Harvesting sunlight in a biodegradable polymer
A key process in the upcoming bio-based economy is the conversion of the primary product of the Calvin cycle of phototrophic organisms into products that can be harvested efficiently in terms of economy and yield. Different approaches have been proposed to achieve this. One is to harvest biomass and convert it thermo-chemically into syngas. Another is to grow photosynthetic organisms with high triglyceride content, followed by conversion of this lipid fraction, via trans-esterification, into biodiesel. A third approach aims at direct conversion of CO2 into butanol. Without natural or artificial selection criteria it is difficult to optimize these processes through genetic screening and all three approaches, suffer from significant problems in the down-stream processing of the ‘fixed- carbon’ product.
In this project we explore optimization of downstream processing of a fixed carbon product. We aim for accumulation of the primary product of carbon fixation in photosynthesis at high yield, more than 90% of dry weight, in an extra-cellular polysaccharide (EPS, i.e. cellulose). Cellulose is optimally suited to be converted to biofuel. The Culture Collection Yerseke (CEME) contains a collection of natural isolates of cyanobacteria that will be screened for maximal cellulose production. At least one strain, Crinalium epipsammum, has been demonstrated to naturally synthesize high amounts (20% of dry weight) of crystalline cellulose. It is expected that this yield can be considerably increased through manipulating culture conditions. Additional strains will be isolated from environments where cyanobacteria with cellulose synthase genes are found.
Furthermore, the literature has revealed details of the regulation of EPS synthesis via cyclic-di-GMP, including a photoreceptor regulating this process in Escherichia coli. Also, heterologous expression of genes encoding cellulose synthase in the cyanobacterium Synechococcus UTCC 100 has been demonstrated. These developments now allow for a synthetic biology approach to isolate and/or engineer a strain with high cellulose yield by the addition of a crystalline cellulose extracellular accumulation system. Our vision is that such an approach has the potential of becoming widely applied for improving downstream processing of carbohydrates across different species. As we learn more about the cellulose secretion process, procedures to deposit the EPS in crystalline form in the growth medium become feasible. These procedures will be tested through directed engineering and random mutagenesis. The project will be a success if the natural production of cellulose in C. epipsammum and other isolates can be significantly increased by engineering and manipulation of the cultivation conditions and if stable cellulose producing transformants of Synechocystis PCC6803 or other candidate strains are obtained that convert 90% of the fixed CO2 into crystalline cellulose that is exuded into the medium.
The team that will carry out this project is composed of staff members of the Molecular Physiology Group of Swammerdam Institute for Life Science and the Aquatic Microbiology Group (IBED) of the University of Amsterdam.