Evaluating Swedish seaweeds for biorefinery
Doctoral thesis, 2020
A sustainable future demands a transition from oil to biomass to produce fuels, chemicals, commodities, and energy. However, to generate the predicted amount of bioenergy required by 2050, all the biomass harvested today should be utilized. One type of biomass that has received relatively little attention is seaweeds. Seaweeds have many advantages over land-based biomass in that they do not require arable land or fertilizer, and their cultivation does not generally compete with food production. Only 20 out of 10 000 species are cultivated and the content of many of them remains poorly characterized. Understanding how their chemical composition is affected by abiotic factors will help determine which biomass component is most valuable and should be maximised.
This thesis aims to enable future research and development of a seaweed industry in Sweden. A screening of 22 species of seaweeds concluded that the most relevant species for biorefinery applications in Europe (Saccharina latissima, and Laminaria digitata) were also among the most interesting for Sweden. In addition, a few poorly investigated species were found to have interesting properties, such as high mannitol content (Halidrys siliquosa) as well as high sugar and low ash content (Chondrus crispus and Dilsea carnosa).
Composition of Ulva intestinalis, which grows all around Sweden, was studied in natural populations on the west coast and southern half of the east coast. Significant effects were found on carbohydrate, fatty acid and ash content between the sites. Rhamnose and iduronic acid were higher on the east coast, but not significantly. However, an elevated sulphate content motivates further investigation of the effect of salinity (the major differential environmental factor) on the potential high-value component ulvan in Ulva intestinalis. There were large variations in yields and composition of the oil, aqueous and solid phases in hydrothermal liquefaction processing within the different sites. Generally, the yields of bio-oil were low, and the quality of the oil was poor due to high contents of S, N and Fe. Considering the low quality of the oil, hydrothermal liquefaction should probably be utilized to treat side streams from a biorefinery after extraction of more high-value compounds.
Finally, abiotic factors and their effect on the growth and composition of Ulva fenestrata were studied. To overcome the low profitability projection of many biorefinery concepts, the data in this thesis could help maximize the value of algal biomass and launch a successful seaweed biorefinery industry. It was found that the content of the valuable monosaccharide rhamnose and the sugar acid iduronic acid could be increased by 26 and 70% respectively at elevated temperature and irradiance. This indicated an increase in ulvan content.