The energy obtained by biological oxidation of autotrophic microorganisms is mainly used for CO2 fixation. In microorganisms, there are three ways to fix CO2 *, namely calvin cycle, anaerobic acetyl-CoA pathway and reducing tricarboxylic acid cycle, which are briefly described as follows. 1.Calvincycle is also called Calvin-Bussham cycle, rubp pathway or pentose reduction cycle. This cycle is the main way for photoautotrophs and chemoautotrophs to fix CO2. Phosphoribokinase and ribulose carboxylase are the only enzymes in this pathway. In addition to green plants, cyanobacteria and most photosynthetic bacteria, the organisms using calvin cycle for dark fixation of CO2 also include all aerobic autotrophic bacteria, so it is very important.

The whole process of calvin cycle can be divided into three stages.

(1) Carboxylation Reaction Tri-ketose-1, 5- diphosphate (RU- 1, 5-P) immobilized three CO2 by ribulose diphosphate carboxylase, and converted it into six 3- phosphoglyceric acid molecules.

In the cycle, this basic reaction is carried out three times, and the net yield of 1 C3 molecule can be obtained by using the doped three CO2 molecules.

(2) Reduction Reaction After carboxylation reaction, the hydroxyl group on 3- phosphoglyceric acid was immediately reduced to aldehyde group. This transformation is carried out through the reverse EMP pathway, that is, it is phosphorylated to 1, 3- diphosphoglyceric acid by 3- phosphoglycerate kinase and ATP, and then reduced to glyceraldehyde -3- phosphate by NAD(P)H2 in glyceraldehyde-3-phosphate dehydrogenase. It takes 6 ATP and 6 NAD(P)H2 to form 1 glyceraldehyde -3- phosphate molecule.

(3) Regeneration of CO2 receptor In the cycle, except the net production of 1 glyceraldehyde -3- phosphate can be further reversed by EMP pathway to form glucose molecules, the other five molecules undergo complex reactions, consume 3 ATP, and finally regenerate 3 ribulose-1, 5- diphosphate molecules, thus accepting CO2 molecules again.

If calculated according to the production of 1 glucose molecule, the general formula of calvin cycle is:

6 CO2+ 12 nad(P)H2+ 18 ATP→c6h 12o 6+ 12 nad(P)+ 18 ADP+ 18 pi

Now the streamlined calvin cycle path is shown in Figure 6-37.

The initial product of calvin cycle is glyceraldehyde -3- phosphate, and then various components needed by cells are further synthesized, namely:

2. Anaerobic acetyl coenzyme A pathway, also known as activated acetyl coenzyme A pathway. This is a new autotrophic CO2 reduction pathway found in some strict anaerobic bacteria that can utilize hydrogen in recent years, including methanogenic bacteria, sulfate-reducing bacteria and acetic acid-producing bacteria. They do not have calvin cycle, because they play a role in reducing CO2 through acetyl-CoA pathway. The experiment was carried out by thermophilic methanobacterium and isotope method. The results of the preliminary study are shown in Figure 6-38.

As can be seen from Figure 6-38, in the CO2 reduction pathway of anaerobic acetyl coenzyme A, 1 molecule CO2 is firstly reduced to methanol level (methyl -X) by reducing force [H] (transferred by enzyme containing F420 factor ① or NADP), and the other molecule CO2 is reduced to carbon monoxide by carbon monoxide dehydrogenase. Acetyl -X is produced by carboxylation of methyl -X, and then acetyl-CoA is formed. Catalyzed by pyruvate synthase, acetyl-CoA receives the third CO2 molecule and carboxylates it to pyruvate. Then all kinds of organic substances needed by cells can be synthesized from pyruvate through known metabolic pathways.

3. The way to reduce TCA cycle is to reduce TCA cycle, and the way to fix CO2 only exists in a few photosynthetic bacteria, such as green thiosulfate bacteria. In this way, CO2 is fixed by reductive carboxylation of succinyl coenzyme A, that is:

It is worth pointing out that after comparing the above three types of autotrophic microorganisms, it is found that anaerobic CO2 fixation is more economical and effective than aerobic CO2 fixation. For example, the synthesis of 1mol glyceraldehyde -3- phosphate from 3molCO2 via anaerobic acetyl coenzyme A pathway only requires 3 mol ATP; By reducing TCA cycle, 5 moles of total phosphorus is needed accordingly; And calvin cycle needs 9molATP.