study because HSPs found in hyperthermophiles are the most stable of their kind. HSPs are also able to prolong the life of a hyperthermophile even beyond its optimal growing temperature. By studying these proteins it may be possible to learn the mechanisms proteins use to stabilize other proteins, which may help in biosynthesis of new molecules. HSPs act as chaperone proteins that help enzymatic proteins maintain their proper conformation for longer than they would by themselves at such high temperatures. This is part of what allows P. fumarii to exist at temperatures that were long believed much too hot for life to exist. The most common organisms that harvest CO2 to build biomolecules are plants and photosynthetic bacteria. Those particular organisms use the Calvin cycle for their carbon fixation. However, P. fumarii and other similar organisms contain particular enzymes that allow them to harvest CO2 at temperatures well above those tolerated by plants and photosynthetic bacteria with slightly different mechanisms. The alternate pathways used by these extremophiles are either the rTCA cycle, 3-HP cycle, 3-HP/4-HP cycle, or DC/4-HP cycle. These are likely some of the first pathways to evolve because the bacteria and archaea who use them live in environments that mirror the early Earth environments. Therefore, it is likely that these are some of the first carbon fixation pathways to evolve. The rTCA cycle is usually seen in organisms living at temperatures between 20 and 90 °C, while organisms living at temperatures above 90 °C most often use the DC/4-HP cycle. The 3-HP/4-HP cycle is most often used by thermoacidophiles in the Sulfolobales order. The rTCA, 3-HP and 3-HP/4-HP cycles are all very important to particular extremophiles, but the DC/4-HP cycle is the one used by P. fumarii. For this reason, this article discusses the DC/4-HP cycle in further depth, and includes more information on two alternate carbon fixation cycles, with associated links. The DC/4-HP cycle is a combination of the rTCA cycle and the 4-HP half of the 3-HP/4-HP cycle. Both the DC/4-HP and the 3-HP/4-HP cycles begin with acetoacetyl-CoA, which is a molecule containing two acetyl-CoA groups allowing this pathway to run twice with only one starting molecule. The acetyl-CoA is then converted to pyruvate, then PEP. The DC/4-HP cycle then follows the rTCA cycle, converting PEP to oxaloacetate, then to malate, then fumarate, and then to succinate. Once the succinate is formed, the pathway follows the