Enzymes for microalgal biorefineries
Microalgae represent a more sustainable feedstock for producing functional ingredients. However, their commercial application is hindered due to the high processing costs such as cell disruption. Enzyme-assisted extractions have been proven to be a more sustainable way of performing these processes. However, the high costs of enzyme cocktails represent major bottlenecks. To overcome this hurdle, we propose to include in the microalgal cells a self-destruct mechanism triggered by an external kill-switch signal such as exposure to a certain light spectrum.
In cases of a microalgae multiproduct biorefinery, downstream processing costs are estimated between 50-60 %, almost double the costs of bulk industrial biotechnology processes. This mainly arises from the excessive organic solvent use as well as the energy-intensive mechanical cell disruption methods required to recover and fractionate their various targeted products. The rigid and complex microalgal cell wall also reduces the extraction efficiency of the intracellular active components produced by the algae. Using cell wall-free mutants would not always provide a solution as their rigidity will be impacted and make them susceptible to the shear forces induced by mixing and harvesting.
The precise composition of the microalgal cell wall offers an opportunity for enzymatic hydrolysis. Enzymatic disruption of the microalgae cell wall has been proven to be efficient, less energy-consuming, and environmentally benign. However, commercial adoption of this approach is low due to the high cost of enzyme cocktails and the lack of enzyme reuse options in the biorefineries. To overcome this hurdle, we propose inducible enzymatic autolysis of microalgae as an alternative. We hypothesize that by taking advantage of the available microalgae genetic engineering toolbox, an autolytic system can be integrated into the microalgae genome to allow inducible “self-destruction”. Induction of production of the autolytic enzyme can be triggered by a specific signal, such as chemical compounds, pH, or light, when the microalgae have accumulated sufficient secondary metabolites (carbohydrates, fatty acids, proteins, pigments, etc.). This will minimize the metabolic cost that arises from the expression of non-essential genes.
Thus, to achieve this, several specific objectives have been identified for tRIggERS DIGEST:
- identifying the enzymes required to hydrolyze the microalgal cell wall and determine a suitable kill-switch expression system;
- generating microalgal cell-factory mutants able to express these enzymes when a specific signal is activated;
- develop a proof-of-concept process involving these cell-factories to produce food ingredients such as functional proteins or omega 3 fatty acids;
- determine the techno-economic feasibility of the novel approach.
Besides reduced complexity, and chemical and energy intensity in downstream processing, the selectivity of this system will provide the optimal microalgal cell factory with a wider platform for multiproduct extraction, especially regarding the secondary metabolites that are entangled with the cell wall polysaccharides. In addition, the extracts obtained by enzymatic hydrolysis are safe for use in food and nutraceutical products.