Page 57 - Biofuels Refining and Performance

40 Chapter One

found to be symbiotic to cycads, lichens, and so forth. Some are hetero-
cystous, lacking photolysis of water, and produce H 2 through the nitro-
genase step (when N 2 is low). The nonheterocystous species produce H 2
at higher efficiency at low N 2 and O 2 concentrations. Some of the species
favor anoxic and dark conditions, but with the presence of organic sub-
strates. They may even use sulfides as a source of electrons under an
anaerobic environment. They are highly adaptable to a changing envi-
ronment and are widely found in salty or sweet water, deserts, hot
springs (up to 75 C), as well as Antarctica. Some heterocystous
Anabaena exhibit H 2 production in an atmosphere of argon and absence
of molecular nitrogen. This was the clue to the knowledge that the
enzyme nitrogenase, the main biocatalyst for molecular nitrogen fixa-
tion, is present in cyanobacteria and is the key route of H 2 production:

N 2 8H 8e 12ATP → 2NH 3 H 2 12ADP 12Pi
A “reversible hydrogenase” (in photolysis of water, 2H 2 O → 2H 2 O 2 ),
is present in both heterocyst and vegetative cells and produces H 2 at a
lower rate than a nitrogenase. An “uptake hydrogenase” also operates

(minor) connected to cytochrome chain, providing both H and elec-
trons. H 2 evolution is common, but the photolytic O 2 is inhibitory to
nitrogenases, which is protected by other biochemical and structural
alternatives existing in heterocysts.
Large amounts of ATP, which is required for the reaction are gener-
ated in the event of photosynthesis and respiration. The electron (reduc-
tant) supply in the nitrogenase equation comes from metabolites, i.e.,
amino acids, mainly from carbohydrates (maltose, glucose, fructose,
other pentoses, tetroses, etc.), produced and stored in the vegetative
cells through photo I and II systems.
Nitrogenase Co II, i.e., NADPH (gained through the pentose phosphate
route) happens to be an electron donor through NADP oxidoreductase/
ferredoxin or flovodoxin. Other electron-supplying batteries are also
envisaged.
1. Through uptake hydrogenase–ferredoxin (photoactivated)
2. Through pyruvate–ferredoxin oxidoreductase
3. Reduced ferredoxin from isocitrate dehydrogenase
4. NADH generated in the glycolytic route

Under anaerobic or low aerobic conditions, nitrogenase activity may
exist in vegetative cells, but H generation is of poor order.
2
Photosynthetic bacteria. Hydrogen production is guided by the surplus
of ATP and reductant organic metabolites (carbon sources from the