Is there a heat-based alternative to photosynthesis?

Question

I can imagine one main disadvantage of a process using sunlight to metabolize sugars would be that wherever there is little/no sun (nighttime), plants don't grow. Heat is more abundant than sunlight and could theoretically be an alternative (think units buried deep underground near geothermally active areas).

Is there a heat-based (artificial or natural) replacement for photosynthetic processes?

Note: my understanding of chemistry involving molecules with more than a few atoms is severely limited.

Answer 1

First, it's important to distinguish between the synthesis of sugars from carbon dioxide (photosynthesis) and the catabolism of those sugars to provide energy for growth, which is a process similar to that employed by all sugar consuming organisms. Some plants are able to acquire sugar from other sources (typically through parasitism of other plants or fungi) and so are able to grow just fine without photosynthesis.

Similarly, there are quite a few alternative metabolic pathways that organisms can use to provide energy for growth that do not involve metabolism of sugars. Almost any soluble organic molecule can be used by some organism somewhere, and a many organisms are able to oxidize inorganic compounds such as metals or hydrogen or sulfide for energy. (They still need a supply of organic compounds as biosynthetic precursors, but do not derive energy for growth from them.)

I am going to assume, however, that your specific question is whether the synthesis from carbon dioxide and water of a reduced organic compound which can support life can be achieved with heat (ie infrared radiation) rather than higher energy visible radiation (ie sunlight).

The wavelengths most important for photosynthesis fall between 400 and 700 nm. The infrared radiation emitted even by a quite hot object on earth will have a wavelength of at least a couple of micrometers. It is an unfortunate consequence of quantization of electron energy levels that one cannot substitute a lot of low energy (long wavelength) radiation for a small amount of high energy (short wavelength) radiation in a reaction, even if the amount of energy provided is equivalent. (There are some minor exceptions such as two-photon absorbances, but those are very special cases.) So we can conclude that it is at least not possible to drive existing photosynthetic processes with heat rather than light.

The follow-up question is whether it is possible through other reactions. A logical choice is use of electricity to reduce the carbon dioxide, since electricity can be produced using heat. One of the simplest organic compounds to make from carbon dioxide (and which requires a fairly low voltage) is methanol, which can be produced with voltages around 300 mV. Many bacteria and archaebacteria are able to metabolize methanol. Ethanol is even more easily metabolized and can be produced with similar voltages. Since bacterial metabolic pathways exist for sugar synthesis, one could theoretically engineer a bacterium to direct much of the energy from ethanol or methanol metabolism towards the synthesis of glucose, ultimately achieving the same end as photosynthetic light reactions.

An electric potential can be produced from heat via a standard steam turbine generator, and geothermal heat sources are sufficient for this purpose, so using that electricity to synthesize methanol and feeding that to glucose-synthesizing bacteria would technically achieve your goal of heat-mediated synthesis of sugar from carbon dioxide.

Answer 2

Note: my understanding of chemistry involving molecules with more than a few atoms is severely limited.

An answer to your question requires understanding of concepts in physical chemistry and biochemistry, and careful use of technical terms such as "metabolize" and "heat". Thus, this answer might be a bit frustrating today, but maybe helpful to motivate further study.

I can imagine one main disadvantage of a process using sunlight to metabolize sugars would be that wherever there is little/no sun (nighttime), plants don't grow.

When we say "metabolize a substance", usually we mean starting with that substance and turning it into something else. The process that makes use of sunlight results in the biosynthesis of sugars. The plant (and this is something you would usually not say) is "metabolizing water and carbon dioxide to produce dioxygen and sugar".

Over their lifespan, photosynthetic plants utilize light to grow. However, they make sugars during the day and store them (and distribute them to non-photosynthetic parts of the plant, e.g. the roots). The plant grows just fine in the absence of light as long as there is some light every now and then. So leaves can grow at night, and roots can grow in the absence of sun exposure. In fact, seeds grow to plants using the nutrients (usually mostly starch or fat) stored in the seeds combined with oxygen in air while photosynthesis is not yet possible.

So plants can grow without light, but photosynthetic plants rely on light to capture most of the (Gibbs) energy that they expend.

Heat is more abundant than sunlight and could theoretically be an alternative (think units buried deep underground near geothermally active areas).

"Heat" is a technical term reserved for the transfer of thermal energy. From the context of the question, I would think you mean "thermal energy".

Sunlight, wind, and electricity are more "useful" sources of energy than thermal energy. You can turn sunlight, wind, and electricity into thermal energy, but you can't "extract" thermal energy from a sample to make e.g. electricity. What is possible is to make e.g. electricity using a hot and a cold sample, where the cold sample gets heated up. You see that in coal, gas or oil-fired electrical power stations, that often are next to rivers to have access to cooling water.

