Klaus Benedikt Möllers:
Photosynthetically converted solar energy has been stored for millions of years in its most energy dense organic states: petroleum, coal and natural gas. The accessibility and refinement of this fossil energy source has revolutionized our modern day society, its economy, and its technological capacities. On the other hand, the extensive use of fossil fuels, the increasing emissions of greenhouse gasses from industries as well as today’s inefficient food chain have produced a climate change scenario that is threatening our survival on planet Earth. However, in order to reduce CO2-emissions and to evolve from a wasteful petrochemical system into a sustainable bio-based society, biofuels and the introduction of bio-refineries play an essential role. Aquatic phototrophs are promising organisms to employ photosynthetic capacities as well as the derived carbohydrates for the production of biofuels and bio-based products.
This thesis shows two examples of the applicability of cyanobacterial biomass as a renewable substrate for industrially relevant biofuel fermentations, i.e. ethanol fermentation by Saccharomyces cerevisiae and acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum ATCC 824 (Paper 1 and paper 2). Furthermore, photo-pigments or the entire photosynthetic apparatus, i.e. the thylakoid membrane system of cyanobacteria were applied for the oxidative degradation of plant biomass by a novel light dependent electron transfer to a metalloenzyme (Paper 3). In particular, the industrially relevant lytic polysaccharide monoxygenases (LPMOs) were employed to further improve their capacity to oxidize the most recalcitrant polysaccharides, i.e. cellulose, for subsequent hydrolysis to monomeric sugars. This auxiliary oxidation is a crucial step to optimize the, otherwise costly, enzymatic conversion of lignocellulosic biomass for further fermentation or as a platform chemical in a bio-refinery concept.
Autotrophically cultivated cells of the marine model cyanobacterium Synechococcus sp. PCC 7002 (Synechococcus) were exposed to mild nitrogen starvation which has been identified as the most favorable growth condition to induce intracellular accumulation of poly-glucose (mainly glycogen) of up to 60% per dry weight. These carbohydrates were efficiently released by a set of enzymatic pretreatments (lysozyme and α-glucanases) and successfully fermented into promising alternative transportation fuels, i.e. ethanol and butanol (Paper 1 and 2). The obtained results can serve as ‘proof of concept’ works for further investigations in this field. Also, if not applied as carbohydrate feedstocks, it was found that cyanobacterial biomass holds a great potential to be used as nutrient source for fermentation of plant waste material or a substitute for yeast extract.
By mimicking photosynthetic electron transport from light excited photo pigments to LPMOs in combination with a reductant and cellulose as substrate, a 100-fold increase in catalytic activity of LPMOs was observed. Also, it was found that the substrate specificity of a LPMO could be broadened to be active on cellulose and hemicellulose.
These findings may pave the way for new applications and novel biotechnological processes, and are important insights for the development of a sustainable bio-based platform for biofuel production and chemical processing.