a b s t r a c tMost organic matter can be used for bioenergy generation via anaerobic fermentation. Today, crop plants like maize play the dominant role as substrates for renewable biogas production. In this work we investigated the suitability of six dominant microalgae species (freshwater and saltwater algae and cyanobacteria) as alternative substrates for biogas production. We could demonstrate that the biogas potential is strongly dependent on the species and on the pretreatment. Fermentation of the green alga Chlamydomonas reinhardtii was efficient with a production of 587 ml (±8.8 SE) biogas g volatile solids −1 (VS −1 ), whereas fermentation of Scenedesmus obliquus was inefficient with only 287 ml (±10.1 SE) biogas g VS −1 being produced. Drying as a pretreatment decreased the amount of biogas production to ca. 80%. The methane content of biogas from microalgae was 7-13% higher compared to biogas from maize silage. To evaluate integrative biorefinery concepts, hydrogen production in C. reinhardtii prior to anaerobic fermentation of the algae biomass was measured and resulted in an increase of biogas generation to 123% (±3.7 SE). We conclude that selected algae species can be good substrates for biogas production and that anaerobic fermentation can seriously be considered as final step in future microalgae-based biorefinery concepts.

SummaryThe main function of the photosynthetic process is to capture solar energy and to store it in the form of chemical 'fuels'. Increasingly, the photosynthetic machinery is being used for the production of biofuels such as bio-ethanol, biodiesel and bio-H 2 . Fuel production efficiency is directly dependent on the solar photon capture and conversion efficiency of the system. Green algae (e.g. Chlamydomonas reinhardtii ) have evolved genetic strategies to assemble large light-harvesting antenna complexes (LHC) to maximize light capture under low-light conditions, with the downside that under high solar irradiance, most of the absorbed photons are wasted as fluorescence and heat to protect against photodamage.This limits the production process efficiency of mass culture. We applied RNAi technology to down-regulate the entire LHC gene family simultaneously to reduce energy losses by fluorescence and heat. The mutant Stm3LR3 had significantly reduced levels of LHCI and LHCII mRNAs and proteins while chlorophyll and pigment synthesis was functional. The grana were markedly less tightly stacked, consistent with the role of LHCII. Stm3LR3 also exhibited reduced levels of fluorescence, a higher photosynthetic quantum yield and a reduced sensitivity to photoinhibition, resulting in an increased efficiency of cell cultivation under elevated light conditions. Collectively, these properties offer three advantages in terms of algal bioreactor efficiency under natural high-light levels: (i) reduced fluorescence and LHC-dependent heat losses and thus increased photosynthetic efficiencies under high-light conditions; (ii) improved light penetration properties; and (iii) potentially reduced risk of oxidative photodamage of PSII.

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