Converting Heat to Nutrition?

[Mr. Mask]

We were discussing ways of getting nutrition/energy for creatures. Someone brought up the idea of converting heat.

I can't think of any particular principles where you could sustain yourself on warmth. I thought I'd check, all the same. Is there any speculation or hard science for something like this?

[Eldan]

I've been looking around a bit, but couldn't find anything useful. But there are apparently some new ideas on reactor design that directly transfer heat into electricity without turning heat into steam (pressure) first and driving turbines. So maybe it's possible. But then, I doubt that living things would ever have gold surfaces and single organic molecules suspended between them.

[Mr. Mask]

Thank you for answering the question. Thought it might be that, but just as well to check.

[Ashtar]

The closest things I can find to pure heat to energy example on earth are:

Riftia - Found next to deep sea vents

Moreover, a completely unexpected community of life was found around these aptly named hydrothermal vents, with not only high densities of numerous new species, but also a new kind of ecosystem flourishing in the dark that had never been imagined by scientists — an ecosystem based on toxic gas! The most amazing of the new species was a giant tubeworm, named Riftia. Growing rapidly in dense clusters, these 2-meter-tall worms were found to have no digestive tract. The revolutionary finding was that they subsist on energy-rich hydrogen sulfide in vent water and generated in the Earth's crust. Hydrogen sulfide (rotten-egg gas) is normally toxic to animals, but these worms avoid the problem in a spectacular manner. They harbor bacteria known as chemoautotrophs (in a large sac replacing a digestive system), which can use the energy in hydrogen sulfide to convert carbon dioxide into sugars, just as plants do using sunlight. The worm's blood picks up and delivers sulfide, carbon dioxide and oxygen to these bacterial symbionts, which in turn "feed" their hosts with the excess sugars they make (while turning the sulfide into a non-toxic waste product). Thus, the ecosystem was found to run on the Earth's geothermal energy rather than sunlight. Many scientists now think that life on Earth began at such vents over 3 billion years ago.

Radiotrophic fungi - Found around Chernobyl


Radiotrophic fungi are fungi which appear to use the pigment melanin to convert gamma radiation[1] into chemical energy for growth.[2] This proposed mechanism may be similar to anabolic pathways for the synthesis of reduced organic carbon (e.g., carbohydrates) in phototrophic organisms, which capture photons from visible light with pigments such as chlorophyll whose energy is then used in photolysis of water to generate usable chemical energy (as ATP) in photophosphorylation or photosynthesis. However, whether melanin-containing fungi employ a similar multi-step pathway as photosynthesis, or some chemosynthesis pathways, is unknown.

[NichG]

For this to work, you need a heat source that is close-by to a cold place, and the organism basically has to span that gap. That may be why its relatively rare - bacteria would generally be too small to get much of a heat gradient going, so you'd expect to find it first in multicellulars - but multicellulars don't have nearly as much metabolic variation as bacteria do.

On the other hand, something like a strong infrared source could allow bacteria coating a surface to profit from the transfer of that radiative energy into the object they're on, so biofilms might be a place to look for this kind of mechanism.

Finally, depending on how generous your definition of 'life' is, the polymerase chain reaction can be thought of as a mechanism that uses heat as its input energy source. In PCR, you've got DNA chains and an enzyme which catalyzes their replication, but for the thing to work efficiently you have to temperature cycle the PCR chamber. That temperature cycling provides the energy that makes the whole thing go forward.

[noparlpf]

RE: Hydrothermal vents: Technically the heat there is just what keeps things moving enough to sustain the chemical reactions that are life. The energy those critters actually use for metabolism is derived from oxidation of (usually) hydrogen sulfide by chemoautotrophic bacteria (and probably archaea too).

I don't think it's particularly likely that any organic life form* would be able to convert heat to useful energy. Heat is generally what other forms of energy deteriorate to and it's pretty hard to convert it back to electric or kinetic energy.

*I also don't think it's particularly likely that inorganic life would evolve naturally, so don't ask about that. Silicon-based life has been proposed so so many times, but due to silicon's size, it doesn't interact with other atoms the same way carbon does. It's relatively inert. Silicon is also relatively rare compared to carbon. And silicon is probably the next most likely base atom after carbon, with boron following.
Alien life would likely be significantly different from our own, but chemistry limits what can actually yield life. It's highly likely that alien life would consist of carbon-based cells, likely formed by something very like a lipid bilayer or monolayer, although the specific composition of lipids and such would naturally be different. Archaea, bacteria, and eukaryotes all have different plasma membrane compositions, and that's just on Earth. They would probably use something like proteins for at least part of their metabolism. The backbone might be formed differently, maybe using something else in place of nitrogen, or not forming the carboxyl group. RNA and DNA are possible but loads of other compounds are also possible. I'd bet on something similar to PNA (not the Pern stuff, that seems super unlikely to me**) being used for genetic storage in some alien species. Peptide nucleic "acid" (not actually acidic) is basically a polyglycine chain with nitrogenous bases stuck onto every other nitrogen instead of having different organic R groups attached to the non-carboxyl carbons. An aminoethyl polyglycine chain is produced by cyanobacteria, so it's possible an information-storing version could evolve in some lineages. I also think it would be likely because it's already so similar to proteins, and I'd also bet on proteins existing in some alien species.
**I doubt the triple helix of Pernese Nucleic Acid would be likely. I'd have to know the structures of its bases to say, but they never got that far. Also, I seriously doubt complex life would evolve a. from ancestors with an inherently low mutation rate and b. on a planet that's frequently seeded with an alien organism that destroys all organic material on the planet before starving to death itself. Also, I'm not sure PernNA would make a good information storage molecule because they only have 23 amino-acid-coding codons, and depending on the structure of the bases, redundancy would be chemically necessary, so they might not have enough amino acids to make really diverse proteins.

