Biocatalytic ulvan degradation by Pseudoalteromonas marina: exploring a marine polysaccharide bioconversion system

Ulvan depolymerization by marine bacteria is a specialized metabolic capability with significant implications for marine carbon cycling and biotechnological applications. While ulvan degradation systems have been extensively characterized in Bacteroidota, their occurrence and mechanisms in Pseudomonadota remain poorly understood. This study proposed a putative ulvan utilization system in the marine bacterium Pseudoalteromonas marina PUA1001, representing a notable metabolic expansion beyond the typical Pseudomonadota repertoire. Comparative genomic analysis with the closely related strain P. marina mano4 (97.5% OrthoANI) revealed a complete absence of the ulvan utilization locus in mano4, suggesting a possible role and strain-level metabolic specialization. The model PUA1001 ulvan utilization locus comprises three polysaccharide lyases (PL24, PL25, PL40), seven glycoside hydrolases, including previously unreported GH106 and GH115 families, two sulfatases, and seven accessory genes, predicted to enable stepwise depolymerization of ulvan to monosaccharides. Biochemical characterization of the three recombinantly expressed ulvan lyases revealed optimal activities at 35-50 °C and pH 8.0, with significant Mn²⁺-dependent activity enhancement (1.25 - 2 fold). Molecular docking analysis of PmUL24 and PmUL25 confirmed conservation of the His/Tyr-dependent β-elimination mechanism. This polysaccharide utilization system may contribute to the ecological success of Pseudoalteromonas species in ulvan-rich marine environments, highlighting adaptive flexibility and strain-specific potential for exploiting distinct polysaccharide niches within marine ecosystems.