Investigating cultivation and biorefinery process techniques on the microalgae Porphyridium aerugineum and Chromochloris zofingiensis

  • Eleanor Elizabeth Wood

Student thesis: Doctoral ThesisDoctor of Philosophy (awarded by UHI)

Abstract

Biorefinery processes offer the opportunity to improve the economic viability and sustainability of the microalgae industry by concurrent production of multiple products from a single source of biomass. In this thesis, two species, Porphyridium aerugineum and Chromochloris zofingiensis, were selected through a literature review and refining system as potential novel candidates for biorefining. One strain of each species was then selected through growth rate comparison experiments. The two selected strains, P. aerugineum UTEX LB755 and C. zofingiensis SAG 211/14, were then investigated in terms of upstream cultivation and downstream processing with the aim of establishing a biorefinery pathway for each, a first for both species.
The most promising approach for obtaining multiple products from P. aerugineum began with phototrophic growth that was scaled-up from 900mL bottles to a 66L airlift PBR. In the PBR, a maximal biomass concentration of 1.47g/L and a phycocyanin concentration of 139.3mg/L were obtained over 21 days. Both values show the potential competitiveness of this strain with Spirulina sp., the traditional source of phycocyanin. The preferable method of harvesting the cells was centrifugation since it simultaneously concentrated the cells and separated EPS in the supernatant, which was not possible using membrane filtration. After testing four cell disruption techniques, the most successful method of mild, selective cell disruption was freeze thawing the centrifuged pellet. Subsequently, phycocyanin was obtained from the resuspended pellet via centrifugation or microfiltration. This allowed recovery of the intracellular and water-soluble compounds with little impurities. Centrifugation was favourable as starch was simultaneously separated from the rest of the cell debris. Ultrafiltration allowed concentration of solubilised phycocyanin and the separation of soluble carbohydrates. The most promising biorefinery route proposed obtained five fractions that were largely composed of 1) EPS, 2) phycocyanin, 3) soluble carbohydrates, 4) carotenoids, chlorophyll, and lipids, and 5) starch. The findings suggest that P. aerugineum could be the next commercial source of phycocyanin due to the biomass and phycocyanin concentrations achieved and separation of multiple additional products.
The first stage for obtaining multiple products from C. zofingiensis was a two-step phototrophic-mixotrophic upstream process which generated orange phase biomass. This was successfully scaled from 100ml flasks to 65L PBRs where a maximal biomass concentration of 5.13g/L and 14.7mg/L astaxanthin were produced after 8 days of mixotrophic cultivation. Both values show the potential competitiveness of this strain with Haematococcus sp., the traditional source of astaxanthin and provide insight for larger industrial applications of mixotrophy as a method of initiating carotenogenesis.
After harvesting via membrane filtration and centrifugation, five cell disruption techniques were tested where optimal selective disruption was achieved using bead beating (medium sized beads, 5 minutes, 50g/L). The most effective separation route proposed was to firstly use centrifugation of the disrupted biomass to obtain three fractions: top, liquid, and pellet. This should be followed by membrane filtration of the liquid fraction and solvent extraction of the pellet. Five fractions will be obtained, three of which can be combined, giving three products containing: 1) lipids and astaxanthin, 2) soluble proteins and carbohydrates, and 3) insoluble carbohydrates. The results suggest that C. zofingiensis could be the next alternative source of natural astaxanthin due to the high biomass concentrations achieved and separation of multiple additional products.
This thesis demonstrates that species specific biorefinery processes could contribute towards the transition to a sustainable bioeconomy. The species and product specific biorefinery routes proposed provide a baseline for further research and commercialisation. P. aerugineum and C. zofingiensis offer robust and resilient production of microalgal biochemicals, specifically the high value natural pigments phycocyanin and astaxanthin as well as medium value products including EPS, proteins, lipids, starch, and other carbohydrates.
Date of Award2 Aug 2024
Original languageEnglish
Awarding Institution
  • University of the Highlands and Islands
SponsorsXanthella Ltd. & IBioIC- Industrial Biotechnology Innovation Centre
SupervisorMichele Stanley (Supervisor) & Michael Ross (Supervisor)

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