Fermentative hydrogen production is the fermentative conversion of organic substrates to H₂. Hydrogen produced in this manner is often called

biohydrogen Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass. Many challenges characterize this technology, including those intrinsic to ...

. The conversion is effected by

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

and

protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

, which employ

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s. Fermentative hydrogen production is one of several anaerobic conversions.

Dark vs photofermentation

Dark fermentation reactions do not require light energy. These are capable of constantly producing

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

from organic compounds throughout the day and night. Typically these reactions are coupled to the formation of carbon dioxide or formate. Important reactions that result in hydrogen production start with

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

, which is converted to

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

: :C₆H₁₂O₆ + 2 H₂O → 2 CH₃CO₂H + 2 CO₂ + 4 H₂ A related reaction gives

formate Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ''Ull ...

instead of

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

: :C₆H₁₂O₆ + 2 H₂O → 2 CH₃CO₂H + 2 HCO₂H + 2 H₂ These reactions are exergonic by 216 and 209 kcal/mol, respectively. Using

synthetic biology Synthetic biology (SynBio) is a multidisciplinary area of research that seeks to create new biological parts, devices, and systems, or to redesign systems that are already found in nature. It is a branch of science that encompasses a broad ran ...

, bacteria can be genetically altered to enhance this reaction. Photofermentation differs from dark fermentation, because it only proceeds in the presence of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

Electrohydrogenesis Electrohydrogenesis or biocatalyzed electrolysis is the name given to a process for generating hydrogen gas from organic matter being decomposed by bacteria. This process uses a modified fuel cell to contain the organic matter and water. A smal ...

is used in

microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxid ...

Bacteria strains

For example, photo-fermentation with ''

Rhodobacter sphaeroides ''Rhodobacter sphaeroides'' is a kind of purple bacterium; a group of bacteria that can obtain energy through photosynthesis. Its best growth conditions are anaerobic phototrophy (photoheterotrophic and photoautotrophic) and aerobic chemoheterot ...

'' SH2C can be employed to convert small molecular fatty acids into hydrogen.High hydrogen yield from a two-step process of dark-and photo-fermentation of sucrose
/ref> ''Enterobacter aerogenes'' is an outstanding hydrogen producer. It is an anaerobic facultative and mesophilic bacterium that is able to consume different sugars and in contrast to cultivation of strict anaerobes, no special operation is required to remove all oxygen from the fermenter. ''E. aerogenes'' has a short doubling time and high hydrogen productivity and evolution rate. Furthermore, hydrogen production by this bacterium is not inhibited at high hydrogen partial pressures; however, its yield is lower compared to strict anaerobes like ''Clostridia''. A theoretical maximum of 4 mol H₂/mol glucose can be produced by strict anaerobic bacteria. Facultative anaerobic bacteria such as ''E. aerogenes'' have a theoretical maximum yield of 2 mol H₂/mol glucose.

References

External links

HYDROGEN PRODUCTION VIA DIRECT FERMENTATIONDevelopments and constraints in fermentative hydrogen production
{{DEFAULTSORT:Fermentative Hydrogen Production Biofuels technology Catalysis Environmental engineering Fermentation Hydrogen biology Hydrogen economy Hydrogen production

Dark vs photofermentation

Bacteria strains

See also

References

External links