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Answers
The Krebs cycle is simply another name for the Citric Acid Cycle, so named for the researcher who identified the complete cycle in 1937. This cycle describes a series of chemical reactions that take place in all aerobic organisms (aerobic meaning that they use oxygen to facilitate their metabolism, like we do).
The reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle) is a sequence of chemical reactions that are used by some bacteria to produce carbon compounds from carbon dioxide and water.
The reaction is the citric acid cycle run in reverse: Where the Krebs cycle takes complex carbon molecules in the form of sugars and oxidizes them to CO2 and water, the reverse cycle takes CO2 and water to make carbon compounds. This process is used by some bacteria to synthesise carbon compounds, sometimes using hydrogen, sulfide, or thiosulfate as electron donors.[1][2] In this process, it can be seen as an alternative to the fixation of inorganic carbon in the reductive pentose phosphate cycle which occurs in a wide variety of microbes and higher organisms.
The reaction is a possible candidate for prebiotic early-earth conditions and, so, is of interest in the research of the origin of life. It has been found that some non-consecutive steps of the cycle can be catalyzed by minerals through photochemistry,[3] while entire two and three-step sequences can be promoted by metal ions and iron (as reducing agent) under acidic conditions.[4]