In general, cyanobacterial H2 production is difficult to sustain for long time periods in liquid cultures in photobioreactors. Utilization of suspension cultures in a two-stage system for H2 production is an even more complicated and energy-consuming process due to the centrifugation or sedimentation steps required for cell harvesting, media changes, and dilutions of cell density during the switch between the different phases. Suspen­sion cultures require intensive mixing, which in turn causes damage to the fragile cyanobacteria filaments. This system is hard to scale up. Use of immobilized cyanobacterial cells in specially designed laboratory — scale photobioreactors would be a good solution to the above-mentioned problems of liquid cultures.

Immobilization of biomolecules and whole cells on various substrates and into different gels, such as solid surfaces like porous glass, supported films, (nano) porous materials, (nano)fibers, foams, inorganic and organic hydrogels, latex, nanotubes, and nanoparticles, has been studied extensively (see for review Meunier et al., 2011). Application of immobilization usually im­proves the stability of the enzymes, and increases light-utilization efficiency. Immobilization of algal cells on a solid phase made of glass has been used for extended H2 production (Laurinavichene et al., 2006). A green alga, C. reinhardtii entrapped in thin alginate films demonstrated extended H2 photoproduction due to increased light-utilization efficiency and better toler­ance against O2 (Kosourov and Seibert, 2009). Several attempts also have been made to immobilize cyanobac — terial cells in order to improve H2 production. These include Plectonema boryanum within alginate beads (Sar — kar et al., 1992), Oscillatoria in agar matrix (Phlips and Matsui, 1986) Phormidium valderianum together with Halobacterium halobium and E. coli within polyvinyl alcohol-alginate beads (Bagai and Madamwar, 1998). Recent immobilization of the Calothrix 336/3 strain in thin alginate film resulted in extended production of H2 even after 40 days of immobilization (Leino et al., 2012). Immobilization has also been found to have a positive effect also on viability of cells. Twelve weeks after initial immobilization, entrapped cells from recov­ered films produced H2 nearly as efficiently as the fresh cells in newly made films. Moreover, the immobilized cells of Calothrix 336/3, Anabaena PCC 7120 and DhupL mutant of Anabaena were viable for over 10 months in the initial nutrient medium without addition of CO2. The demonstrated long-term viability of entrapped cells is a very important issue for economical use of cyano — bacterial in H2 production systems.