Dottoressa di ricerca

ciclo: XXXIV

relatore: Simone Ferrari
co-supervisore: Simone Ferrari

Titolo della tesi: “Energetic potential of lignocellulosic materials: waste or treasure? Development of bio-based approaches for the valorization of agro-food waste biomasses"

Environmental pollution, energy shortage and geopolitical tension issues demand the production of more eco-friendly biofuels to replace the conventional fossil ones. In this context, lignocellulosic waste biomass represents a promising alternative, being an extremely abundant and unexpensive source of sugars suitable for many industrial processes. In addition to lignocellulose, microalgae represent a promising source of renewable fuels and other industrial products. Mixotrophic growth enables microalgae to grow using light as the energy source as well as CO2 and organic carbon as the carbon sources. The use of lignocellulosic waste biomass as carbon source for algal growth is therefore of interest to advance the objective of a circular bioeconomy. In this work, the potential of different types of agro-food waste lignocellulosic biomass to support growth of Chlorella vulgaris, a strain suitable for biodiesel production and well known for its ability to grow under mixotrophy, was evaluated. Initially, three different waste biomasses, namely barley straw (BS), park and garden waste (PGW) and citrus processing waste (CPW), were characterized in terms of monosaccharides released upon enzymatic saccharification. Treatment of BS and PGW with a cocktail of cellulases and pectinases mainly released glucose (Glc) and xylose (Xyl), whereas digestion of CPW yielded high amounts of Glc, arabinose (Ara), galactose (Gal) and galacturonic acid (GalA). The use of the white rot fungus Phanerochaete chrysosporium as a cheap, bio-based alternative to commercial enzymes for saccharification of waste biomass was also evaluated. Cell-wall degrading enzymes secreted by the fungus in the presence of waste biomasses were identified by proteomics approaches, indicating that P. chrysosporium secretes largely overlapping sets of enzymes, including glucanases, xylanases and pectinases, but not ligninolytic enzymes, when grown on the BS or PGW. Digestion of BS with fungal filtrates yielded amounts of Glc comparable to those released by commercial enzymes. However, when the biomass was incubated directly with P. chrysosporium, rather than the culture filtrate, very low amounts of Glc accumulated in the medium, indicating rapid assimilation by the fungus, whereas Ara and GalA accumulated to significant amounts, suggesting that the latter monosaccharides are not significantly metabolized. Analysis of the oligosaccharides released by the fungus in the culture medium indicated extensive degradation of cellulose, mixed-linked β-1➝3/β-1➝4-glucans and pectins. These results indicate that BS and CPW are suitable sources of different monosaccharides that can be potentially used to grow microalgae under mixotrophy. Moreover, the secretome of P. chrysosporium can be used to efficiently convert these biomasses into fermentable sugars, though this fungus can be used to obtain Ara and GalA, but not Glc or Gal. The ability of C. vulgaris to grow using sugars derived from different waste biomasses as organic carbon sources was then evaluated. Sugars released from BS and CPW upon enzymatic saccharification were able to efficiently support C. vulgaris growth and resulted in similar biomass production. C. vulgaris could consume all Glc and Gal present in the digestate within few days, and was partially able to assimilate also Xyl, but not Ara or GalA. However, when these monosaccharides were provided as sole organic carbon sources, only Glc and Gal could be consumed by the alga. Notably, greater biomass production was observed when algae were grown on digestate from CPW than from BS, suggesting that additional factors, beside monosaccharide composition, make the former a more suitable substrate for mixotrophic growth of C. vulgaris. These results suggest that sugars from agro-food waste, and in particular citrus processing by-products, represent a suitable source of organic carbon for mixotrophic growth of C. vulgaris.

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