Oops my footnote turned into a huge thing like ten times longer than my actual post.

[factotum]

I also don't think it's particularly likely that inorganic life would evolve naturally, so don't ask about that. Silicon-based life has been proposed so so many times, but due to silicon's size, it doesn't interact with other atoms the same way carbon does. It's relatively inert.

I'm pretty sure that any practical silicon-based life form would have to be based on silicon oxides--those can arrange themselves into long chain molecules just like carbon can. Of course, basing your life-form on a molecule rather than an atom definitely reduces the likelihood of it happening, but since we're still not 100% sure how carbon-based life got started, I for one won't rule it out.

[NichG]

The reason people generally go to carbon for a good core component of life is that it can form four bonds (or three with a double-bond). That means you can make a lot of different structures with that one atom, compared to what you can make with e.g. chlorine or something. In principle, three bonds per site would be enough to have polymers with side-chains, so you could imagine pulling it off with nitrogen, phosphorous, boron, etc. You might have problems making stable cyclic molecules though, which probably limits your chemical repertoire quite a bit.

One consideration though is, lets say chemical life is possible with each of those substrates. You'd still expect some to be easier or more difficult to pull off (either because it requires rare environments, or because the different bond-energies imply different timescales for reactions). That means that there might be a factor of 10 or 100 or 1000 between how long it'd take to see carbon-based life emerge versus how long it'd take to see boron-based life emerge. Which means that on a planet where both are possible, carbon would tend to win just because it consistently gets there first. If on the other hand you had a planet where carbon-based life were impossible but boron-based life might work, you'd still have to wait much longer to see boron-based life emerge; rather than a billion years, it might take ten billion on average, in which case we'd have an outside chance of finding it; or it might take a hundred billion years, in which case we're not going to find it out there even if it is possible.

On the other hand, maybe that '1 billion years' thing is driven more by geological processes taking a long time for the primordial Earth to cool down to a point where its suitable, and the actual emergence only takes a few hours after it gets there. In which case we might see all sorts of kinds of life all over the place.

A lot of it depends on what you're willing to call life as well. Self-perpetuating 'reactions' that consume some sort of external resource to amplify themselves are pretty common in the universe, and don't require anything as complex as carbon chemistry to obtain. Combustion processes do this, and I recently learned that there are autocatalytic reactions even in stellar nucleosynthesis - so for that simple of a definition of life, even the processes in the hearts of stars are 'alive'. But those things never complexify beyond the particular autocatalytic reactions that compose them - you don't get information carrying excitations and heredity and things like that.

[noparlpf]

Yeah, don't remember if I said it, but boron is often mentioned sometime after silicon. Silicon is the traditional hypothetical alternative because it's in the same group as carbon, but boron is also a possibility. I think the issue with boron-based life is that boron compounds aren't as stable as carbon compounds, at least in an environment like ours. Boron is also less abundant than carbon.

There was that one experiment a while back where a guy put a bunch of organic junk in a closed system and supplied light, heat, and electricity to see if life would happen. He got a bunch of organic compounds but nothing really "life", so I'm gonna go ahead and say that life doesn't pop up within a few hours of geological stability. :smalltongue:
(Edit: It's called the Miller-Urey Experiment and it was done in the '50s. It's been reproduced a few times since. Starting with water, methane, ammonia, and hydrogen gas, they got stuff like various (racemic) amino acids in about two weeks, including several beyond the twenty that our DNA codes for. Of course any separate organic lineage would evolve to use a slightly different set of amino acids in its proteins and whether it would use left- or right-handed amino acids is basically up to chance. Glycine was of course the most common amino acid present. They also got some sugars and a bunch of less-complex junk, but no nucleic acid bases. Later experiments did yield some nucleic acid bases. They'd need to add some phosphorus-bearing compound to get actual nucleic acid chains, obviously. I don't think that the RNA world hypothesis was accepted yet in the '50s so they might not have thought they'd need nucleic acids to yield life. Life might still evolve in a phosphorus-starved system, though, using something else as an information storage molecule. I already mentioned PNA, which is a modified protein backbone, but that's pretty rare in nature on Earth, so I dunno.)

