The 3-Hydroxypropionate/4-hydroxybutyrate cycle, also known as the 3HP/4HB cycle, is a specialized carbon fixation process used by some archaea, including Thermoproteota.[1] For these organisms to survive and grow autotrophically in hostile settings, such as hydrothermal vents, this cycle is essential.[2] Carbon dioxide (CO2) is effectively transformed by the process into organic chemicals like acetyl-CoA, which can then be utilized for growth and energy production.[3] This route is specific to organisms that fix CO2 in high-temperature, low-oxygen settings, in contrast to the more well-known Calvin cycle which does not perform as well at fixing CO2 under these conditions.[2]