Six new bacterial species isolated from the phycosphere of marine macroalgae: a joint analysis based on taxonomy and polysaccharide utilization loci

Marine macroalgae-associated <i>Bacteroidota</i> play crucial roles in global carbon cycling through polysaccharide degradation, yet their taxonomic and functional diversity remains understudied. Here, we describe six novel species (strains 3-376^T, 4-2040^T, 2-473A^T, 4-528^T, 4-911^T and 463^T) within the families <i>Flavobacteriaceae</i>, <i>Crocinitomicaceae</i>, and <i>Cytophagaceae</i> isolated from macroalgal surfaces in the coastal area of Weihai, China. Metagenomic read recruitment and 16S rRNA abundance analyses demonstrated host-specific associations. Integrative taxonomic analyses, including phylogenomics (120 conserved proteins), 16S rRNA sequencing, and chemotaxonomy (e.g., MK-6 quinones, phosphatidylethanolamine lipids, and iso-C_15:0 fatty acids), confirmed their novel status, with average amino acid identity (AAI), percentage of conserved proteins (POCP) distinguishing them from related taxa. Genomes (3.3-7.1 Mb; G + C 31.7-45.3%) revealed diverse polysaccharide utilization loci (PULs) targeting algal glycans like laminarin, alginate, and sulfated polymers (ulvan, chondroitin sulfate). <i>Cytophagaceae</i> 463^T harbored the richest CAZyme/PUL repertoire (131 CAZymes, 15 PULs), while <i>Crocinitomicaceae</i> 4-911^T lacked PULs, highlighting family-level specialization. This study expands the known diversity of core phycosphere <i>Bacteroidota</i>, linking PUL evolution to habitat specialization. The novel species' distinct degradative capacities underscore their ecological roles in algal carbon processing and potential for biotechnological applications. Our integrated taxonomy-genomics approach advances understanding of microbial contributions to marine ecosystem dynamics.