Environmental Engineering Reference
In-Depth Information
In photosynthesis, the reduced carbon is stored as endogenous carbohydrates,
such as starch in microalgae and glycogen in cyanobacteria. Studies on the mecha-
nisms involved in hydrogen evolution have found that the electrons or reducing
equivalents of hydrogenase and nitrogenase do not always come from water, but
may sometimes originate from the intracellular energy reserve including carbo-
hydrates. The stored energy is released through fermentation of the endogenous
carbohydrates in dark conditions, and the excess reducing power can be deposited
by hydrogenase on protons (H
+
) forming molecular hydrogen. Hydrogen evolution
from endogenous carbon reserve under dark anaerobic conditions looks very similar
to the conventional anaerobic hydrogen fermentation, but the endogenous carbon
reserve is made in vivo during photosynthesis. In this sense, the electrons or reduc-
ing equivalents in indirect bio-photolysis are derived from water by photoautotro-
phic cells. This indirect bio-photolysis, therefore, consists of two stages in series:
photosynthesis for carbohydrate accumulation, and dark fermentation of the carbon
reserve for hydrogen production. This way the oxygen and hydrogen evolutions are
temporally and/or spatially separated. This separation not only avoids the incompat-
ibility of oxygen and hydrogen evolution (e.g., enzyme deactivation and the explo-
sive property of the gas mixture), but also makes hydrogen purification relatively
easy because CO
2
can be conveniently removed from the H
2
/CO
2
mixture.
In cells of certain green algae (e.g.
Chlamydomonas reinhardtii, Chlorella fusca
)
and blue-green algae (cyanobacteria), hydrogen production occurs as a result of
light-driven splitting of water during photosynthesis. In direct bio-photolysis, the
photosynthetic apparatus captures light and the recovered energy is used to couple
water splitting to the generation of a low-potential reductant, which can be used to
reduce a hydrogenase enzyme. This is an inherently attractive process since solar en-
ergy is used to convert a readily available substrate, water, to oxygen and hydrogen:
2
HO light energy HO
+
→
2
+
2
2
2
This reaction was first demonstrated with a cell free chloroplast-ferredoxin-hydrog-
enase system, although the existence of such a reaction in green algae had been sug-
gested earlier (Spruit
1958
). Anaerobic conditions are indispensable for this process
to occur. A stream of electrons and protons originating from water is generated upon
the light energy with wavelength lower than 680 nm and absorbed by photosystem
II (PSII). On the other hand, photosystem I (PSI) is induced with light wavelength
lower than 700 nm which allows the transportation of electrons from PSII to PSI via
chain of reductors called cytochrome bf. Electrons from PSI system are transferred
via ferrodoxine to hydrogenase (algae or cyanobacteria) or nitrogenase (cyanobac-
teria) and these enzymes reduce protons to molecular hydrogen. In direct biopho-
tolysis, neither CO
2
nor liquid metabolites are observed. The constant removal of
oxygen is required since oxygen inhibits hydrogenase activity irreversibly (Das and
Veziroglu
2008
).
Photoautotrophic microorganisms, either prokaryotic cyanobacteria or eukary-
otic green microalgae, possess chlorophyll
a
and other pigments to capture sun-
light energy and use photosynthetic systems (PSII and PSI) to carry out oxygenic
