Biofuels in Perspective

212 Biofuels
Concentration (mM)
45
40
(a)
25 45
40
(b)
25
35
20 35
20
30
25
15
30
25
15
20
15
10
20
15
10
10
5
10
5
5
5
0
0 10 20 30 40
0
50
0
0 10 20 30 40
0
50
Time (h)
Time (h)
Hydrogen (%)
Figure 11.4 Growth of Tt. elfii at high (a) and low (b) xylose concentrations in mixtures of glucose and
xylose. Symbols: * hydrogen; ♦, glucose; �, xylose; �, acetate; �, lactate. (72)
Several Thermotoga species are able to grow on complex carbohydrates, like cellulose
or xylan and various endocellulases, cellobiohydrolases and xylanases have been purified
and characterized. 86 The broad hydrolytic capacity combined with the high hydrogen producing
potential makes the Thermotoga species good candidates for hydrogen production
from biomass, although some species need ‘rich’ medium and high salt. The production
of hydrogen by Tt. neapolitana, Tt. elfii and Tt. maritima on different biomass types, like
sorghum and miscanthus, has been investigated (24). In general hydrolysates were not
toxic and high levels of hydrogen were reached. Experiments with Tt. elfii (Figure 11.4a
and 11.4b) revealed that xylose and glucose were used simultaneously if the xylose concentration
was high (∼40 mM). At low xylose concentration (∼ 5 mM) glucose repressed
xylose utilization similar to the pattern observed for Ca. saccharolyticus, although no
lactate production was found.
11.10 The Genus Pyrococcus/Thermococcus
The genera with the highest temperature optimum that will be discussed here for hydrogen
production are Pyrococcus and Thermococcus. These closely related genera represent a
group of hyperthermophilic sulfur-reducing heterotrophs and the only archaea considered
here. Various sugar- or amino acid based oligomers and polymers can be fermented by the
different species, however, cellulose and xylan cannot be used. Most data on hydrogen production
are obtained for P. furiosus, which is one of the best studied hyperthermophiles. As
research during the past decade has shown, many archaea possess non-canonical versions
of the glycolytic pathways. 87 Although Pyrococcus and Thermococcus species appeared
to have an EM-like pathway, 88 various steps in the pathway were modified compared to
the classical version. 87 Most importantly, the oxidation of glyceraldehyde-3-phosphate is
not NAD-dependent but uses ferredoxin instead. 89 Moreover, the oxidation of GAP to
3-phosphoglycerate was not dependent on Pi and did not involve the intermediate formation
of 1,3-bisphophoglycerate, and therefore it does not yield ATP by substrate-level
phosphorylation. 90 At the level of pyruvate a second oxidation step occurs which is similar
to the classical pyruvate:ferredoxin-oxidoreductase observed in e.g. clostridia. Thus,
the main difference between the glycolytic pathways of Pyrococcus/Thermoccus and all
other hydrogenic species is that all reducing equivalents are released as ferredoxin. Since
Concentration (mM)
Hydrogen (%)