As for "what is life?" That came up in the giant virus thread a month or two ago. I'd say life is a. self-replicating, b. capable of transmitting information between generations, and c. capable of evolution by selection. Something like a virus is right on the edge of "life" because it only really fulfills at most two and a half of those conditions. Viruses evolve, transmit information between generations, and some viruses do carry some of their own replicative enzymes. I think we've even gotten viruses to go through their replication cycle in vitro, but only by supplying cellular replication machinery. So they can't actually replicate independently. Something like fire or stellar processes also don't count as life. They may reproduce themselves or multiply, but there's no information transfer or evolution. If you keep a fire burning for a thousand years the chemical reactions will still be exactly the same as they were at the start. With cell cultures you can see changes in DNA or physiology in a matter of days, sometimes even without adding selective pressure, just because of random mutations and neutral molecular evolution.

[NichG]

Yeah, don't remember if I said it, but boron is often mentioned sometime after silicon. Silicon is the traditional hypothetical alternative because it's in the same group as carbon, but boron is also a possibility. I think the issue with boron-based life is that boron compounds aren't as stable as carbon compounds, at least in an environment like ours. Boron is also less abundant than carbon.

There was that one experiment a while back where a guy put a bunch of organic junk in a closed system and supplied light, heat, and electricity to see if life would happen. He got a bunch of organic compounds but nothing really "life", so I'm gonna go ahead and say that life doesn't pop up within a few hours of geological stability. :smalltongue:
(Edit: It's called the Miller-Urey Experiment and it was done in the '50s. It's been reproduced a few times since. Starting with water, methane, ammonia, and hydrogen gas, they got stuff like various (racemic) amino acids in about two weeks, including several beyond the twenty that our DNA codes for. Of course any separate organic lineage would evolve to use a slightly different set of amino acids in its proteins and whether it would use left- or right-handed amino acids is basically up to chance. Glycine was of course the most common amino acid present. They also got some sugars and a bunch of less-complex junk, but no nucleic acid bases. Later experiments did yield some nucleic acid bases. They'd need to add some phosphorus-bearing compound to get actual nucleic acid chains, obviously. I don't think that the RNA world hypothesis was accepted yet in the '50s so they might not have thought they'd need nucleic acids to yield life. Life might still evolve in a phosphorus-starved system, though, using something else as an information storage molecule. I already mentioned PNA, which is a modified protein backbone, but that's pretty rare in nature on Earth, so I dunno.)


Just today I was reading a paper that showed that adding a small amount of Fe-II and temperatures consistent with hydrothermal vents causes the various chemistry you get to whittle down and become much more efficient at producing the components of a citric-acid-cycle-based metabolism (which is autocatalytic). So it seems like things can get up and running pretty fast compared to geological timescales once the conditions are right. That would be an example of a pre-biotic, non-information carrying life precursor. Of course, the concentrations aren't enough in these experiments to get the cycle to be self-perpetuating, so we're still missing things in our understanding of it. Mineral surface catalysis, vesicle formation, things like that are sometimes brought up as possibilities for getting the concentrations high enough to make things go forward.



As for "what is life?" That came up in the giant virus thread a month or two ago. I'd say life is a. self-replicating, b. capable of transmitting information between generations, and c. capable of evolution by selection. Something like a virus is right on the edge of "life" because it only really fulfills at most two and a half of those conditions. Viruses evolve, transmit information between generations, and some viruses do carry some of their own replicative enzymes. I think we've even gotten viruses to go through their replication cycle in vitro, but only by supplying cellular replication machinery. So they can't actually replicate independently. Something like fire or stellar processes also don't count as life. They may reproduce themselves or multiply, but there's no information transfer or evolution. If you keep a fire burning for a thousand years the chemical reactions will still be exactly the same as they were at the start. With cell cultures you can see changes in DNA or physiology in a matter of days, sometimes even without adding selective pressure, just because of random mutations and neutral molecular evolution.

There's a lot of gradations I think. Something like fire is closer to life than, say, a bucket of water. There's at least three major competing points of view on the subject that I'm aware of. There's 'RNA world' which argues that the information-carrying molecule was first, and basically doubled as a catalyst - in that viewpoint template-based replication is the fundamental necessary condition, and things like metabolism emerged as a product of slow-replicating information carriers tweaking the chemistry by catalysis.

The metabolism-first view is that something like a chemical 'fire' is a necessary precursor to generate a rich enough sampling of the chemical space in order to actually find or produce a sufficient number of the information-carrying compounds to step your way up to an operating evolutionary process. For example, if you have something like the citric acid cycle then the particular distribution of related compounds in solution with it can bias what byproducts are produced, so you can in principle get a sort of quasi-heredity emerging from that. The argument for this is that prebiotic chemistry gets you most of the way to a functioning citric acid cycle, which would be an autocatalytic process that'd throw off all sorts of useful byproducts, and then information carriers could modulate that process to improve its efficiency and thereby increase the availability of the compounds needed to make more of the information carriers.

