Imbimbo, Paola (2020) Microalgae as factory of high value bioproducts. [Tesi di dottorato]

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The aim of this PhD thesis was the use of red thermo-acidophilic microalgae, Galdieria phlegrea (stain 009) to develop a biorefinery to obtain three different high value bio-products. First, a preliminary cascade approach has been designed to recover phycocyanin and lipids from the wet biomass of the microalga grown in autotrophic conditions. Extractions were performed in two sequential steps: a conventional high-pressure procedure to recover phycocyanins (PCs) and a solvent extraction to obtain fatty acids. PCs were isolated by a single purification step obtaining a high purity grade and they fully retained their antioxidant activity on a cell-based model. From the residual biomass, fatty acids were recovered, with a higher yield with respect to those obtained from the raw biomass. Then, a green approach was used to recover PCs, carotenoids and lipids in cascade. PC extraction was performed as mentioned above and the isolation was optimized by ultrafiltration. Carotenoids were obtained by a pressurized liquid extraction (PLE) on the residual biomass and finally lipids were extracted by supercritical fluid extraction (SFE). The used extraction procedures are green, fast, and cost-effective and allow to improve the extraction yield up to 40% for carotenoids and up to 12% for lipids in comparison with conventional extractions. The carotenoids extracted by PLE were tested as antioxidants, thus proving that the procedure employed for the extraction did not affect their bioactivity. Finally, a complete biophysical and biochemical characterization of purified phycocyanin was carried out. The results show that the protein is stable under different pHs and unfolds with an apparent melting temperature higher than 80°C within a pH range between 5.0-7.0. The protein also exerts interesting in cell antioxidant properties even after the pasteurization process, and is cytotoxic for cancer cells, whereas it is not toxic for non-malignant cells. In conclusion, in the present PhD thesis, a green biorefinery was set up, thus completely valorizing the biomass of G. phlegrea. The proposed approach to recover in a cascade approach PCs, carotenoids and PUFAs could be easily scaled up at industrial level. The obtained results could also contribute to increase the revenue streams of the process, thus compensating the large cultivation and downstream costs for biomass production. Furthermore, the development of a green process could also increase the social acceptance of industrial microalgal products.

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