Cyanobacteria considered to besuitable feed-stocks for blue biorefineries and

Cyanobacteria and green algae are widely distributedphotosynthetic microorganisms, which are capable ofharnessing solar energy and converting it into chemical energyby assimilating atmospheric CO2. They are considered to besuitable feed-stocks for blue biorefineries and attractivebiocatalysts for the production of biofuels, such as biodiesel,isoprene, alcohols, ethylene and other valuable industrialcompounds.1–3 Theoretical photosynthetic light conversionefficiencies to carbon-based products of approximately 4.6 %have been reported,4 which would satisfy the feasibility ofindustrial applications. Besides carbon-based biofuels,cyanobacteria and green algae can also produce molecularhydrogen (H2), which has the highest energy content ofreported biofuels and a zero CO2 release index.5 These qualitiesmake H2 an ideal energy carrier for the sustainable bioeconomyof the future. The photosynthetic conversion efficiency to H2 ismuch higher than to other carbon-based biofuels. For example,nitrogenase-driven H2 photoproduction in cyanobacteria has amaximum efficiency of around 6 – 7%,6 while direct waterbiophotolysis in green algae can drive H2 production with apotential efficiency of 10 – 12%.7,8 In reality, however, only asmall fraction of these values has been achieved, even inlaboratory scale photobioreactors. This is due to variousmetabolic and production process hurdles, such as the low lightutilization efficiency of suspension cultures, sensitivity ofphotosynthetic organisms to environmental factors, thecompetition for photosynthetic reductant from wastefulmetabolic pathways and the high energy demand of cellcultivation, harvesting and maintenance of photobioreactors.Thin-layer immobilization is a novel technology recentlyproposed to ensure uniform light distribution to high-densityphototrophic cultures fixed within a controllable volume, andfor increasing light-to-product conversion efficiency.9,10 Besidesbetter light utilization (as compared to suspensions), theentrapment of the dense cultures within the immobilizationmatrix strongly limits cell division and engages more efficientdistribution of photosynthetic reductant to the production ofdesired end-products, instead of biomass accumulation.

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