Isolation and characterization of Polaribacter sargassicola sp. nov. and Algibacter ulvanivorans sp. nov.: two novel species with ability to degrade algal polysaccharides

The genera Polaribacter and Algibacter, which are members of the Bacteroidota family Flavobacteriaceae, are well known for their arsenal of carbohydrate-active enzymes (CAZymes) and specialized polysaccharide utilization loci (PULs). In this study, two aerobic, gram-negative, and rod-shaped bacterial strains, designated strains DS7-9 ^T and PT7-4 ^T, were isolated from the surface of seaweed collected from the coastal zone of Fujian Province, China. The two strains exhibited broad tolerance ranges to salinity (0–4%), temperature (10–40 °C) and pH (5.0–10.0). The predominant fatty acids in strain DS7-9 ^T were iso-C_15:0 (53.7%) and iso-C_15:0 3-OH (10.9%), whereas those in strain PT7-4 ^T were iso-G-C_15:1 (33.2%) and iso-C_15:0 (27.1%). Additionally, the predominant menaquinone was MK-6. The 16S rRNA gene of strain DS7-9 ^T shares 97.5% similarity with that of Polaribacter haliotis KCTC 52418 ^T, whereas that of strain PT7-4 ^T is 97.0% sequence similarity with that of Algibacter mikhailovii KCTC 12710 ^T. Phylogenomic analysis revealed that the highest average nucleotide identity (ANI) value between strain DS7-9 ^T and its closest relative reached 84.7%, whereas the corresponding value for strain PT7-4 ^T was 84.2%. Genomic characterization revealed DNA G + C contents of 29.5% and 31.9% for strains DS7-9 ^T and PT7-4 ^T, respectively. Both strains possessed complete circular chromosomes, with genome sizes of 3.7 Mb (DS7-9 ^T) and 3.5 Mb (PT7-4 ^T). Bioinformatic analysis revealed that strain DS7-9 ^T harbors 5 PULs putatively associated with laminarin, ulvan, fucoidan, and alginate catabolism, whereas strain PT7-4 ^T possesses 7 PULs predicted to target ulvan, alginate, and starch metabolism. On the basis of their distinct physiological characteristics, two strains are suggested for classification into novel species: Polaribacter sargassicola sp. nov. (type strain DS7-9 ^T = MCCC 1K08670^T = KCTC 102076 ^T) and Algibacter ulvanivorans sp. nov. (type strain PT7-4 ^T = MCCC 1K08669^T = KCTC 102077 ^T).