What biochemical reactions might be employed by plants to harvest heat energy when light energy is scarce or even missing? What is the temperature range over which this biochemical reaction can work?
Background: A red-dwarf M-star, emitting most of its energy in the red and infrared side of the spectrum. The planet is tidally locked to its star. Some red-shifted light irradiates on one side of the planet while the other side is in perpetual darkness...
Constraints: According to the laws of thermodynamics, you cannot convert heat into useful energy, you can convert HEAT DIFFERENCE into useful energy. Alas, tidally-locked planets don't seem to allow much temperature fluctuations which create enough HEAT DIFFERENCE in one area. I will soon suggest possible solutions to circumvent the problem without violating the laws of thermodynamics.
CREATING HEAT DIFFERENCE IN THE FIRST PLACE
1- Capturing radiative heat: A leaf exposed to the heat of the star absorbs more heat. If the leaf is black, it heats-up above the AMBIENT AIR TEMPERATURE. The other side of the leaf loses heat and there is a heat difference between the two sides of the leaf. Leaf thickness is a significant factor, here. This strategy works well for land plants and plants floating on the high seas.
2- Doing the "Yo-Yo": This is a good strategy for aquatic plants on the dark side (receiving no radiative heat from the star): If the plants had a controllable buoyancy bladder, they can do the yo-yo in cycles: By adjusting the bladder's buoyancy, the plant moves up and down to juggle between currents of hot and cold water.
BIOCHEMICAL PRINCIPLE OF HEAT ENERGY
This is what I'm looking for -- What chemical reactions could harness heat energy in that manner? The thermo-chemical energy harvesting is a reversible process which works as follows: Assuming there are two substances, named A and B. The two molecules recombine to form a molecule AB.
Upon exposure to heat, the molecule AB decomposes into two constituents A and B:
AB + Heat --> A + B
The molecules cannot recombine as long as heat exposure continues. Upon exposure to colder temperatures, the opposite happens:
A + B --> AB + energy.
This recombination can be used as an electron donor, just as oxygen and hydrogen combine to make water in a fuel cell. The electron donor drives photosynthesis.
HARNESSING HEAT DIFFERENCE AS SOURCE OF ENERGY
There are two modes of making use of heat difference:
Passive transfer: The example of aquatic plants being able to move up and down between hot and cold regions. The constituents A and B are "locked" via specialized enzymes so that the plant allows their recombination in a controlled manner. Single-celled organisms do the same as they are carried away between hot and cold currents.
Active transfer: Plants under the red sun may have the equivalent of blood vessels moving a fluid between the side facing the sun (absorbing radiative heat) and the other side (radiating heat back to the environment). This means that any "plant" will be more complex and comparable to animals.