Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 106

over a long time is difficult because water is such an inherently stable molecule and
the product, O 2 , is extremely reactive.
2.3.2 Photosynthetic Dihydrogen
Solar fuel production is the same as artificial photosynthesis and instead of building
synthetic molecular H 2 -producing devices an alternative approach is to re-tune
microbial photosynthesis to achieve efficient solar H 2 production [ 39 ]. The advan-
tage of using a living system is that it is capable of self-repair and self-replication
and it can be assembled from inexpensive growth media, possibly even sewage.
Hydrogen-producing, photosynthetic cyanobacterial and algal strains exist which
can thrive under growth conditions that are inhospitable for other agricultural
species, e.g., very high salt. Microbial solar H 2 culture sites would therefore not
have to compete for areas of land or water supplies which could be used for farming
[ 40 , 41 ]. True water-splitting is achieved in oxygenic photosynthesis via the action
of photosystem II which catalyzes H 2 O oxidation to O 2 at a manganese center
(Figure 5 and Chapter 2 in [ 84 ]). Because H 2 production utilizes electrons directly
produced by the photosystems, the rate of fuel production is not limited by the rate
of the CO 2 -fixing Calvin cycle, which controls C-biofuel production (Figure 5 ). The
challenge is to achieve high efficiency conversion of solar energy into H 2 fuel
production rather than rapid rates of microbial growth.
As described in Section 2.2 , cyanobacteria are [NiFe] hydrogenase-producing
microbes which are capable of photosynthetic H 2 production. In non-N 2 -fixing
cyanobacterial strains like Synechocystis the bidirectional hydrogenase will pro-
duce H 2 under low O 2 using photosynthetically-generated nicotinamide adenine
dinucleotide phosphate (NADPH) and also reduced ferredoxin (Figure 5 )[ 28 , 42 ]
but at high O 2 levels the Calvin cycle dominates. One method for boosting
cyanobacterial hydrogenase solar H 2 production is therefore to perturb the meta-
bolic flux so that photo-generated NADPH is diverted away from CO 2 -fixation and
to the native hydrogenase instead [ 28 ]. This can be achieved by genetically
manipulating the bacteria to over-express the hydrogenase and 20-fold increases
in hydrogenase production have been achieved using this approach in Synechocystis
[ 43 ]. Alternatively, because cyanobacterial H 2 production is inhibited by O 2 ,
attempts are being made to genetically introduce new [NiFe] hydrogenases that
function in air, such as the H 2 enzyme from the aerobe R. eutropha [ 28 ].
Compared to cyanobacteria the cellular organization of algal photosynthesis is
slightly more complex because of the compartmentalization of the thylakoid
membranes into chloroplasts. Rather than [NiFe] enzymes, algae produce [FeFe]
hydrogenases and it is only the reduced ferredoxin which can act as the electron
donating redox partner (Figure 5 ). In contrast to [NiFe] hydrogenases, all native
[FeFe] hydrogenases are irreversibly inactivated by O 2 but they have a greater
catalytic bias for H 2 production over H 2 oxidation. The most commonly studied
algae for H 2 production is Chlamydomonas reinhardtii , and increases in photosyn-
thetic H 2 production levels have been achieved by taking different approaches to
overcoming O 2 inhibition. Firstly, if the hydrogenase machinery is left unchanged then