The third view I'm aware of is that some sort of confinement/compartmentalization is necessary before either of the others, in order to get the concentrations high enough to make anything that can sustain itself against decay pathways. This is the so-called 'Lipid world' hypothesis that suggests that you can get evolution on a sort of 'mechanically replicating' vesicle-based life that would act as the tunable container for the microscopic chemistries of the above two models. Essentially, lipids exist in that environment, spotaneously form vesicles, and spontaneously divide when placed under sufficient internal pressure, so if the lipids already present feed back to control the rate at which new lipids are induced, you have a mechanism that can replicate patterns of lipid composition so long as there's food (free lipids) available to drive it; in simulation at least, that's been shown to be capable of responding to selection pressures and inheriting information, but it hasn't been shown in the lab as far as I'm aware.

[ericgrau]

Doing anything useful with uniform heat itself is a violation of thermodynamics. But what you can do is harness a large temperature difference; this is the basic principle behind engines and power plants. It's hard to be efficient without getting into the thousands of degrees on one side and tens or hundreds on the other.

On the biological side of things a creature such as an animal or bacteria would have to move back and forth between hot and cold which might cause something to expand and contract which then could be used to force certain chemical reactions to take place. And then it might use that directly or use it to power the construction of sugar. A bacteria might also seal off a chamber of cold or hot, then move to the other temperature region. Then utilize the difference in heat across its membrane.

None of this may be possible though, since the enzymes that life depend on tend to only work in certain temperature ranges. Too far outside of range may even destroy them. Even if you had two sets of enzymes, it might be hard to find a location that always stayed within a hundred degrees of 2300 F or some such. Perhaps instead there's a bacteria that lives around a much lower temperature heat source (and cold too) and uses it inefficiently for energy. But it's not practical for any animal.

[Ravens_cry]

Life as we know it, yes. I've heard of some silicon based hypothetical ideas that work best several hundred degrees Celsius or even more.

[NichG]

Doing anything useful with uniform heat itself is a violation of thermodynamics. But what you can do is harness a large temperature difference; this is the basic principle behind engines and power plants. It's hard to be efficient without getting into the thousands of degrees on one side and tens or hundreds on the other.

On the biological side of things a creature such as an animal or bacteria would have to move back and forth between hot and cold which might cause something to expand and contract which then could be used to force certain chemical reactions to take place. And then it might use that directly or use it to power the construction of sugar. A bacteria might also seal off a chamber of cold or hot, then move to the other temperature region. Then utilize the difference in heat across its membrane.

None of this may be possible though, since the enzymes that life depend on tend to only work in certain temperature ranges. Too far outside of range may even destroy them. Even if you had two sets of enzymes, it might be hard to find a location that always stayed within a hundred degrees of 2300 F or some such. Perhaps instead there's a bacteria that lives around a much lower temperature heat source (and cold too) and uses it inefficiently for energy. But it's not practical for any animal.

If anything, I'd think an animal would be more likely to be able to use thermal gradients than a bacterium because they have a much greater spatial extent. For a bacterium, thermal conduction is a really fast process because everything is so small.

That said, hydrothermal vents are an example of a natural process that provides short-timescale cycling between something like 400C and ~0C. If anything is going to be making direct use of thermal energy, thats the sort of environment I'd expect to find it in.

[Icewraith]

Not directly, no. If you simplify a vast number of complications and stifle some objections and squint a little, being alive is being an engine. You make use of an external resource to maintain a body temperature higher than that of the surrounding environment, you lose energy through a number of inefficiencis in the process of obtaining and processing the external resource, and you return unused and depleted resources to the environment. The less of a temperature difference there is between you and the environment, the less capable you are of doing useful work.

Wht we already do have is an entire massive subset of known lifeforms that rely directly on absorbing heat from the environment to maintain their thermodynamic cycle. We call them "cold blooded". However, they require additional sources of energy to maintain their cycle when the environment isn't providing extra energy, and still need to obtain raw materials to either reach sources of free energy they can absorb, complete the thermodynamic cycle, or increase their ability to store and retain heat.

Basically, your question is biased from the perspective of being a mammal. Nutrition is either a resource that an organism can partically convert into heat, a source of secondary material necessary for that process to function, or a source of material used to improve, maintain, or repair the organism; or for the organism to reproduce. You can absorb heat directly from the environment, or at least stop losing heat to the environment, but if you stay for too long at an elevated temperature you reduce the efficiency of your heat cycle or damage the apparatus required for your heat cycle to function.

You can't sustain yourself on warmth, but you CAN live longer in a warmer environment where you don't lose as much energy to your surroundings. If you're lost and starving in a wasteland, you'll still die if you can't eventually get food and water but you'll be in far better shape if the temperature is constantly 70 F rather than fluctuating between -10 and 114.

The other problem with using direct heat to sustain oneself is that there is a certain rate of heat influx that is just as or more disruptive to your cycle as a large rate of heat outflux. Too much heat and you cook, burn, or ignite lifeforms.

Edit: I'm not sure how long an organism could stay alive in an environment that matched its normal internal body temperature. My guess is that, to a limit, the body would adapt by maintaining at least a slightly higher temperature than the environment or the mass flow rate to maintain an effective gradient for doing work and regulating temperature in such an environment would increase drastically. This is again how the cold-blooded creatures do it all the time, but would probably be profoundly uncomfortable for an organism built to maintain its own body temperature at a certain level above the environment.

