Under certain conditions, green algae and cyanobacteria can use water-splitting photosynthetic processes to generate molecular hydrogen. Biophotolysis-based hydrogen production can be carried out via direct or indirect means as identified by whether or not light is irradiated during hydrogen evolution (Benemann, 1998). A brief description of the principles, the systems (Fig. 13.2) and the main bottlenecks for the practical application of both processes are given below.

13.2 Direct (a) and indirect (b) biophotolysis of water.

Process Microorganisms Feedstock Advantages Disadvantages H2 synthesis rate mmol H2/L/h Direct biophotolysis Green algae Chlamydomonas reinhardtii Water, sunlight H2 production directly from water and sunlight Solar conversion energy increased by tenfold as compared to trees, crops High intensity of light 02 can be dangerous for the system Lower photochemical efficiency 0.07 Indirect biophotolysis Cyanobacteria Anabaena variabilis Water, sunlight H2 production from water Ability to fix N2 from atmosphere Removal of uptake hydrogenase enzymes because of H2 degradation 30% 02 present in gas mixture Sunlight requirement 0.355 Photo­ fermentation Photosynthetic bacteria Rhodobacter spheroides Biomass, sunlight A wide spectral light energy can be used Different organic wastes as substrates Inhibitory effect of 02 on nitrogenase Low light conversion efficiency 0.16 Biological water gas shift Photo-heterotrophic bacteria Rhodospirillum rub rum Rubrivivax gelatinosus CO Low temperatures and pressures High conversion efficiency Demand of CO source Inhibitory effect of 02 96 Dark fermentation Fermentative bacteria Enterobacter aerogenes Clostridium butyricum Mixed microbial cultures Biomass H2 production all day, without light independence Variety of carbon sources as substrates Valuable metabolites, as byproducts No 02 limitation Low obtained H2 yields 02 is a strong inhibitor of hydrogenase C02 present in the gas Requirement of further treatment of the fermentation effluent, before disposal 8.2-121 Microbial Electrolysis Cells Electrode reducing microorganisms Geobacter, Shewanella sps Biomass, electricity Higher yields No use of 02 Electricity supply Competitive methane generation 5.8

Direct biophotolysis

In this process, electrons are generated from water through photosynthesis and then transferred via an electron carrier — ferredoxin (Fd), to hydrogen producing enzyme — hydrogenase, to produce hydrogen. Microalgae, such as green algae and Cyanobacteria (blue-green algae), containing hydrogenases, have the ability to produce hydrogen. Well-known cyanobacteria that have been found to produce hydrogen in lab scale bioreactors are Anabaena sp. such as Anabaena cylindrical (Weissman and Benemann, 1977), Anabaena variabilis (Sveshnikov et al., 1997; Borodin et al., 2000) and Synechococcus (Howarth and Codd, 1985). Chlamydomonas reinhardtii is the representative of green microalgae for biohydrogen production (Tsygankov et al., 2006; Griesbeck et al., 2006; White and Melis, 2006). Other algal species such as Chlorococcum littorale and Platymonas subcordiformis have also been investigated for hydrogen production (Schnackenberg et al., 1996; Guan et al., 2004).

The main drawback of direct biophotolysis is that the process is limited because of the strong inhibition of hydrogenase by the oxygen produced. Thus, for the sustainability of hydrogen production, it is necessary to maintain the oxygen content at a low level, below 0.1% (Hallenbeck and Benemann, 2002). In practice, it is very difficult to maintain such low partial pressures of oxygen, without additional energy and cost demands. For example, neutral gases such as helium could be sparged in the reactor, in order to eliminate oxygen, but the supplemental cost of helium and hydrogen dilution, makes this solution unacceptable. Another approach involves the addition of oxygen absorbers (Hallenbeck and Benemann, 2002) but now this seems not practical at larger scale.

Other limitations, such as the low light conversion efficiencies and the requirement for large photobioreactors, make the process impractical for large — scale application as it becomes inefficient from an economical point of view. It ought to be mentioned that a number of approaches to improve H2 production by green algae are currently under investigation. These include genetic engineering of light gathering antennae (Polle et al., 2002), optimization of light input into photobioreactors (Gordon, 2002) and improvements to the two-phase H2 production systems used with green algae (Laurinavichene et al., 2002a; Tsygankov et al., 2002). Another challenge is the modelling and simulation of photolytic systems to support systems design and optimization.