Environmental Engineering Reference
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cyanobacteria to fragmentation by mixing, which greatly diminishes H2 evolution capability
[12]. Nevertheless, the O 2 -tolerance of the process is uniquely attractive.
2.4.1. H 2 recycling . Uptake hydrogenases are found in apparently all nitrogen-fixing
cyanobacteria, where they efficiently recover H 2 energy, and they thus present one of the
primary limitations of indirect photolysis [75, 13]. Cyanobacterial uptake hydrogenases
appear to be conserved, encoded by hupL (large subunit) and hupS (small subunit) genes; the
recombinase xisC is also essential to uptake hydrogenase expression because it facilitates
excision by site- specific recombination of a 10.5 kilobit element within the hupL structural
gene during heterocyst differentiation [75]. In addition, bidirectional hydrogenases are present
in many but not all N 2 -fixing cyanobacteria. These are heterotetrameric enzymes consisting of
a hydrogenase part encoded by hoxYH and a diaphorase part encoded by hoxFU, typically
function in the H 2 - uptake direction, and can oxidize nitrogenase-generated 112 in the absence
of uptake hydrogenases.
In attempts to improve 112 evolution by indirect photolysis, several mutants have been
generated in which the activity of uptake hydrogenases has been impaired [78] or in which
the hupSL structural genes have been inactivated [79, 80]. These have shown significantly
improved 112 production [32, 80-81], with 112 evolution rates enhanced by 3- to 10-fold
over those achieved with the corresponding wild-type strains [12].
2.4.2. Cultivation conditions . Photobioreactors for hydrogen production are undergoing
rapid development in attempts to provide light evenly, abundantly, and efficiently to
photosynthetic cultures while facilitating gas removal (e.g., [50, 83-86]). At the same time,
experimental work in indirect photolysis is investigating effects of variations in dissolved
oxygen, light, temperature, nutrients and waste products, and suspension versus
immobilization of cells, not to mention cell morphologies and genetic compositions [32, 75,
80]. It is not surprising, therefore, that an optimal range of conditions has not emerged, and
indeed, such optimization may need to follow the identification and/or engineering of a few
outstanding 112- producing organisms.
2.4.3. Rate and efficiency . Reports of photosynthetic conversion efficiency (energy in 112
produced divided by energy in photosynthetically active radiation [400-700 nm
wavelengths]) vary, with the highly active strain Synechococcus sp. Miami BG04351 1
yielding a performance of 3.5 percent [82]. 112 synthesis rates achievable by indirect
photolysis, however, compare quite favorably to those presently achievable by direct
photolysis and by photofermentation (Table 5), at 0.355 millimoles 112 per liter culture per
hour, compared to 0.07 millimoles per liter per hour for direct photolysis and 0.16 millimoles
per liter per hour for photo-fermentation [5].
2.4.4. Research priorities . 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 CO 2 . In
addition, several promising avenues are available for the improvement of 112 production.
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 [75]. 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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