[NichG]

In practice, the reason there's a temperature difference between living organisms and their environment is that heat is always being produced by the organism's internal processes, and if the organism is the same temperature as their environment then there's no heat leaving the organism, which means that its own temperature must increase as heat is being produced.

I don't think the correct temperatures to use to determine the Carnot efficiency of an organism are its steady-state body temperature versus the environment. An example of this is solar power collection. If you have a photovoltaic cell as your engine, the 'high' temperature of the engine isn't the steady-state temperature of the solar cell, its the temperature associated with the blackbody spectrum of light that the cell absorbs. Which means that its basically something like 4000K - much higher than the photovoltaic cell can operate. The same would be true of an organism using photosynthesis as a power source.

Another example would be an engine whose physical extent and heat capacity that you could see were much larger than the combustion chamber. If you measured the average temperature of the entire engine block throughout a cycle, it would look much smaller than the peak temperature in the chamber (which is actually what sets its Carnot efficiency). Now imagine an engine where the 'combustion chamber' is the vicinity of a covalent bond that has just broken. Covalent bonds have energies around 1eV, and ATP hydrolosis is about 0.5eV, so we're looking at a range of maximum operating temperatures from about 5000K to 10000K.

Compared to that, whether the organism's ambient temperature is at 330K or 300K won't make much of a dent in the efficiency.

[The Grue]

Radiotrophic fungi - Found around Chernobyl


Radiotrophic fungi are fungi which appear to use the pigment melanin to convert gamma radiation[1] into chemical energy for growth.[2] This proposed mechanism may be similar to anabolic pathways for the synthesis of reduced organic carbon (e.g., carbohydrates) in phototrophic organisms, which capture photons from visible light with pigments such as chlorophyll whose energy is then used in photolysis of water to generate usable chemical energy (as ATP) in photophosphorylation or photosynthesis. However, whether melanin-containing fungi employ a similar multi-step pathway as photosynthesis, or some chemosynthesis pathways, is unknown.

That is actually a little bit terrifying.

http://i255.photobucket.com/albums/hh158/vividwings/Warhammer%20motivational%20Posters/exterminatus2.jpg

[Icewraith]

In practice, the reason there's a temperature difference between living organisms and their environment is that heat is always being produced by the organism's internal processes, and if the organism is the same temperature as their environment then there's no heat leaving the organism, which means that its own temperature must increase as heat is being produced.

I don't think the correct temperatures to use to determine the Carnot efficiency of an organism are its steady-state body temperature versus the environment. An example of this is solar power collection. If you have a photovoltaic cell as your engine, the 'high' temperature of the engine isn't the steady-state temperature of the solar cell, its the temperature associated with the blackbody spectrum of light that the cell absorbs. Which means that its basically something like 4000K - much higher than the photovoltaic cell can operate. The same would be true of an organism using photosynthesis as a power source.

Another example would be an engine whose physical extent and heat capacity that you could see were much larger than the combustion chamber. If you measured the average temperature of the entire engine block throughout a cycle, it would look much smaller than the peak temperature in the chamber (which is actually what sets its Carnot efficiency). Now imagine an engine where the 'combustion chamber' is the vicinity of a covalent bond that has just broken. Covalent bonds have energies around 1eV, and ATP hydrolosis is about 0.5eV, so we're looking at a range of maximum operating temperatures from about 5000K to 10000K.

Compared to that, whether the organism's ambient temperature is at 330K or 300K won't make much of a dent in the efficiency.


If you simplify a vast number of complications and stifle some objections and squint a little

Well, that didn't take long. If we take a more detailed approach, then the body is more of an engine of engines. However, in a cold-blooded creature, its internal body temperature absolutely is what affects its performance, and the particular temperature of the hot source merely determines how long it waits to "warm up". The carnot cycle assumes the gas reaches thermal equilibruim with the hot source before expanding. If we're worrying about efficiency, we use the ultimate temperature of the hot source. Howver, if we're more interested in the system's theoretical ability to perform work, we're better off using the "temperature of the gas".

Edit: It's also telling that the gradient between your internal body temperature and the surroundings dropping too much is not merely an effect or definition of death but can also be a cause.

[NichG]

Well, that didn't take long. If we take a more detailed approach, then the body is more of an engine of engines. However, in a cold-blooded creature, its internal body temperature absolutely is what affects its performance, and the particular temperature of the hot source merely determines how long it waits to "warm up". The carnot cycle assumes the gas reaches thermal equilibruim with the hot source before expanding. If we're worrying about efficiency, we use the ultimate temperature of the hot source. Howver, if we're more interested in the system's theoretical ability to perform work, we're better off using the "temperature of the gas".


Well, except that all the engines are using very local chemical processes. If there's a 'gas', its more like a phonon gas of protein oscillations for the body's ion pumps, or an electron gas for its electrochemical stuff. I'm not convinced that any of those processes notice the thermal gradient between where they occur and the outside of the body - that particular transport is way too slow to make a difference.

