Environmental Engineering Reference
In-Depth Information
tolerant R. gelatinosus hydrogenase to understand the basis for its resilience would be
valuable to all efforts to engineer improved O2-tolerance in hydrogenases.
Further priorities, several of which the water-gas shift pathway shares with dark
fermentation, are described in Section 2.7 below.
2.7. Dark Fermentation
Dark fermentation refers to the light-independent production of H
2
by anaerobic
heterotrophic bacteria. While the water-gas shift process is thus technically a dark
fermentation, and green algae and cyanobacteria are also capable of light-independent
fermentations as described above, in the context of H2 production this term most often refers
to non-phototrophic bacteria unless otherwise specified. These microbes may be mesophilic
(with optimal metabolic temperatures of 25-40°C), thermophilic (40-65°C), extremely
thermophilic (65-80°C) or hyperthermophilic (>80° C). They typically ferment carbohydrates
(glucose, starch, or cellulose) and generate a gas of mixed composition, including not only H2
but also CO2, CH4, and/or volatile fatty acids, depending on the fermentation pathway used
(Figure 17). In practice, highest H2 yields are associated with fermentations yielding a
mixture of acetate and butyrate, while lower yields are associated with more reduced end-
products such as propionate, lactate, and alcohols. Most fermentative bacteria are capable of
multiple fermentation pathways, and culture conditions strongly influence the pathway(s)
used; in particular, H2 concentrations must be kept very low to avoid end-product inhibition
[93, 5].
2.7.1. Genetics
. Fermentation pathways are numerous, diverse, and well-characterized
genetically and physiologically in numerous organisms, particularly in gram-negative bacteria
[93]; their regulatory mechanisms have been thoroughly investigated and widely-modeled
(94); they have been metabolically engineered for the synthesis of numerous organic products
(e.g., [95]); and numerous fermentative enzymes have been structurally characterized (e.g.,
[96]). As a result, the genetic, genomic, proteomic, metabolomic, and physiological
groundwork has been done to facilitate a vast array of genetic and metabolic engineering
efforts directed to the improvement of H2 production by dark fermentations.
2.7.2. Substrates
. Since fermentations are, by definition, supported by organic substrates,
an important question for dark fermentative H2 production is the availability of sufficiently
inexpensive and abundant carbon sources that the resulting H2 could be commercially viable.
Organic wastes are an attractive option, yet are typically complex and variable and therefore
challenging to combine with sophisticated metabolic engineering approaches. In response to
this problem, Logan and colleagues at the University of Pennsylvania decided to attempt 112
production with a similarly complex microbial community obtained from soil and achieved
promising success [97], indicating that highly diverse, low-cost substrates may be
accommodated with appropriate microbial inocula.
2.7.3. Rate and efficiency
. A number of different dark fermentation systems have reported
112 synthesis rates well above 1 millimole 112 per liter culture per hour, using both pure and
undefined cultures, with values reaching 121.0 millimoles 112 per liter culture per hour for
undefined mesophilic cultures (Table 5). Dark fermentation, even by quite diverse systems,
thus produces 112 at rates that are up to two orders of magnitude greater than those currently
achieved by any of the phototrophic mechanisms.
At the same time, these high laboratory rates are achieved at the expense of purified
organic substrates that would be prohibitively expensive at larger scales, especially with a
