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 Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...

. Many challenges characterize this technology, including those intrinsic to H₂, such as storage and transportation of a noncondensible gas. Hydrogen producing organisms are poisoned by O₂. Yields of H₂ are often low.

Biochemical principles

The main reactions involve

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

of sugars. Important reactions start with glucose, 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 CH3COOH + 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: :C6H12O6 + 2 H2O -> 2 CH3COOH + 2 HCOOH + 2 H2 These reactions are exergonic by 216 and 209 kcal/mol, respectively. H₂ production is catalyzed by two hydrogenases. One is called eFehydrogenase; the other is called iFehydrogenase. Many organisms express these enzymes. Notable examples are members of the genera

Clostridium ''Clostridium'' is a genus of anaerobic, Gram-positive bacteria. Species of ''Clostridium'' inhabit soils and the intestinal tract of animals, including humans. This genus includes several significant human pathogens, including the causative ag ...

, Desulfovibrio, Ralstonia, and the pathogen '' Helicobacter''. ''

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

'' is the workhorse for genetic engineering of hydrogenases. It has been estimated that 99% of all organisms utilize

dihydrogen 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, and ...

(H₂). Most of these species are microbes and their ability to use H₂ as a metabolite arises from the expression of H₂ metalloenzymes known as hydrogenases. Hydrogenases are sub-classified into three different types based on the active site metal content: iron-iron hydrogenase, nickel-iron hydrogenase, and iron hydrogenase. ActiveSitesCorrected

Production by algae

The biological hydrogen production with algae is a method of photobiological water splitting which is done in a Closed system, closed Bioreactor#Photobioreactor, photobioreactor based on the Hydrogen production, production of hydrogen as a solar fuel by algae. Algae produce hydrogen under certain conditions. In 2000 it was discovered that if ''Chlamydomonas reinhardtii, C. reinhardtii'' algae are deprived of sulfur they will switch from the production of oxygen, as in normal photosynthesis, to the production of hydrogen.

Photosynthesis

Photosynthesis in cyanobacteria and green algae splits water into hydrogen ions and electrons. The electrons are transported over ferredoxins. Hydrogenase#.5BFeFe.5D hydrogenase, Fe-Fe-hydrogenases (enzymes) combine them into hydrogen gas. In ''Chlamydomonas reinhardtii'' Photosystem II produces in direct conversion of sunlight 80% of the electrons that end up in the hydrogen gas. Light-harvesting complex photosystem II light-harvesting protein LHCBM9 promotes efficient light energy dissipation. The Fe-Fe-hydrogenases need an Anaerobic organism, anaerobic environment as they are inactivated by oxygen. Fourier transform infrared spectroscopy is used to examine metabolic pathways. In 2020 scientists reported the development of algal-cell based micro-droplets for multicellular spheroid microreactor, microbial reactors capable of producing Hydrogen fuel#Production, hydrogen alongside either oxygen or CO₂ via photosynthesis in daylight under air. Enclosing the microreactors with synergistic bacteria was shown to increase levels of hydrogen production. CC-BY icon

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Specialized chlorophyll

The chlorophyll (Chl) antenna size in green algae is minimized, or truncated, to maximize photobiological solar conversion efficiency and H₂ production. The truncated Chl antenna size minimizes absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic productivity by the green alga mass culture.

Economics

It would take about 25,000 square kilometre algal farming to produce biohydrogen equivalent to the energy provided by gasoline in the US alone. This area represents approximately 10% of the area devoted to growing soya in the US.

Bioreactor design issues

* Restriction of photosynthetic hydrogen production by accumulation of a proton gradient. * Competitive inhibition of photosynthetic hydrogen production by carbon dioxide. * Requirement for bicarbonate binding at photosystem II (PSII) for Photosynthetic efficiency, efficient photosynthetic activity. * Competitive drainage of electrons by oxygen in algal hydrogen production. * Economics must reach competitive price to other sources of energy and the economics are dependent on several parameters. * A major technical obstacle is the efficiency in converting solar energy into chemical energy stored in molecular hydrogen. Attempts are in progress to solve these problems via bioengineering.

History

In 1933, Marjory Stephenson and her student Stickland reported that cell suspensions catalysed the reduction of methylene blue with H₂. Six years later, Hans Gaffron observed that the green photosynthetic alga ''Chlamydomonas reinhardtii'', would sometimes produce hydrogen. In the late 1990s Anastasios Melis discovered that deprivation of sulfur induces the alga to switch from the production of oxygen (normal photosynthesis) to the production of hydrogen. He found that the enzyme responsible for this reaction is hydrogenase, but that the hydrogenase lost this function in the presence of oxygen. Melis also discovered that depleting the amount of sulfur available to the algae interrupted their internal oxygen flow, allowing the hydrogenase an environment in which it can react, causing the algae to produce hydrogen. ''Chlamydomonas moewusii'' is also a promising strain for the production of hydrogen.

Industrial hydrogen

Competing for biohydrogen, at least for commercial applications, are many mature industrial processes. Steam reforming of natural gas - sometimes referred to as steam methane reforming (SMR) - is the most common method of producing bulk hydrogen at about 95% of the world production. :CH4 + H2O <-> CO + 3 H2

References

External links

DOE - A Prospectus for Biological Production of HydrogenMaximizing Light Utilization Efficiency and Hydrogen Production in Microalgal CulturesDIY Algae/Hydrogen Bioreactor 2004EERE-CYCLIC PHOTOBIOLOGICAL ALGAL H2-PRODUCTION
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