Certainly if their operating temperature goes far away from optimal they notice that, but see the next point for why.



Edit: It's also telling that the gradient between your internal body temperature and the surroundings dropping too much is not merely an effect or definition of death but can also be a cause.

This is more because the rate of chemical reactions depends exponentially on the inverse temperature. Outside of a range of +/- 5K is enough to completely screw up carefully tuned biochemistry, both because relative rates get screwed up but also because certain thermally suppressed conformation changes of proteins become feasible, so proteins fold incorrectly or unfold and refold into shapes which don't actually do what the protein is supposed to do.

[Grim Portent]

We were discussing ways of getting nutrition/energy for creatures. Someone brought up the idea of converting heat.

I can't think of any particular principles where you could sustain yourself on warmth. I thought I'd check, all the same. Is there any speculation or hard science for something like this?

From what I recall the tube worms that live around volcanic vents in the deepest reaches of the ocean live off of bacteria that carry out biological processes using thermal energy to serve the role that solar energy does in the normal food chain. The bacteria use heat energy to power their metabolic processes in a rough equivalent to photosynthesis, they secure their chemical needs form the mineral content of the volcanic water and the detritus that drifts from above, and in turn are fed upon by the various lifeforms that live around the vents such as the tube worms. I understand it to be a very slow ecosystem due to the lack of resources and cold temperature gradient towards the edge of the community.

[noparlpf]

From what I recall the tube worms that live around volcanic vents in the deepest reaches of the ocean live off of bacteria that carry out biological processes using thermal energy to serve the role that solar energy does in the normal food chain. The bacteria use heat energy to power their metabolic processes in a rough equivalent to photosynthesis, they secure their chemical needs form the mineral content of the volcanic water and the detritus that drifts from above, and in turn are fed upon by the various lifeforms that live around the vents such as the tube worms. I understand it to be a very slow ecosystem due to the lack of resources and cold temperature gradient towards the edge of the community.

The primary producers around hydrothermal vents do not use heat to power metabolism. They get their energy from hydrogen sulfide gas (or other inorganic sources) and use that energy to fix carbon dioxide (usually) to produce organic molecules. The only thing the heat does is keep chemistry working fast enough to be life.

[Knaight]

Wht we already do have is an entire massive subset of known lifeforms that rely directly on absorbing heat from the environment to maintain their thermodynamic cycle. We call them "cold blooded". However, they require additional sources of energy to maintain their cycle when the environment isn't providing extra energy, and still need to obtain raw materials to either reach sources of free energy they can absorb, complete the thermodynamic cycle, or increase their ability to store and retain heat.

The vast majority of the energy they use to actually do things (e.g. move) comes from food. All the absorbing heat from the environment does is prevent the chemical energy from the food having to go into keeping them warm. It's not a matter of needing additional sources of energy to "maintain their cycle when the environment isn't providing extra energy", it's a matter of needing food regardless to actually do things, as they have absolutely no way to convert the heat from the environment into movement, and even less way to magically turn it into actual nutrients.

[Ravens_cry]

The vast majority of the energy they use to actually do things (e.g. move) comes from food. All the absorbing heat from the environment does is prevent the chemical energy from the food having to go into keeping them warm. It's not a matter of needing additional sources of energy to "maintain their cycle when the environment isn't providing extra energy", it's a matter of needing food regardless to actually do things, as they have absolutely no way to convert the heat from the environment into movement, and even less way to magically turn it into actual nutrients.
It's also why they can go longer without eating.
I have had some pondering about what the mentality of a cold blooded sentient would be like. I see no reason they'd be the cold calculating stereotype any more than any other creature, but I wonder how temperature would affect their mental processes. I had an idea that cold blooded creature colonized an alien (to them) planet that was colder than they were used to, and they had a problem that they turned feral in cold (for them) weather, and the rest spent as much time in hot mud baths and, when they went outside, in heated suits.

[Icewraith]

Well, except that all the engines are using very local chemical processes. If there's a 'gas', its more like a phonon gas of protein oscillations for the body's ion pumps, or an electron gas for its electrochemical stuff. I'm not convinced that any of those processes notice the thermal gradient between where they occur and the outside of the body - that particular transport is way too slow to make a difference.

Certainly if their operating temperature goes far away from optimal they notice that, but see the next point for why.



This is more because the rate of chemical reactions depends exponentially on the inverse temperature. Outside of a range of +/- 5K is enough to completely screw up carefully tuned biochemistry, both because relative rates get screwed up but also because certain thermally suppressed conformation changes of proteins become feasible, so proteins fold incorrectly or unfold and refold into shapes which don't actually do what the protein is supposed to do.

So if there's not enough ambient energy available, proteins won't "expand" or "contract" correctly.

The really cool thing about thermodynamics is that you can describe everything generally in terms of heat and mass transfer and not worry too much about your overall description being that far off when someone comes along with details.

I shouldn't have used "maintain" again the second time though, that's misleading. Food is the primary energy source, "maintain" is an understatement there.

