is basically just our definition of life, something animated and composed of chemicals. Chemical energy makes for great lifeforms because it’s a way of storing energy stably over long periods of time, and of course it’s easy for chemical energy to be utilized to catalyze chemical reactions.

So, for an autotroph to exist it needs to be able to convert energy into chemical energy. And that’s really our only requirement. Any energy source that an organism can use to generate chemical energy can potentially be used to create and sustain life. Of course, we know that theoretically all sources of energy are interchangeable and could in some way be converted to chemical energy, that conversation may not be particularly efficient or simple life really doesn’t need much to get by.

While I think the answer “all possible forms of energy can be used to feed autotrophs” is technically correct we can expand on our answer with more specifics by looking at how chemical energy is generated by life as we know it.

In photosynthesis light is converted into chemical energy. Fortunately for us it’s something of a roundabout process. Specifically I’m referring to the generation of an electrochemical gradient and its use by a protein complex called ATP synthase. The light energy captured by chlorophyll isn’t all transferred directly into chemical energy, some of it is used to create a proton gradient across a membrane which ATP synthase, a protein that spans that membrane utilizes to create a very useful molecule called ATP. ATP is one of the primary ways in which chemical energy is stored and expended in a cell. What this means for us is that we know that organisms don’t need to convert energy directly into chemical energy, but could instead create an electrical or chemical gradient to then convert into chemical energy. What is particularly interesting about the function of ATP synthase though is that it also converts the energy from the electrochemical gradient into an intermediate form before it becomes chemical energy. That intermediate form is kinetic energy! ATP synthase actually uses the electrochemical gradient to spin a piece of itself sort of like a turbine and it is this movement that actually translates into the “charging” of ATP. What this tells us is that theoretically any form of kinetic energy can also be converted into chemical energy and thus utilized by autotrophs to sustain life. If you want a very thorough explanation of the ATP synthase complex you can look here.

So, from that analysis we know that any source of energy that we can convert directly into chemical energy, into an electrical or chemical gradient, or into kinetic energy could potentially be used by a lifeform. Let’s take a look at potential energy sources.

Kinetic: Our planet has tons of kinetic energy lying around and I imagine it won’t be unique in this. Atmospheres have wind, hydrospheres have evaporation, rain, rivers, waves, tides. Even the ground can have earthquakes, landslides, or tides. All of these are potentially usable forms of energy. We know it’s possible to convert kinetic energy into chemical energy with proteins, it’s just a matter of an organism finding a sufficiently efficient way to do it.

Gravitational: While this overlaps somewhat with kinetic (like rivers flowing downhill) I wanted to make a special mention of tides and elliptical orbits. When a gravitational field changes substantially like on a moon orbiting closely to a large planet and therefore having strong tides it could provide an opportunity for a lifeform to steal some energy. Imagine an organism that can pump a fluid up and down very efficiently. Pumping the fluid up it expends energy. Allowing the fluid to flow back down it is able to extract energy. If the organism pumps fluid up while the tides are in and gravity is relatively weak, and lets it flow back down while the tides are out and gravity is strong there is the potential for a net profit in energy. The system would need to be incredibly efficient and the changes in the gravitational field significant, but in theory it could work.

Heat: Presumably any heat differential could be harnessed to produce energy. This heat differential could come directly from the sun, or from the ground as geothermal energy. Heat differentials like those between sun and shade or between thermal layers in a body of water or an atmosphere could be used to create electrochemical gradients or kinetic movement.

Pressure: Buried liquids in the form of artesian aquifers, geysers, or petroleum could provide a source of energy. By releasing the pressure the organism would be generating kinetic energy that could be converted to chemical.

Radiation: Radiation in the form of alpha and beta and gamma particles all generate high energy chemicals. In water-based organisms these are peroxides, but the principle should be similar with other chemistries. Radiation thus directly generates chemical energy, it’s just up to the organism to efficiently harvest it.

Electrical: Natural processes can generate electrical potentials, such as lightning. An electrical gradient can easily be used to generate chemical energy as seen by the electrochemical gradient.

Chemical gradients: Chemical gradients can be formed by natural processes. Fresh water flowing into salt water for example. Evaporation can concentrate solutes and rainfall can dilute them. Organisms could potentially derive energy from any such gradient.

Hopefully this is what you were looking for, let me know if you'd like more concrete examples of how certain things might work and I can elaborate, but I wanted to avoid rambling about hypotheticals for pages and pages.

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