Genome-based identification of a putative ulvan-related gene cluster in Celeribacter sp.

Ulvan is a sulfated polysaccharide that constitutes a major component of the cell wall of green macroalgae in the genus Ulva and plays an important role in coastal carbon and sulfur cycling. Although ulvan-modifying enzymes and gene systems have been characterized primarily in Bacteroidetes and, to a lesser extent, Gammaproteobacteria, their occurrence in Alphaproteobacteria remains poorly understood. In this study, strain ULVN23_4 was isolated from the surface of Ulva lactuca collected in Nha Trang Bay, Vietnam, and its genome was sequenced using the Illumina platform, followed by genome annotation, phylogenomic analysis, and carbohydrate-active enzyme (CAZyme) profiling. Genome-based relatedness analyses using average nucleotide identity (ANI) and digital DNA–DNA hybridization (dDDH) supported its classification as a putative novel genomic species within the genus Celeribacter . Genome analysis identified a candidate ulvan-associated gene cluster containing a putative polysaccharide lyase family 25 (PL25)-like protein, a putative glycoside hydrolase family 105 (GH105)-like protein, and an S1-family sulfatase assigned to subfamily 7 (S1_7), co-localized with an adenosine triphosphate-binding cassette transporter. Notably, this locus lacks the canonical starch utilization system proteins C and D (SusC/D) transport pair typically found in Bacteroidetes polysaccharide utilization loci (PULs), suggesting a non-canonical, transporter-associated genomic organization. Plate-based assays produced clear halos on ulvan-containing medium and blue coloration on medium supplemented with 5-bromo-4-chloro-3-indolyl sulfate (X-SO 4 ), observations consistent with modification or depolymerization of the ulvan-containing substrate and sulfatase-associated activity, respectively. Collectively, these findings support the hypothesis that this genomic region may participate in the processing of ulvan-derived or related sulfated substrates. This study expands the phylogenetic range of bacteria potentially involved in ulvan transformation and highlights Alphaproteobacteria as an underexplored group in the turnover of sulfated polysaccharides in marine environments. Further biochemical characterization will be required to validate the predicted enzymatic functions.