Page 58 - Biofuels Refining and Performance

Energy and Its Biological Resources 41

Krebs cycle) and reduced nitrogen sources (glutamate/aspartate).
Interactions of hydrogenase and nitrogenase may be complementary or
competitive in different species or mutants. Nitrogenase (Mo, Ni, or Fe)
also with mixed isozymes are reported. Some mutants liberate H more
2
efficiently, utilizing DL-malate, D-malate, and L-lactate. Photoautotrophic
growth is found to be less efficient in producing H 2 than photoheterotrophic
growth with limited nitrogen in nutrients. Normally, in photosynthetic
fix-
bacteria, hydrogenase utilizes the hydrogen as a reductant for CO 2
ation and also for fixing molecular nitrogen. Nitrogenase reduces molec-
ular nitrogen, along with the production of molecular hydrogen at the
expense of almost six stoichiometric equivalents of ATP. This means that
concurrent nitrogenase activity during photosynthesis competitively con-
sumes the ATP that is produced and lowers the CO -fixing efficiency.
2
Rhodospirillum and Rhodopseudomonas grow aerobically in the dark.
But Rhodospirillum rubrum growing on glutamate (a nitrogen source)
exhibit good hydrogen release during photosynthesis. Quantitative pro-
duction of hydrogen has also been observed, growing on acetate, succi-
nate, fumarate, and malate, by photosynthesis, initially in the presence
of limited ammonium salts.
In Rhodopsuedomonas acidophilla, hydrogenase and nitrogenase are
genetically linked. Several species of Rhodospirillaceae can perform
nonnitrogenase-mediated hydrogen production in the absence of light,
using glucose and organic acids including formates. Different strains of
Rhodopseudomonas gelatinus and Rhodobacter sphaerolides exhibit
highly efficient production of hydrogen [90 µL/(h
mg) cell] grown in a
glutamate–malate medium.
In some cultures of Rhodopseudomonas capsulata, R. rubrum, and
gas
Rhodomicrobium vannielli, replacement of glutamate by N 2
improved productivity of H (760 mL/d, 10 days) decreasing a little on
2
aging. The model of a nozzle loop bioreactor, with immobilized R.
rubrum KS–301 in calcium alginate, initial glucose concentration of
5.4 g/L, 70 h at 30 C, showed production of hydrogen 91 mL/h (dilu-
tion rate of 0.4 mL/h). Improvement was suggested by using an agar
gel for immobilization.

Aerobes.
1. Bacillus licheniformis isolated from cattle dung showed production
of H 2 in mixed culture media. Immobilized on brick dust, the aerobe
maintained H 2 production for about 2 months in a continuous system,
with an average bioconversion ratio of 1.5 mole of H 2 per mol of
glucose.
2. Alcaligenes eutrophus, when grown on gluconates or fructose anaer-
. Hydrogenase directly reduces the coenzyme
obically, produces H 2