Environmental Engineering Reference
In-Depth Information
long-term (incorporating reasonable research targets), and in the best case (incorporating
improvements that are theoretically possible but which would require major accomplishments
in several areas). The H
2
selling prices were estimated at $5,300 per gigajoule, $930 per
gigajoule, $110 per gigajoule, and as low as $9 per gigajoule, respectively, for these scenarios
[45].
2.3.7. Research priorities
. Direct photolysis is not yet able to compete with other
mechanisms of biohydrogen production in rate or cost, but the necessary advances are well
within reach of modern biotechnology. To achieve practicality, the most important challenges
to be overcome are the light-saturation effect and the strong repression of the Fe
hydrogenases,
both transcriptionally and post-translationally, by molecular oxygen. In
addition, improvements to specific H
2
yield will be important, achievable through either
modification of production hydrogenases or possibly through elimination of H
2
uptake
activity. More ambitious ideas foresee advances in the light-harvesting capability of the algae,
possibly by adding light-harvesting pigments to cover additional portions of the solar
spectrum. The land-use issue is also extremely important, which in turn presumes that
effective scale-up is achieved. While direct photolysis is a long-term prospect, its potentially
low-energy requirements and the approachability of its challenges by established molecular
techniques cause it to be well worth the continued research investment.
2.4. Indirect Photolysis
Indirect photolysis is the light-dependent evolution of H
2
by nitrogenases that occurs in
cyanobacteria. In this process, nitrogenases evolve H
2
as they fix N
2
into NH3,
simultaneously consuming ATP supplied by respiration of organic carbon fixed through
photosynthesis at the rate of 16 ATP per H
2
produced [74, 5]. Although nitrogenases are
highly oxygen-sensitive, like hydrogenases, they are protected to a great extent from O2, in
filamentous cyanobacteria such as Anabaena and Nostoc, in specialized cells known as
heterocysts. As a result, cyanobacteria undergoing indirect photolysis can evolve H2 quite
rapidly and continuously under oxic conditions [12], in contrast to microbes using direct
photolysis. Because N2 fixation is an inducible process that requires an absence of fixed
nitrogen, cultivation in nitrate-free media is required to induce H
2
production. In addition,
published H2 evolution rates are typically acquired under anoxic, light-saturated conditions
that follow aerobic and frequently lower-light cultivation conditions [12].
The nitrogenase complex consists of two proteins—the dinitrogenase and the
dinitrogenase reductase. The former is an alpha
2
beta
2
heterotetramer, with alpha and beta
subunits encoded by the genes nifD and nifK, respectively. The dinitrogenase reductase,
encoded by nifH, is a homodimer and mediates the transfer of electrons from the external
electron donor, typically a ferredoxin or flavodoxin, to the dinitrogenase [75]; mechanistic
understanding of nitrogenase action is well-understood [76]. Several distinct variations of the
nitrogenase structural genes exist, although they are highly conserved, and many
cyanobacteria possess multiple nitrogenases. Interestingly, the unusual non-molybdenum-
containing variants appear to allocate more electrons to H2 production (75). Numerous
additional genes have been identified and characterized that are involved with nitrogen
fixation and regulation, recently reviewed in [77].
Indirect photolysis requires a great deal of energy—16 ATP per H
2
molecule generated
[34]. Furthermore, it is limited by the H
2
-recycling activity of uptake hydrogenase found in all
known N2-fixing cyanobacteria [13]. Another limitation is the vulnerability of filamentous
