Introduction
Utilization
Conclusions
Fermentation
Non-photosynthetic organisms get their energy from glucose. If an organism uses oxygen, the glucose will be used through respiration. If the organism is anaerobic, the glucose will be used through fermentation. Pyruvate is first made from glucose. Then, the pyruvate can react further. One of the more familiar fermentation processes is alcoholic fermentation. In this process, pyruvate, an intermediate, produces ethanol and carbon dioxide. However, other fermentation processes can produce molecular hydrogen.
Dark fermentation is one process being explored for hydrogen production. In this process, pyruvate reacts to form acetyl-CoA and carbon dioxide. In this reaction, ferredoxin acts as the oxidizing agent. In the next step, ferredoxin is oxidized back to its original form, and molecular hydrogen is formed. The net reaction is:
C6H12O6 + 2H2O = 2CH3COOH + 2CO2 + 4H2
Or
C6H12O6 = CH3CH2CH2COOH + 2CO2 + 2H2
These hydrogen yields are the ideal yields, and actual experiments have given lower yields. This is because often the hydrogen is recycled in cells. The amount of hydrogen produced from this fermentation reaction is too low to make sense economically.
However, attempts are being made to increase the hydrogen yield. Some methods would involve genetically modifying the fermentative bacteria. These modifications could include overexpression of the enzymes that produce glucose from more complex carbohydrates, or eliminating the enzymes that uptake hydrogen. Another issue is that acids are produced during fermentation, which decreases the pH in the organism. If the pH gets too low, bacteria will try to reduce the concentration of H+, and they will not produce H2. Therefore, bacteria that can function at lower pH could have a higher hydrogen yield, or the reactions that produce the acids could be blocked.
Photosynthetic bacteria also produce hydrogen from small organic acids. If these bacteria are combined with fermentative bacteria, the fermentative bacteria could produce the small organic acids, which the photosynthetic bacteria could then use. Since photosynthetic bacteria get energy from light, they are capable of undergoing reactions with higher energy barriers, so can get a higher hydrogen yield.
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