Ulvan from Ulva lactuca L.: a multitarget approach for chemical characterization combined with in vitro tests on human gut microbiota

This doctoral thesis aimed to provide valuable insights into the transformation of the seaweed Ulva lactuca L., harvested in aquaculture tanks of the Orbetello lagoon (Tuscany, Italy), from a disposal challenge into a promising source of bioactive compounds, focusing on its main polysaccharide, ulvan. A multi-methodological approach was applied to develop an effective extraction protocol, characterize the chemical and structural features of ulvan, and assess its fermentability and potential prebiotic effects. Firstly, extraction conditions were optimized using Design of Experiment (DoE) and Response Surface Methodology (RSM), achieving high quality ulvan in high yield. The criteria identified through RSM may also guide future extraction studies on different Ulva species or environmental contexts. The extracted ulvan was characterized for monosaccharide composition, sulfation degree, molecular weight, and structural features. Subsequently, an optimized enzymatic hydrolysis method was developed using a commercial ulvan lyase to produce ulvan oligosaccharides suitable for biological applications and fermentation tests by reducing medium salt content and selecting a biocompatible buffer. The enzyme, kinetically characterized, showed high efficiency and robustness, supporting its potential for innovative applications. Finally, in vitro digestion and subsequent fermentation studies revealed that ulvan is utilized by gut microbiota, leading to the production of short-chain fatty acids and modulation of tryptophan metabolism, with increased levels of health-promoting metabolites such as indole-3-propionic acid and indole-3-acetic acid. Untargeted metabolomics confirmed ulvan utilization and highlighted potential ulvan-specific fermentation products. Overall, the extraction and hydrolysis protocols established in this thesis lay the groundwork for future valorization of Ulva biomass, while the positive results on ulvan fermentation suggest its potential as a dietary supplement or functional food ingredient.