This study explores a multi-resource approach for extracting and characterizing l-rhamnose-rich polysaccharides from nine natural biomasses, including green macroalgae (Ulva spp.), sumac species (Rhus spp.), and agro-industrial by-products such as sea buckthorn and sesame cakes. Hot-water and alkaline extractions were performed by biomass type, and the resulting fractions were analyzed using biochemical assays, monosaccharide profiling (HPAEC/PAD and GC/MS-EI), FTIR, and antioxidant activity tests. Extraction yields ranged from <1% in sea buckthorn residues to 15.48% in Ulva spp., which showed the highest recovery. l-rhamnose enrichment varied across biomasses: the highest proportions were found in Ulva extracts and Rhus semialata galls (PRS), reaching up to 44% of total sugars by HPAEC/PAD and 58% by GC/MS-EI. Antioxidant activities also differed markedly. In DPPH assays, the most active extracts were those from sea buckthorn berry cake (PTBA), Rhus coriaria seeds (PRC), and commercial sea buckthorn powder (PPA), with IC₅₀ values of 32, 43, and 42 µg/mL, respectively. Hydroxyl-radical inhibition was also substantial, reaching 83.0% for PTBA, 79.4% for PRC, and 79.9% for Ulva lactuca at 1 g/L, compared with 97.5% for ascorbic acid. These results highlight specific biomasses as promising dual sources of l-rhamnose and natural antioxidants for valorization within a circular bioeconomy.

L-Rhamnose (6-deoxy-l-mannose) is a rare deoxyhexose recognized for its significant value in the food, pharmaceutical and nutraceutical industries due to its unique physiological effects. While primarily utilized as a reagent in organic synthesis, l-Rha and l-Rha-enriched fractions are conventionally sourced from terrestrial plants (fruits, buds, leaves, barks, and stems) and bacteria (including Klebsiella sp., Pseudomonas aeruginosa, and Mycobacterium spp.). Yet, l-Rha co-extraction often leads to contamination with other monosaccharides or aglycone compounds prior to hydrolysis, which is typically carried out using chemical and/or enzymatic methods. Furthermore, the purification steps necessary to obtain high-purity l-Rah generally involve organic solvents and long separation processes that are poorly suited for large-scale applications. Future biotechnological processes must therefore be critically re-evaluated through intelligent sourcing, specifically tailored downstream processing technologies (particularly for purification), and innovative chemo-enzymatic approaches to boost l-Rha production. This review provides a comprehensive overview of current aquatic sources, structural features, production processes, and properties of l-Rha and l-Rha-enriched fractions. Emphasis is placed on novel insights into l-Rha-enriched polysaccharides produced by macroalgae, microalgae, and bacteria. Finally, perspectives on next-generation bioprocessing strategies and key challenges affecting the future of efficient l-Rha production are discussed.