Microalgae as photosynthetic cell factories for biofuel production
The microalgae Botryococcus has an outstanding capacity to produce and excrete large quantities of hydrocarbons (levels of up to 85% dry weight have been reported) that can be used to substitute hydrocarbon based fuels. Although Botryococcus braunii represents one of the most interesting organisms for the direct production of biofuels, its utilization as an industrial platform for this purpose has been limited by its extremely slow growth rate and present lack of knowledge on both biological (genome, metabolism, composition and regulation of the photosynthetic apparatus) and bioprocess engineering aspects (continuous production and extraction of hydrocarbons at high productivities).
The main objective of the project is to develop scientific and applied knowledge required to implement Botryococcus as an industrial platform for sustainable production of hydrocarbons. The challenge is to bring the organism in a biological steady state with high solar to hydrocarbon conversion rates by phenotypic engineering and selection for high product productivities through efficient milking. A coherent strategy is followed based on four tasks:
1. Understanding the molecular mechanisms and regulation of photosynthesis in Botryococcus for growth improvement;
2. Understanding the efficiency of nutrient supply to the cells and eventual mass transfer limitations due to the formation of colonies for growth enhancement:
3. Growth enhancement by strain improvement and
4. Development of a process in which the cells do not need to grow but are instead used as a photocatalyst for the continuous production of hydrocarbons. Ideally, in such a process the major part of supplied photosynthetic energy is converted directly in high quality oil, which is excreted by the cells and can be continuously recovered.
One common aspect in the four tasks is that high productivities of the desired hydrocarbons are required and that the product needs to be recovered. The metabolism must be therefore directed towards the desired hydrocarbons and the productivity must be further optimised. To assess the performance of the organisms under various environmental conditions, we will acquire knowledge on the metabolic pathways and how they lead to the different hydrocarbons, under different steady state conditions. To obtain more insight in the metabolic pathways the genome of Botryococcus will be sequenced (requirement to set up a genome based metabolic model), the metabolome will be measured (to allow the calculation of flux distributions) and physiological studies will be done in combination with gene expression to allow for the identification of key genes controlling hydrocarbon biosynthesis and therefore of targets for manipulation, which will as well be done in the project. As an alternative approach, depending on the success and time required for developing an efficient transformation system, the reprogramming of the transcriptome with artificial transcription factors to put stress directly on the transcriptome, and then perform directed evolution, will be done. Finally, a process for the in situ continuous recovery of the hydrocarbons will be developed.
The team that will carry out this project is composed of tenured and temporary staff members of the Bioprocess Engineering Group, Wageningen UR, the Elementary Events in Biophysics Group of the Free University, Amsterdam, Plant Research International, Wageningen UR, and Food & Biobased Research, Wageningen UR.