Environmental Engineering Reference
In-Depth Information
protons to be pumped through the thylakoid membrane, contributing to the chemiosmotic
gradient that drives ATP production [7].
2.1.1.1. Green algae
. Green algae are unicellular eukaryotes in which the majority of H
2
-
producing capability lies in the use of photosynthetically-activated electrons by Fe-
hydrogenase enzymes, requiring anaerobiosis in light. This process is known as direct
photolysis. Green algae typically also have significant H2 uptake activity, although the
enzymes responsible have not yet been identified. In general terms, therefore, the primary
enzymes of concern for H
2
production in green algae are Fe-hydrogenases, known to produce
H2, and uptake hydrogenases of unknown composition.
Green algae, as well as cyanobacteria (below), must withstand periods of anaerobiosis
during darkness and therefore are also capable of fermentation (Figure 17). While reducing
power generated by fermentation of endogenous substrates can enter the plastoquinone pool,
undergo activation by PS I, and lead to H2 production, H2 generation through fermentation is
estimated to have only 1/100 of the potential of direct photolysis in green algae [9, 10].
Nevertheless, the presence of this fermentative metabolism is physiologically significant for
H
2
production by direct photolysis, as it appears that the ability of these organisms to evolve
H
2
by means of hydrogenases originated with the necessity of disposing of excess reducing
power during fermentation [11].
Figure 17. The mixed-acid fermentation of Escherichia coli, an example of a common H
2
-producing
fermentative pathway, showing H2 production from formate [8].
2.1.1.2. Cyanobacteria
. Cyanobacteria or blue-green algae, in contrast to green algae, are
unicellular or multi-cellular (filamentous) prokaryotes that generate H
2
by three distinct
mechanisms. These pathways can, under certain conditions, operate simultaneously. Among
N2-fixing cyanobacteria, the majority of H
2
-producing capability lies in the activity of
nitrogenase enzymes that use photosynthetically-generated ATP to reduce or “fix” molecular