[noparlpf]

Note: There's no such thing as a "cold-blooded" animal. Many reptiles actually function at temperatures above 37°C. Call them ectotherms.


The vast majority of the energy they use to actually do things (e.g. move) comes from food. All the absorbing heat from the environment does is prevent the chemical energy from the food having to go into keeping them warm. It's not a matter of needing additional sources of energy to "maintain their cycle when the environment isn't providing extra energy", it's a matter of needing food regardless to actually do things, as they have absolutely no way to convert the heat from the environment into movement, and even less way to magically turn it into actual nutrients.

Yeah. The only reason we spend energy to stay warm is because chemistry happens better at high temperatures (but not too high, for organic molecules). There is no example of any terrestrial organism (that I'm aware of) that actually converts heat into metabolic energy. And I have a BSci in Bio with a minor in Chem. I've taken Microbio, Zoology, Cell Bio, and Physiology, among others. I'm pretty sure one of those many professors would have mentioned such a novel organism.

[Ravens_cry]

Note: There's no such thing as a "cold-blooded" animal. Many reptiles actually function at temperatures above 37°C. Call them ectotherms.

True enough, but most people, at least geek who would respond to a science thread, probably know that. Cold blooded is just another way of saying ectotherm in this case, even though it isn't literally true.

[NichG]

So if there's not enough ambient energy available, proteins won't "expand" or "contract" correctly.

The really cool thing about thermodynamics is that you can describe everything generally in terms of heat and mass transfer and not worry too much about your overall description being that far off when someone comes along with details.

I shouldn't have used "maintain" again the second time though, that's misleading. Food is the primary energy source, "maintain" is an understatement there.

You do have to get the inputs to the thermodynamic calculation right though, which is my point. If you do a calculation about a heat engine operating at a high temperature of 37C and a low temperature equal to the ambient environmental temperature, and then try to use it to predict the usable work generated per calorie for an organism living in various ambient temperatures, you'll get something that is very far from the real value.

The thing you can predict robustly without worrying about the details is the generation of waste heat - that is to say, you know that no process will be 100% efficient so whatever the internal effective temperature is of the microscopic engines that are processing fuel for the body, there will always be some waste heat generated. Without knowing that internal operating temperature (which is set by local chemical considerations), you can't actually predict what fraction of the energy input will actually be waste heat though.

What you might be able to pull off is backing out what the effective internal operating temperature must be to explain the efficiency of converting a specific molecule (say glucose) into ATP, since thats a very quantized process and you can get a good estimate of the yield just by counting atoms. That means you can use the chemistry to determine the efficiency, and therefore figure out what the minimum possible internal temperature would be in order to achieve that as the Carnot efficiency.

A quick trip to google suggests that the efficiency is about 40-50% for aerobic conversion of glucose to ATP. That means that the minimum possible effective operating temperature for the internal processes of an organism to convert glucose into ATP would be 517K-620K or 244C-347C. That's much higher than the body temperature of most things on earth, although I suppose things living near a hydrothermal vent would have to deal with that kind of temperature (as the environmental temperature though, not their internal temperature, which means their internal operating temperature would have to actually be much higher to achieve that efficiency).

Edit: For comparison, the maximal efficiency of a heat engine operating between body temperature (37C) and room temperature (23C) is about 4.5%, which is actually fairly close to the efficiency of anaerobic respiration.

[Icewraith]

Note: There's no such thing as a "cold-blooded" animal. Many reptiles actually function at temperatures above 37°C. Call them ectotherms.



Yeah. The only reason we spend energy to stay warm is because chemistry happens better at high temperatures (but not too high, for organic molecules). There is no example of any terrestrial organism (that I'm aware of) that actually converts heat into metabolic energy. And I have a BSci in Bio with a minor in Chem. I've taken Microbio, Zoology, Cell Bio, and Physiology, among others. I'm pretty sure one of those many professors would have mentioned such a novel organism.

I was actually specifically trying to avoid using that terminology in a conversation about biology and thermodynamics, since the usage in one (endo/ecto) is counter-intutive to the meaning in the other (exo/endo).

Stealing heat from the environment to make your metabolism work significantly better was the closest I could come to what the OP was asking for.

What I probably should have focused more on (although I did mention it) was that life doesn't just require maintaining a heat flow rate, but a (not necessarily constant) mass flow rate through the organism.

[ericgrau]

If anything, I'd think an animal would be more likely to be able to use thermal gradients than a bacterium because they have a much greater spatial extent. For a bacterium, thermal conduction is a really fast process because everything is so small.
It also produces far too little energy for an animal to use it. So I was thinking highly specialized bacteria that survive where nothing else can on a very low amount of energy. It's possible to turn the heat into pressure too and that could be retained better.

[NichG]

It also produces far too little energy for an animal to use it. So I was thinking highly specialized bacteria that survive where nothing else can on a very low amount of energy. It's possible to turn the heat into pressure too and that could be retained better.

