Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 65

Hydrogen production by cyanobacteria has been studied for over three
decades and has revealed that efficient photoconversion of H 2 O to H 2 is
influenced by many factors. Rates of H 2 production by nonnitrogen-fixing
Cyanobacteria range from 0.02 μmol H 2 ·mg -1 chl a/h ( Synechococcus PCC
6307) to 0.40 μmol H 2 ·mg -1 chl a/h ( Aphanocapsa montana ) [75].

4.4.2 Photofermentation

Purple nonsulfur bacteria evolve molecular hydrogen catalyzed by nitroge-
nase under nitrogen-deficient conditions using light energy and reduced
compounds (organic acids) [76]. These bacteria themselves are not powerful
enough to split water. However, under anaerobic conditions, these bacteria
are able to use simple organic acids, like acetic acid, or even hydrogen disul-
fide as electron donor. These electrons are transported to the nitrogenase by
ferredoxin using energy in the form of adenosine triphosphate (ATP). When
nitrogen is not present, this nitrogenase enzyme can reduce proton into
hydrogen gas again using extra energy in the form of ATP [72]. The reaction
can be given as

C H O + 12 H O Light energy + → 12 H + 6 CO .

(4.15)

6 12 6 2 2 2

Among the various bioprocesses capable of hydrogen production, photo-
fermentation is favored due to relatively higher substrate-to-hydrogen yields
and, its ability to trap energy under a wide range of the light spectrum and
versatility in sources of metabolic substrates with promise for waste stabili-
zation [77]. In photofermentation processes, the yield of the order of 80%
has been achieved [72]. However, these processes have three main drawbacks
[23]: (1) use of nitrogenase enzyme with high-energy demand, (2) low solar
energy conversion efficiency, and (3) demand for elaborate anaerobic pho-
tobioreactors covering large areas.

4.4.3 Dark Fermentation

Hydrogen can be produced by anaerobic bacteria, grown in the dark on
carbohydrate-rich substrates. Dark fermentation of carbohydrate-rich
substrates as biomass presents a promising route of biological hydrogen
production, compared with photosynthetic routes. Anaerobic hydrogen fer-
menting bacteria can produce hydrogen continuously without the need for
photoenergy. Dark hydrogen fermentation can be performed at different
temperatures: mesophilic (298-313 K), thermophilic (313-338 K), extreme-
thermophilic (338-353 K), or hyperthermophilic (>353 K). While direct and