Which groups of organisms typically use the Calvin cycle? Which groups use reverse TCA? How are these two mechanisms of CO2 fixation related to different types of phototrophy conducted by these different groups of organisms?

Question

What will be an ideal response?

Answer 1

The Calvin cycle is performed by several categories of organisms: bacteria and chloroplasts. It is not found in archaea or in the cytoplasm of eukaryotes. Therefore, it appears to have evolved after the divergence of the three domains of life. It requires a large amount of energy (9 ATP x 2 cycles to generate 2xG-3-P to make one 6-carbon glucose molecule) and reducing power (NADH) to run, so is typically coupled to oxygenic photosynthesis (i.e. "Z" pathway).

In some anaerobic bacteria and archaea, the entire TCA cycle runs in reverse. This is believed to be the most ancient form of CO2 fixation - the original cycle of biomass generation in the ancestors of all three living domains. In the reverse TCA cycle, the acetyl group enters biosynthesis, recycling the coenzyme A. The Reverse TCA cycle requires less energy to run (3 ATP as opposed to 9 ATP in the Calvin Cycle), so it is usually used by organisms that don't use the Z pathway for photosynthesis. For example, reverse TCA is used by Chlorobium (green sulfur bacteria), which conducts anoxygenic photosynthesis by photolyzing H2S (electron donor) using only photosystem I.