Fanina, Silvia (2026) Bacterial and algal polysaccharides from marine sources: potential bioactive compounds. [Tesi di dottorato]

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Abstract

Marine environments represent a vast and largely untapped reservoir of unique glycoconjugates and extracellular polysaccharides (EPSs) with remarkable physicochemical and biological properties. These compounds hold promising potential for pharmaceutical, cosmetic, and biotechnological applications. The present study aimed to explore the structural diversity, chemical features, and potential bioactivities of polysaccharides from multiple marine organisms, including bacteria, red macroalgae, and microalgae. In second chapter, the surface polysaccharides of Shewanella vesiculosa HM13 nfnB, including capsular polysaccharides (CPS) associated with extracellular membrane vesicles, were investigated. Extraction and purification revealed a previously unreported disaccharide repeating unit consisting of a 4-substituted glucose alternated with a 3-substituted N- acetylglucosamine. This unique CPS structure expands the known chemical diversity of marine bacterial polysaccharides and provides insights into the functional roles of surface glycoconjugates and their impact on vesicle cargo loading. The thrid chapter focused on the CPS of Vibrio crassostreae, a marine gram-negative bacterium potentially associated with Bald Sea Urchin disease. Detailed structural characterization demonstrated that the CPS is composed primarily of L-fucose, D-acetylated 2-amino-2-deoxy-L-fucosamine, and D-mannose arranged in a trisaccharide repeating unit. Preliminary analysis of the LPS fraction revealed phosphorylated Kdo residues. These findings provide a molecular framework for understanding the role of surface polysaccharides in pathogenicity and host interactions, laying the groundwork for future functional studies. In the fourth chapter, the EPS produced by Alteromonas macleodii Mo169 was isolated and characterized. Chemical and NMR analyses revealed a linear disaccharide repeating unit composed of glucosamine and gulaminuronic acid, with an O-acetyl substitution at position 3 of the gulaminuronic acid residue. The study provides a coherent structural description of this marine bacterial EPS, highlighting the occurrence of uronic acids and acetyl groups within Alteromonas polysaccharides and expanding knowledge of their structural diversity. The fifth chapter explored the polysaccharides of the red macroalga Ellisolandia elongata under three environmental conditions: submerged, exposed, and re-submerged. Aqueous and acidic extraction methods were compared, with the acidic procedure providing higher yield and purity. Structural analyses revealed the coexistence of α-1,4-glucan (Floridean starch) and a sulfated xylogalactan. Environmental stress influenced both polysaccharide yield and sulfation patterns. Functional assays demonstrated significant antioxidant and antibiofilm activities, supporting the potential of E. elongata extracts for biomedical and biotechnological applications. Furthermore, preliminary thermal analyses (DSC) and incorporation into sodium alginate films indicated that purified extracts exhibit defined thermal transitions and good VIII compatibility with polymeric matrices, highlighting their potential for material and functional applications. Finally, the sixth chapter focused on EPSs from three Tetraselmis strains, cultivated under autotrophic, heterotrophic, and mixotrophic conditions. Results highlighted strain- and trophic mode-dependent variability in biomass accumulation and EPS production. Tetraselmis chuii produced the highest EPS yield in mixotrophic conditions, with a galactan polysaccharide consisting of a linear 1,3-β-D-galactan backbone and sparsely distributed 3,6-linked branch points. Structural features were influenced by cultivation conditions, demonstrating the interplay between metabolism and polysaccharide biosynthesis, and supporting potential biotechnological applications. This work provides a comprehensive investigation of marine polysaccharides from diverse organisms, elucidating their chemical structures, environmental influences, and preliminary bioactivities. The findings contribute to expanding the understanding of marine glycobiology, offering new perspectives for sustainable biotechnological exploitation of marine biodiversity. The study was supported by the European Marine Biological Resource Centre (EMBRC), which promotes the discovery of novel marine resources while minimizing environmental impact.

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