Biohydrogen is H2 that is produced biologically.
Besides the promising possibilities of biological hydrogen production, many challenges characterize this technology. First challenges include those intrinsic to H2, such as storage and transportation of an explosive noncondensible gas. Additionally, hydrogen producing organisms are Oxidative stress and yields of H2 are often low.
Biochemical principles
The main reactions driving hydrogen formation involve the oxidation of substrates to obtain electrons. Then, these electrons are transferred to free
to form molecular hydrogen. This proton reduction reaction is normally performed by an enzyme family known as
.
In heterotrophic organisms, electrons are produced during the fermentation of sugars. Hydrogen gas is produced in many types of fermentation as a way to regenerate NAD+ from NADH.
- C6H12O6 + 2 H2O -> 2 CH3COOH + 2 CO2 + 4 H2
A related reaction gives
formate instead of
carbon dioxide:
- C6H12O6 + 2 H2O -> 2 CH3COOH + 2 HCOOH + 2 H2
These reactions are exergonic by 216 and 209 kcal/mol, respectively.
It has been estimated that 99% of all organisms utilize or produce dihydrogen (H2). Most of these species are microbes and their ability to use or produce H2 as a metabolite arises from the expression of H2 known as hydrogenases.[
]
Enzymes within this widely diverse family are commonly sub-classified into three different types based on the active site metal content: FeFe-hydrogenases (iron-iron), NiFe-hydrogenases (nickel-iron) hydrogenases, and Fe-hydrogenases (iron-only).
Production by algae
The
biological hydrogen production with
algae is a method of photobiological
water splitting which is done in a
Closed system photobioreactor based on the production of hydrogen as a
solar fuel by
algae.
[ 2013 - Gimpel JA, et al Advances in microalgae engineering and synthetic biology applications for biofuel production] produce hydrogen under certain conditions. In 2000 it was discovered that if
C. reinhardtii algae are deprived of
sulfur they will switch from the production of
oxygen, as in normal
photosynthesis, to the production of hydrogen.
[ Wired-Mutant Algae Is Hydrogen Factory ]
Green algae express FeFe hydrogenases, being some of them considered the most efficient hydrogenases with turnover rates superior to 104 s−1. This remarkable catalytic efficiency is nonetheless shadowed by its extreme sensitivity to oxygen, being irreversibly inactivated by O2
Photosynthesis
Photosynthesis in
cyanobacteria and
green algae splits water into hydrogen ions and electrons. The electrons are transported over
.
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.
In 2020 scientists reported the development of algal-cell based micro-emulsion for multicellular spheroid microreactor capable of producing 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 via reduction of O2 concentrations.[ Available under CC BY 4.0.]
Improving production by light harvesting antenna reduction
The
chlorophyll (Chl) antenna size in green algae is minimized, or truncated, to maximize photobiological solar conversion efficiency and H
2 production. It has been shown that Light-harvesting complex photosystem II light-harvesting protein LHCBM9 promotes efficient light energy dissipation.
The truncated Chl antenna size minimizes absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic efficiency when the green alga are grown as a mass culture in bioreactors.
Economics
With current reports for algae-based biohydrogen, 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.
[ Growing hydrogen for the cars of tomorrow]
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 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.
Production by cyanobacteria
Biological hydrogen production is also observed in
Diazotroph cyanobacteria. This microorganisms can grow forming filaments. Under nitrogen-limited conditions some cells can specialize and form
, which ensures an anaerobic intracellular space to ease N
2 fixation by the
nitrogenase enzyme expressed also inside.
Under nitrogen-fixation conditions, the nitrogenase enzyme accepts electrons and consume ATP to break the triple dinitrogen bond and reduce it to ammonia.
N2 + 8 H+ + 8NAD(P)H + 16 ATP-> 2 NH3 + H2 + 16 ADP + 16 Pi + 8 NAD(P)+
Nevertheless, since the production of H2 is an important loss of energy for the cells, most of nitrogen fixing cyanobacteria also feature at least one uptake hydrogenase.
History
In 1933, Marjory Stephenson and her student Stickland reported that cell suspensions catalysed the reduction of
methylene blue with H
2. Six years later,
Hans Gaffron observed that the green photosynthetic alga
Chlamydomonas reinhardtii, would sometimes produce hydrogen.
[ Algae: Power Plant of the Future?] 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.
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.
[P. Häussinger, R. Lohmüller, A. M. Watson, "Hydrogen, 2. Production" in Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH, Weinheim. ]
- CH4 + H2O <-> CO + 3 H2
See also
External links
target="_blank" rel="nofollow"> EERE-CYCLIC PHOTOBIOLOGICAL ALGAL H2-PRODUCTION
