Genomic and phenotypic insights into Pseudocolwellia antarctica sp. nov., a novel psychrotolerant bacterium with symbiotic potential from Antarctic zooplankton.

Zooplankton in the Southern Ocean represent unique microhabitats for microbial communities that contribute significantly to polar biogeochemical cycles. In this study, we performed a polyphasic and genomic characterization of two Gram-negative, psychrotolerant bacterial strains, HL-MZ7T and HL-MZ19, isolated from Antarctic zooplankton. Phylogenetic and phylogenomic analyses clearly placed these isolates within the genus Pseudocolwellia, forming a sister clade to Pseudocolwellia agarivorans QM50T. The complete genomes of HL-MZ7T (4.5 Mbp) and HL-MZ19 (4.6 Mbp) exhibited DNA G + C contents of 36.0% and 35.9%, respectively. Average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values confirmed that both strains represent a novel genomic species, distinct from P. agarivorans QM50T (79.5% ANI and 24.0% dDDH). Comparative genomic analysis revealed a rich repertoire of genes associated with cold environmental adaptation and host-microbe interactions. Notably, we identified biosynthetic gene clusters of aryl polyenes and homoserine lactones, alongside genes involved in surface adhesion and nutrient acquisition, suggesting a specialized symbiotic potential within the zooplankton micro-niche. Phenotypically, the strains are characterized as strictly aerobic, with optimal growth at 15-20 °C and 3.0% (w/v) sea salts. These findings expand the genomic and ecological understanding of zooplankton-associated bacteria in polar marine ecosystems, and support the proposal of Pseudocolwellia antarctica sp. nov.