Microbiology and Enzymology

Abstract

Microorganisms present in biological hydrogen production system can be categorized into hydrogen producers and non-hydrogen producers. Dark fermentative microorganisms are rich in species and widely distributed. In this chapter, the microbiology, biochemistry, and enzymology for biological hydrogen production were briefly introduced. The inoculum for dark fermentative hydrogen production system can be mixed cultures, like anaerobic sludge, compost, soil, leachate, etc., or pure cultures, like Clostridium sp., Enterobacter sp., etc. In the practical application, mixed cultures are more widely used because of the broader choice of feedstock, cheaper operation, and easier control. Through the metabolism of bacteria present in the system, complex polymers are hydrolyzed to glucose. Subsequently, pyruvate is produced via the glycolytic pathway to generate adenosine triphosphate (ATP). And then, according to hydrogen-producing strains present in the system (obligate anaerobes like Clostridia or facultative anaerobic enteric bacteria like E. coli.), pyruvate is involved in two different biochemical reactions leading to the formation of hydrogen. According to the main volatile fatty acids, widely accepted fermentation types include butyrate-type fermentation, propionate-type fermentation, ethanol-type fermentation, and mixed-type fermentation. The key enzyme involved in catalyzing H2 formation from protons or oxidation to protons is hydrogenase. According to the metal content of the active site, the hydrogenases can be categorized into three classes, [Fe]-, [FeFe]-, and [NiFe]-hydrogenases. To genetically and metabolically modify the hydrogenase is a very promising strategy to improve the biological hydrogen production from water or organic substances through optimizing the flow of reducing equivalents to it by redirecting the electron paths. Metabolic engineering could be used to modify metabolic pathways to increase the biological hydrogen production, to overcome limiting factors for hydrogen production in various systems by increasing the flow of electrons to hydrogen-producing pathways, increasing substrate utilization, and engineering more efficient and/or oxygen-resistant hydrogen-evolving enzymes.