Environmental Engineering Reference
In-Depth Information
approachable by creative use of metabolic and chemical engineering. Given continued
investment, each of the major biohydrogen pathways should be able to find a niche in a future
sustainable-energy economy by delivering competitively-priced H2 at commercial scales.
•
Direct Photolysis
. 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 112 yield will be important,
achievable through either modification of production hydrogenases or possibly
through elimination of 112 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.
•
Indirect Photolysis
. While the conversion efficiency of indirect photolysis is low
compared to the theoretical efficiency of direct photolysis, it nevertheless presents
the advantages of sustained 112 production over longer periods of time, great
tolerance of aerobic conditions, and requirement for little input other than sunlight,
minerals, and CO2. In addition, several promising avenues are available for the
improvement of 112 production:
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Engineering of cyanobacteria
. The engineering of cyanobacteria to include
alternative, non-molybdenum-containing nitrogenases, as well as to inactivate
uptake hydrogenases, are paramount, and overexpression of nitrogenase as well
as nitrogenase engineering to improve its catalytic rate have also been
suggested. Engineering efforts directed toward antenna improvements,
minimizing light- saturation effects as well as enabling photon collection from
wider regions of the solar
spectrum, would also be applicable to indirect
photolysis.
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Investigation of alternative forms of cyanobacteria
. Among experimental
organisms, the filamentous cyanobacteria have received the majority of
investigation to date, but non-filamentous cyanobacteria such as Oscillatoria
that protect nitrogenases from O2 by temporal rather than physical separation
of photosynthesis and N2 fixation also deserve further examination. In
addition, a great diversity of cyanobacteria adapted to a wide variety of
environments exists that should be investigated for potential contributions to
the 112 production effort. The ability of some cyanobacteria to grow
heterotrophically is especially interesting in light of the possibility it presents
to convert organic wastes into 112.
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Optimizing cultivation conditions
. Finally, further work directed toward
optimizing cultivation conditions for highly desirable organisms, particularly
outdoor and marine cultivation, is essential to the realization of commercially
viable indirect photolysis.
