Exploiting Photosynthesis for Biofuel Production - Biophotonics - page 23

2.3.2 Eukaryotic Algae
Gaffron and coworkers over 60 years ago discovered that green algae, when
illuminated after an anaerobic incubation in the dark, have the ability to
produce H 2 [11, 12]. This hydrogen production is directly coupled with wa-
ter oxidation through Photosystem II (PSII) and the photosynthetic electron
transport chain. Thus, H 2 is synthesized directly from water, using sunlight
as a source of energy, with a totally clean process, making very promising the
perspective of exploiting green algae for a large scale H 2 production. More-
over, differently from the cyanobacteria, H2 production is achieved by Fe-Fe
hydrogenases, thus in a more e cient way.
2.4 Challenges in Algal Hydrogen Production
The exploitation of algae for hydrogen production is probably the method with
the best perspectives in the long term among those that are analyzed here.
However, it has two major limitations that prevent its large scale exploitation:
(1) the great sensibility of the process to the oxygen presence; (2) the ine cient
light energy distribution in the bioreactor.
2.4.1 Oxygen Sensitivity of Hydrogen Production
As mentioned, hydrogen is always produced in anaerobic conditions because
all hydrogenases are sensitive to oxygen. This limitation becomes deleterious
when oxygenic photosynthetic organisms such as algae are discussed: in fact,
they use water as electron donor to produce protons and oxygen. The latter is
thus an unavoidable by-product of photosynthesis and even the incubation in
anaerobic conditions is su cient to avoid inhibition of hydrogen production.
Different strategies are possible to obtain a significant level of hydrogen
production by photosynthetic organisms: one is to separate temporally or spa-
tially oxygen evolving process from the hydrogen production. Alternatively,
modified hydrogenase enzymes, less sensitive to O 2 , can be searched.
Some cyanobacteria, as mentioned, can maintain a state of controlled
anaerobiosis in heterocystis; however, it is di cult to imagine modifying green
algae to make them able to build such complex structures. In the case of algae,
thus, the approach of a temporal separation of oxygen and hydrogen evolutions
is more promising. In 2000, Melis et al. [13] demonstrated in Chlamydomonas
reinhardtii that, when sulphur in the growth medium is limiting, rate of oxy-
genic photosynthesis declines without affecting significantly the mitochondrial
respiration. In fact, under illumination Photosystem II (PSII) polypeptides,
in particular the D1 subunit, have a very fast turnover. Under sulphur de-
privation, when the protein biosynthesis is inhibited, PSII is thus among the
first protein complexes affected, causing the decrease of photosynthetic ac-
tivity and oxygen evolution. On the contrary, the mitochondrial complexes,