Environmental Engineering Reference
In-Depth Information
Figure 19. Relationships among photosystems of oxygenic (plants, algae, and cyanobacteria) and
anoxygenic (purple bacteria, green filamentous bacteria, green sulfur bacteria, and heliobacteria) [17].
Fermentations occur in the absence of an electron transport chain or exogenous terminal
electron acceptors such as O 2 [8]. Fermentations are therefore anaerobic processes, but they
may be carried out by either anaerobic or aerobic microorganisms within sufficiently anoxic
microenvironments [20]. Numerous fermentation pathways are known, many of which, like
the mixed acid fermentation pathway (Figure 17), produce H 2 , and all of which generate
multiple products that typically must be separated and purified for commercial use [8, 14, 5].
While members of both the oxygenic and anoxygenic phototrophs possess H 2 -producing
fermentation pathways, only bacteria appear to produce H 2 from fermentation in appreciable
quantities. These bacteria include members of the Bacillus, Clostridia, and Enterobacter, as
well as the extreme thermophiles Caldicellulosiruptor and Thermotoga, all heterotrophs that
thrive on carbohydrates and typically are not capable of anaerobic respirations [5].
2.2. Enzymes
Two categories of enzymes, the hydrogenases and the nitrogenases, are responsible for
H 2 production in the microorganisms discussed above.
2.2.1. Hydrogenases . Hydrogenases catalyze the redox interconversion of protons with
H 2 gas:
They occur in two primary forms, known as uptake and bidirectional hydrogenases.
Uptake hydrogenases oxidize H 2 to H+ to provide reducing power to many anaerobic
respiratory organisms [21], while bidirectional hydrogenases may catalyze either H 2 oxidation
or H+ reduction, although in vivo they typically function in only one of the two capacities
[22, 23]. Hydrogenases may also be classified according to differences in their organometallic
catalytic sites: NiFe(Se)-hydrogenases are characterized by catalytic sites possessing
Search WWH ::




Custom Search