Pressure might be hard since it requires gas-phase stuff. I guess you could play with the CO2(aq)/CO2(g) equilibrium? Another thought would be to use the fact that long polymers tend to contract at high temperatures due to entropic effects (if you heat a rubber band, for example, it shrinks). You could use that combined with a sort of molecular snap-lock to basically store potential energy in a molecular spring when you heat-cycle the system. At high temperature, the protein contracts and then crosslinks itself at some significant point using a disulfide bridge or something, then at low temperature the protein can't automatically re-expand until you break the disulfide bridge.

The thing is, that sort of mechanical energy is pretty hard to use for things the cell generally would want to do, such as setting up proton gradients. Maybe this is something that could be used as a mechanism for cheap thermotaxis (e.g. moving up and down temperature gradients)?

[Jacob.Tyr]

Just utilize the heat to drive the Haber–Bosch reaction and produce nitrogen fertilizer. Give the nitrogen to some crops, let them grow, eat them.

But seriously, this thread has gotten me really confused on the nature of life and energy (I study viruses for a living, I don't know if this makes it easier or harder to get me confused on this topic). For the most part organisms use energy, ATP, to move things. Moving ions and molecules against a gradient. The whole purpose of this, in my opinion, is to just set up situations in which DNA/RNA replication occurs at a faster rate than if these molecules and ions weren't concentrated. Energy is just used to "speed up" the rate at which life is occurring.

With that consideration, if you threw enough of the material components into a big stew I think you'd probably find a fairly slow process that fulfilled the A-B-C outlined previously just using ambient energy for the chemical processes. "a. self-replicating, b. capable of transmitting information between generations, and c. capable of evolution by selection". PCR was mentioned previously as fulfilling the exercise in using ambient heat to fuel life, and would go much faster than just letting things sit, if we're going for just hitting on these three marks. Powered solely by ambient heat, self-replicating molecules that are capable of transmitting information between generation, and there are fairly common errors that crop up just to screw with researchers. If any of these errors create molecules that self-replicate with higher-fidelity or stability, boom evolution by selection.

[NichG]

From what I can tell, the non-trivial thing is the 'transmit information' part of that trio. Autocatalysis appears to be ridiculously common, but often it can at best store a single bit ('present' or 'absent'). The sort of thing you need to get something like a family of different sequences that each replicate themselves faithfully (modulo errors) isn't clear to me, though it does seem you can get a sort of pseudo-heredity from stacked autocatalytic cycles (e.g. cycle 1 creates byproducts that can let cycle 2 be autocatalytic, which creates byproducts that let cycle 3 be autocatalytic), at least in some chemistries - the thing is that the results aren't generalized replicators, so there's a missing refinement where the evolvability of the thing expands massively due to being able to replicate 'any' sequence rather than just specific distributions.

[Jeff the Green]

Finally, depending on how generous your definition of 'life' is, the polymerase chain reaction can be thought of as a mechanism that uses heat as its input energy source. In PCR, you've got DNA chains and an enzyme which catalyzes their replication, but for the thing to work efficiently you have to temperature cycle the PCR chamber. That temperature cycling provides the energy that makes the whole thing go forward.

That's not how PCR works. You cycle between three temperatures. A high one first to denature the DNA (make the strands separate so it's a single helix), a low one for the primers (bits of DNA that mark the ends of the sequence you want to copy) to anneal to the DNA, and a medium one for the DNA polymerase to work and stick dNTPs (deoxynucleotide triphosphates) together to make the complementary strands. The energy comes from the dNTPs; phosphate groups are fine in pairs, but stick another one on and they become unhappy (high energy), so when the DNAPol sticks them together by removing a phosphate they enter a lower energy state. The heat's just there to give the right temperatures for each step since they won't all happen at the same temperature.

[NichG]

That's not how PCR works. You cycle between three temperatures. A high one first to denature the DNA (make the strands separate so it's a single helix), a low one for the primers (bits of DNA that mark the ends of the sequence you want to copy) to anneal to the DNA, and a medium one for the DNA polymerase to work and stick dNTPs (deoxynucleotide triphosphates) together to make the complementary strands. The energy comes from the dNTPs; phosphate groups are fine in pairs, but stick another one on and they become unhappy (high energy), so when the DNAPol sticks them together by removing a phosphate they enter a lower energy state. The heat's just there to give the right temperatures for each step since they won't all happen at the same temperature.

Okay, so I guess that example doesn't work then. I know you can build temperature cycling driven replicators in simulations, but if PCR is driven chemically then I don't know a real-world example of that kind of process.

[Max™]

Hmmm, wonder what you folks more familiar with biology would think of the only two vaguely relevant sci-fi organisms I can recall.

The Outsiders (Niven, from Ringworld and the Fleet of Worlds books) operate at very low temperatures and make use of shadows to set up a temperature differential between one portion of their body and another. Incidentally this is similar to creatures from a Baxter story on Pluto I think, where something like a cross between a spider and tree makes use of the temperature difference at day/night terminator in a similar fashion.

The Qax (Baxter, from the Xeelee Sequence stories) evolved as convection cells in massive swamp areas on their homeworld, with their information store/transfer being encoded in the convective cells themselves somehow.