Publication
Plastics are versatile polymeric materials that are produced for a multitude of useful applications. The world-wide demand for plastics, which are currently mostly made from fossil fuels, is continuously increasing. The development of biodegradable plastic materials from renewable sources is a major challenge for plastic industries to provide sustainable alternatives to petroleum plastic, that is accountable for a large proportion of anthropogenic greenhouse gas emissions and the environmental pollution by micro-plastics. Recently, cellulose and chitin the most abundant fiber-forming biopolymers on earth have gained attention as raw materials for the development of new bio-based plastics. Cellulose, mostly derived from wood and plant biomasses that compete with agricultural land for food supply and chitin from shell waste, can be processed into nano-sized particles, which are favourable building blocks for many sustainable industrial applications for instance as biocompatible and biodegradable packaging materials with useful mechanical and barrier properties. Growing evidence points to the presence of both, cellulose and chitin as structural constituents of the cell wall of a small number of microalgae, among them Chlorella vulgaris, one of the most frequently cultivated unicellular green microalga of commercial relevance. Despite the potential of algae as a feedstock for nanocellulose and nanochitin very little is known how to produce and to process them efficiently from algal biomass. Moreover, there is yet no feasible strategy available for enhancing the productivity of these biopolymers for future commercial applications. In the present thesis some simple and cost-effective abiotic stress approaches were developed to remarkably increase the cell wall thickness in C. vulgaris thereby significantly improving the cellulose and chitin biopolymer productivity by a factor of three. Using a double staining approach, we revealed for the first time the structural organization of chitin and cellulose in alternating layers under specific stress conditions, presumably contributing to a higher cell wall stability. Environmentally friendly downstream processing strategies for the effective co-purification of cellulose and chitin from microalgae are not yet available. In the present study the development of a green biorefinery approach was achieved that allowed on one hand the co-extraction of chitin and cellulose for subsequent mechanical fibrillation, and for the first time the fabrication of free-standing bioplastic films. On the other hand, this biorefinery approach enabled to co-purify lipids, pigments and proteins together with the biopolymers to make the whole procedure economically more viable. This greener fractionation and purification process was shown to reach efficiencies comparable to conventional more toxic solvent extractions. The analysis of the structure and molecular composition of the bioplastic film, produced in the present study, indic