How is this related to making sugar and dioxygen from carbon dioxide and water? This direction of the reaction goes away from equilibrium, and you have to do work to force it in this direction. Work is an umbrella term for electrical work, photochemistry, or mechanical work. (The same is true for making heat flow from a cold to a hot sample, like a refrigerator.)

Is there a heat-based (artificial or natural) replacement for photosynthetic processes?

Yes, you could use a cold and a hot reservoir to make some electricity (for example with a turbine), use that to electrolyze water to dioxygen and hydrogen, and let the hydrogen react with the carbon dioxide to make molecules like formaldehyde or methanol or methane. From there, you still have to make sugar. There are probably some organism that are able to do that in the absence of sunlight.

What you can't do is cool down a hot substance, and use the energy to turn carbon dioxide and water into sugar and dioxygen.

[from the comments] There are devices such as photothermovoltaic cells or thermoradiative cells that can input heat and output electricity.

Yes, these hot-of-the-press devices are pretty cool (pun intended).

The radiative heat part would not work if the system were at thermal equilibrium. So there is still heat transferred from a hot to a cold part. When they calculate the efficiency, they get a familiar result.

So while there are some technical advantages of this over a Carnot cycle, the second law still governs this technology.

Answer 3

Limits on life powered by heat engines

One strict interpretation of your question is whether living organisms could use a heat engine to power their metabolism. Heat engines convert a temperature difference between two bodies to useful work. Hypothetical organisms that live this way could use this work to power chemical reactions that aren't prone to "go" without external power, like protein biosynthesis, growth and division, DNA replication, and other fundamental biochemical process.

There's no fundamental physical rule or law that prohibits this, but (a) all known organisms are made of cells, (b) cells are very tiny, and (c) in natural environments, it is very rare to find temperature differences of appreciable magnitude on the length scale of a cell. Thus its unlikely that there were many environments on early Earth that would favor the evolution of a heat engine.

To get quantitative:

1. Let's suppose the hot temperature of a heat engine was 70 ° C = 343 K, and the cold temperature 20 ° C = 293 K.

2. The hot region and the cold region are separated by about 1 μm.

3. There is a hypothetical cell of around 1 μm in diameter (this is the size of many modern bacteria and archaea) set right between the $T_H$ zone the and the $T_C$ zone poised to use this temperature difference as a heat engine. The temperature gradient required in this case is $\frac{\pu{50 K}}{\pu{10^{-6} m}}$ for 50 million Kelvins per meter.

4. If the cell, like all known life, required water to work, this temperature gradient is not sustainable. Water has a thermal diffusivity of $\alpha = \frac{\pu{0.14 mm^2}}{\pu{s}}$, which means it would take (very approximately) $\frac{L^2}{\alpha} \approx \frac{\pu{(10^{-6} m)^2}}{\pu{0.14\times 10^{-3} m^2//s}} \approx \pu{10 ns}$ for the temperature gradient to smooth itself out in the absence of very large external inputs. Ten nanoseconds isn't very long for these hypothetical cells to be able to do any useful work!

A way out

Photosynthesis short-circuits this calculation because it is effectively using sunlight (effective temperature maybe ~ 5000 K) to do this work, because sunlight isn't in local thermal equilibrium with most solid materials on the Earth's surface.

Life based on sunlight-free chemical processes

Life can certainly live without sunlight. It's not as obvious a statement as you might think though -- oxygen in our atmosphere comes from photosynthesis, so the lives of most familiar organisms (animals, fungi, etc) depends completely on sunlight, because they depend on oxygen for respiration.

However, certain types of chemoautotrophic bacteria can live without oxygen. For example, 2.8 kilometers below the Earth's surface, scientists isolated a microbe called Desulforudis audaxviator, which probably obtains energy for its metabolism by using the hydrogen, sulfate, and formate formed from water and rocks by radiolysis from naturally occurring radioactive decay, primarily from uranium.

This isn't directly a type of metabolism powered by "heat", but since Earth's heat comes primarily from radioactive decay of heavy elements, and since this life is using some of the byproducts of that decay, maybe it's close enough?

Answer 4

The mechanism for photosynthesis has too many steps to be accomplished thermally from just CO2 and water, however, simple reactions involving CO2 and H2O can be promoted thermally. Given enough time, energy, and varied conditions more complicated molecules will eventually be formed. It is thought that is what happened [is happening] over several billion years.