l-Rhamnose from Seaweeds, Microalgae, and Cyanobacteria: Sourcing, Production, and Bioprocessing Strategies

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.