Abstract
Currently, the European seaweed industry mainly relies on alginate extraction from the wild-harvested tangle (Laminaria hyperborea). The remaining biomass is generally not valorized to the same extent – partly due to a limited market – although alginate account for only 20 – 40 % of the dry mass, depending on the part of the seaweed and the season. Given the expected rise in demand for bioproducts and the growing awareness of our shared natural resources, it is no longer considered economically or environmentally justifiable to only use a fraction of raw materials while discarding the rest as waste. In light of this, the overall objective of the thesis has been to develop multicomponent extraction strategies for brown algae, specifically targeting the four main groups of polysaccharides: alginate, fucoidan, laminarin and cellulose. Alginate already has great commercial value due to its viscosifying and gelling properties, while fucoidan and laminarin show potential within the pharmaceutical, nutraceutical, feed and cosmetics industries based on several reported bioactive properties. Cellulose, which is currently sourced mainly from lignocellulosic biomass, is the most widely exploited biopolymer, and its demand is expected to increase in the coming years.
Substantial expansion of wild harvest of seaweed is challenging from a technical and environmental point of view, and cultivation could help meet increasing demands for biomass. The work has therefore focused on the two cultivated species: Saccharina latissima and Alaria esculenta. First, a sequential extraction protocol was established to recover all four main polysaccharides from seaweed. The process was based on mild chemical treatments to not compromise the structural integrity of the polysaccharides, with emphasis on maintaining the molecular weight of alginate.
A mass balance of the sequential extraction process revealed that high concentrations of fucoidan remained after initial fucoidan extraction. Therefore, an enzymatic process was developed using the protease Alcalase®. Enzymes were employed after initial fucoidan extraction using mild chemical treatment, to recover an additional fucoidan fraction. Compositional and structural analysis revealed that the two fractions, defined as ‘easily accessible fucoidans’ and ‘interconnected fucoidans’ had distinct sugar compositions and degree of sulfation (DS) and acetylation. In addition, a fraction of fucoidan that appeared to be covalently linked to polyphenols was identified. Previous studies have shown that the biological activities of fucoidan are linked to its degree of sulfation and fucose content. It may therefore be of value to extract these fractions separately. By combining strategic chemo-enzymatic treatment with detailed analysis of the extracts, we gained new insights into the complex organization of seaweed polysaccharides within the biomass matrix. A thorough understanding of the biomass composition and the connectivity of individual components is crucial for further development of efficient extractions strategies that may valorize the whole biomass.
Immunomodulating activity, specifically complement inhibition, of ultrapurified ‘easily accessible fucoidans’ from L. hyperborea, S. latissima and A. esculenta were assessed using a human whole-blood model. Further, the chemical composition of both crude and purified fucoidans was analyzed using a combination of monosaccharide analysis (with HPAEC-PAD), CHNS analysis and 1D- and 2D-NMR. The study demonstrated that the purification process, combining tangential-flow filtration, ion-exchange chromatography and carbon filtration resulted in fucoidans with relatively higher DS and fucose contents. Further, the fucoidans from SL and LH – resembling homofucans with high DS (1.3 and 1.6 for SL and LH, respectively) –exhibited complement-inhibiting activity, in contrast to the heterofucans from AE with low DS, which showed no such activity.
Lastly, the effect of acetic acid (AA) preservation of SL on subsequently extracted fucoidans and alginate was examined. The study showed that AA at pH 4.5 or 3.2 can stabilize the SL biomass for up to 8 weeks at 13 or 20 °C. Although acid hydrolysis led to polymer degradation, particularly in alginate, the molecular weight of the polysaccharides remained high (> 230 kDa for alginate and > 1 500 kDa for fucoidan), while the DS of fucoidans appeared unaffected. The rate of acid hydrolysis was much greater for alginate, suggesting that alginate stability is the limiting factor for preserving brown algae in AA prior to their extraction.
A future circular bioeconomy will require both efficient biomass utilization and exploration of new feedstocks. The future of brown algae biorefineries will depend on both technological and market developments for seaweed and seaweed-based products. The work conducted within this thesis aims to accelerate the journey toward this future.