Biofuels in Perspective

198 Biofuels
11.1 Introduction
Hydrogen gas has great potential as a future fuel, as during its combustion the greenhouse
gas CO2 is not produced. However, hydrogen is only a true ‘green’ fuel if it is produced
from renewable sources like wind, sunlight, geothermal energy or biomass. A wide variety
of microorganisms is able to form hydrogen in light-dependent and in light-independent
processes, such as dark fermentations. Hydrogen can be formed in the fermentation of
complex biomass, but for large-scale production thus far only hydrogen formation from
polysaccharides is feasible. The various types of microorganisms that can play a role in
hydrogen formation by dark fermentations will be discussed here, with special emphasis
on the thermophilic hydrogen-producing microorganisms. At elevated temperatures hydrogen
formation is thermodynamically more feasible, and less undesired side-products are
produced. The current knowledge on the key enzymes (hydrogenases, oxidoreductases) of
the various hydrogen producing microorganisms is highlighted, making use of the genome
information that is available now for several of the hydrogen producing species. The availability
of the complete genome sequences also offers the possibility to apply genetic tools
to optimize hydrogen formation. Acetate is an obligate end product of dark fermentations,
which limits the hydrogen yield. Options to deal with the acetate problem are discussed.
The application of electricity-mediated electrolysis would broaden the range of compounds
that can be used for hydrogen formation.
11.2 Hydrogen Formation in Natural Ecosystems
In methanogenic environments hydrogen is a key intermediate in the anaerobic decomposition
of organic matter to methane and carbon dioxide. 1 At low temperatures (up to about
45 ◦ C) about 1/3 of the methane is formed by reduction of carbon dioxide with hydrogen
as electron donor:
4H2 + HCO3 − + H + → CH4 + 2H2O �G 0′
=−135.6 kJ/mol methane
while 2/3 of the methane is formed by acetate cleavage:
(acetate − + H2O → HCO3 − + CH4 �G 0′
=−31 kJ/mol methane
As methanogens cannot metabolize complex organic carbon compounds (polysaccharides,
proteins, lipids, nucleic acids, etc), fermentative bacteria are required to funnel the degradation
of complex organic compounds to the methanogenic substrates hydrogen and acetate.
Irrespective the type of microorganisms that are involved and irrespective the nature of
organic compounds, in methanogenic environments methane and carbon dioxide are the
final carbon end products. At moderately high temperatures (50–80 ◦C), even all methane
is formed from the reduction of carbon dioxide by hydrogen. This is because at these
conditions, acetate is first degraded by bacteria to form hydrogen and carbon dioxide:
acetate − → 2HCO3 − + H + + 4H2 �G 0′ =+104.6 kJ/mol acetate
the hydrogen being used by methanogens to reduce carbon dioxide to methane. 2
At first glance it may seem that at high temperatures all organic carbon can be converted
into carbon dioxide and hydrogen, provided that methanogens are inhibited. This indeed