Biomedical Engineering Reference
In-Depth Information
demonstrated by the example of
Thermotoga neapolitana
, which was shown to
produce hydrogen even in the presence of 4-6% oxygen [14]. The gene encod-
ing the hydrogenase of this organism has been isolated and sequence analysis
suggested that its superior resistance could be due the fact that this enzyme
is generally found as a trimer, while all the other hydrogenases known are
monomers. Thanks to this larger molecular size, so that its active site can
be buried and less accessible to oxygen molecules. However, this explanation
alone is insu
cient as a homologous organism,
T. maritima
has an hydro-
genase with similar size but it is not equally resistant to oxygen. In a close
future perspective, the study of this hydrogenase would suggest possible ways
to further improve oxygen resistance, while the expression of this resistant
hydrogenase in
Chlamydomonas
cells would allow understanding the possible
impact of a oxygen-resistant hydrogenase on hydrogen yields.
2.4.2 Optimization of Light Harvesting in Bioreactors
The first step in hydrogen production is light absorption by photosynthetic
complexes. When algae grow in photobioreactors, this process has a poor e
-
ciency because of the bad distribution of light. In fact, to have good produc-
tion yields, biomass concentration need to be high, leading to increased optical
density of the culture where light can only penetrate for a few millimeters,
as shown in Fig. 2.3. As a consequence, the external cell layers are exposed
to a very intense light leading to activation of photo-protective mechanisms
to dissipate energy in excess, to avoid formation of oxygen reactive species
and consequent cellular damages. This cells population, thus, is not e
cient
in the light utilization because absorbed energy is dissipated rather than used
for metabolic reactions. On the contrary, cells located at more internal level
in the culture are exposed to very low light, insu
cient for sustaining intense
metabolic activity.
This unequal light distribution has another negative consequence: the con-
nective fluxes in the medium cause algal cells to stir into the reactor where
they can move from dark to strong light within few seconds. The fast variation
in light intensity leads to an input of reducing power into the photosynthetic
chain at very different rates, without allowing the time for acclimation of
photosynthetic apparatus [15]. These conditions are highly stressful because,
without the delay needed for the activation of photo-protective mechanisms,
the energy absorbed in excess generates oxidative stress and leads to PSII
photoinhibition.
To overcome these problems and increase the productivity of photobiore-
actors, two type of interventions are needed:
1. To reduce the antenna size to lower the optical density of the culture and
allow for a better light distribution within the bioreactor without reducing
the cellular density
2. To strengthen the algal mechanisms of oxidative stress resistance and of
thermal energy dissipation
