(i) The Calvin Cycle: Certain photosynthetic bacteria e.g., Rhodospirillum rubrum make use of this cycle to synthesize carbohydrates by reducing CO 2. The reverse Krebs cycle, also known as reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard Calvin-Benson cycle for carbon fixation. Purple nonsulfur bacteria grow photoheterotrophically by using light for energy and organic compounds for carbon and electrons. The Calvin-Benson-Bassham cycle (Calvin cycle) represents the most important extant autotrophic carbon fixation pathway (43, 50). Where did the extra ATP molecules come from. The Calvin cycle, elucidated by American biochemist Melvin Calvin, is the most widely distributed of these pathways, operating in plants, algae, photosynthetic bacteria, and most aerobic lithoautotrophic bacteria. In autotrophic bacteria, where are the enzymes located that can carry on organic synthesis? For many years, the doctrine prevailed that the Calvin-Benson-Bassham (CBB) cycle is the only biochemical autotrophic CO2 fixation pathway of significance in the ocean. The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathway used for fixation of CO 2, is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. It exists in plants and microalgae, as well as photoautotrophic and chemoautotrophic bacteria. The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%–50% of the total soluble protein in plant chloroplasts. However, since these bacte­ria do not store or utilise carbohydrates, lesser amount of sugar photophates have been detected in them during photosynthesis. The Calvin cycle, also known as the Calvin Benson cycle, is an integral part of the process of photosynthesis in plants, algae, and photosynthetic bacteria. In bacteria: Autotrophic metabolism …the reductive pentose phosphate (Calvin) cycle, the reductive tricarboxylic acid cycle, and the acetyl-CoA pathway. ... Calvin cycle in 1 hour. Despite its global significance, it is restricted to organisms with high-energy yield from a chemotrophic or phototrophic lifestyle. Disrupting the activity of the CO 2-fixing Calvin cycle enzyme, ribulose 1,5-bisphosphate carboxylase (RubisCO), prevents photoheterotrophic growth unless an electron acceptor is provided or if cells can dispose of electrons as H 2. As shown in Fig. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales ) [11] and anaerobic archea . The Calvin cycle is the principal mechanism that leads to the conversion of carbon dioxide into sugars by plants, algae, photosynthetic bacteria, and certain other bacteria that use chemicals as an energy source instead of light. cyclic electron flow. The Calvin cycle requires various enzymes to ensure proper regulation occurs and can be divided into three major phases: carbon fixation, reduction, and; regeneration of ribulose. This energy requirement is less than half of the Calvin cycle, giving bacteria that use the reverse TCA cycle, such as Epsilonproteobacteria, an advantage [26] [27]. (ii) Reductive Carboxylic Acid Cycle: They form an integral part of ecosystems by making an otherwise unavailable form of carbon available to other organisms, a central component of the global carbon cycle. There are various organisms that utilize the Calvin cycle for production of organic compounds including cyanobacteria and purple and green bacteria. Photosynthetic Pigments. 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